ELCOMP COMCOLOR
SPECTROPHOTOMETER AND COLOUR DATA COMPUTER
used as a computer peripheral
USER'S MANUAL
January, 2013.
ELCOMP Kkt.
Vadász u. 107.
H-2800 Tatabánya,
HUNGARY
Phone: +36-34-786-734
COMCOLOR SPECTROPHOTOMETER - USER'S MANUAL
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Copyright  2013. by M. Szabados 2800 Tatabánya, Vadász u. 107.
All rights reserved
CONTENTS
USER'S MANUAL
GENERAL DESCRIPTION .................................................................
APPLICATION AREA .........................................................................
SETTING UP ......................................................................................
PRINCIPLE OF OPERATION .............................................................
HOW TO USE IT ................................................................................
MEASURING ACCURACY .................................................................
CALIBRATION ...................................................................................
SOFTWARE DESCRIPTION ..............................................................
INSTRUMENT SETTINGS .................................................................
HANDLING MEASUREMENT DATA ..................................................
FURTHER USAGE .............................................................................
TECHNICAL DATA .............................................................................
APPENDIX A
COLOUR PERCEPTION ....................................................................
COLOUR MIXTURE ...........................................................................
COLOUR ORDER SYSTEMS ............................................................
COLOUR COLLECTIONS ..................................................................
COLORIMETERS ...............................................................................
CALIBRATION OF INSTRUMENTS ...................................................
COMPARISON OF RESULTS ............................................................
RECOMMENDED LITERATURE ........................................................
APPENDIX B
COMCOLOR MEASURING ACCURACY ...........................................
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IBM and IBM PC is a trademark of IBM Corp.
MSDOS and WINDOWS are trademarks of Microsoft Inc.
Munsell Color System is a trademark of the Munsell Color Company
NCS Natural Color System is a trademark of the Scandinavian Colour Institute
Subjected to changes without notice to improve quality and reliability.
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GENERAL DESCRIPTION
The ELCOMP COMCOLOR spectrophotometer and colour data computer
essentially consists of a measuring head and an application software packet used in
conjunction with an IBM PC (or compatible). The measuring head is to be connected to
one of the USB, or serial communication ports of the PC, and the software packet
enables its use as a spectrophotometer and colorimeter. The instrument can be used
easily and quickly, after placing the measuring head on the sample, only the selected
measurement action has to be started by a click on the computer's mouse or keyboard,
the measurement takes only two second.
For running the COMCOLOR software we anticipate that a variant of Microsoft's
WINDOWS operational systems has been installed. The use of the computer as a
spectrophotometer and colorimeter has no influence on its use for other applications,
the only restriction is that during the running of the COMCOLOR software other
program's undisturbed simultaneous usage is not granted.
The COMCOLOR spectrophotometer measures the reflectance spectra of
opaque materials (surfaces) in the visible wavelength range from 360 nm to 760 nm.
This spectral reflectance function is, beside the spectral power distribution of the
illuminating light source, the main factor influencing colour perception when the surface
is envisioned. Also available a variant of the instrument, working in the near infrared
(600-1200 nm) range.
Colour is a complicated human perception, in this area of science there are still a
number of unknown factors. According to the present knowledge the light rays
penetrating into our eye get absorbed in the retinal layer, where the receptors are
located, and from where the visual information is transmitted to the visual cortex of the
brain, and the perception gets formed here. Although there are considerable variations
in human colour perceptions, it is possible to elaborate certain general rules and
quantities that enable the description of the colour stimulus responsible for the colour
perception. The correlation between the stimulus and the perception is reasonably
good, especially for determining correlates of colour perception differences. A short
overview of colorimetry and a literature survey is found in Appendix A of this Manual.
The application software supplied with the measuring head enables the user to
display the measured reflectance spectra both in graphical and tabular form. The
program displays on the monitor a copy of the measured colour stimulus, it calculates
the tristimulus values, chromaticity co-ordinates and quantitative values in some
uniform colour spaces. The software also enables the user to save the measured
results for later use or to display the results saved before. All the displayed results can
also be printed on a printer connected to the host computer.
APPLICATION AREA
The instrument can be used in most area of daily practice, where the colour of
opaque objects (surfaces), or the colour difference of two objects has to be quickly
determined. Compared to traditional instruments a fundamental advantage is that the
measuring head can be freely moved, thus it is not necessary to sample the tested
material, the measuring head can be positioned at almost any place of the surface to
be measured. Inside the instrument light emitting diodes are used for sample
illumination, so there is neither warm-up time, nor heat production.
The COMCOLOR spectrophotometer uses 45° illumination and perpendicular
observation angles, thus no gloss contribution is included in the measured results. This
has to be taken into consideration if the results obtained by the instrument are
compared to those taken by instruments of other optical layout.
The measuring software can calculate colour stimulus values for any of the
standard CIE illuminants and observers. The parameters to be used in calculations can
COMCOLOR SPECTROPHOTOMETER - USER'S MANUAL
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be selected in combo boxes on the computer's monitor, the selected parameters are
stored, continuously displayed, and used by the computer until their next modification.
The software also permits the direct determination of quantities used in the paper and
printing industries (whiteness grades, opacities, densities, fill factors, etc.).
Examples of application:
Building industry:
selecting and qualifying materials,
determining and keeping colour tolerances.
Food industry:
determining colour of foods and vegetables,
classifying food products,
determining optimal picking time of fruits.
Printing industry:
qualifying basic materials (papers and inks) and end products,
harmonising design and production,
control of printing processes and machines.
Chemical industry:
grading basic materials and end products,
pigment characterisation,
producing materials with required colorimetric characteristics.
Textile industry:
grading basic materials and end products,
recipe calculation to obtain the requested colour.
Education:
visualising reflectance curves and colours associated to them,
studying different colour order systems and their interdependence,
inclusion of measured results into your own calculations.
In shops:
selecting paints for repair,
mixing the selected colorant.
SETTING UP
Before setting up the spectrophotometer and installing the application software,
the user should check whether the PC is correctly set for the main supply and whether
it is connected to the mains according to the local safety requirements via a suitable
protecting device (protecting ground or isolating transformer).
The computer has to be switched off before the measuring head is connected by
the cable supplied to one of the serial ports, or via the supplied adapter to one of the
USB ports of the computer. Take off the protecting cover from the optical input port of
the measuring head and place the head, with the optical entrance downward on a
stable, plane surface.
Note: The serial ports of IBM PCs have usually 9 or 25 pin connectors, to be
found on the rear side of the computer. If the connector on the cable supplied with the
instrument does not match the socket on the PC, use a standard MODEM connector
adapter. At recent computers use the supplied USB - RS232 adapter.
After starting the computer put the supplied data storage device into the
appropriate drive of the computer and copy its content into a new directory on the hard
disk drive where your operating system is also located (usually Drive C:). The program
can now be started from this directory by opening for execution the Comcolor.exe file.
Note: On the supplied data storage device all the necessary files needed for
operating the instrument are located in one folder named ELCOMP. The copy of the
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folder to your computer can be accomplished by the appropriate Windows copy
command. You can also make an icon to the Comcolor.exe file and drag it onto the
desktop screen for easy start at next time.
The supplied software has been carefully tested under WINDOWS ’95,
WINDOWS '98, WINDOWS NT, WINDOWS 2000, WINDOWS XP and WINDOWS 7
operating systems (both English and Hungarian versions) and no problem was found at
running the program. If you wish to use the application software in another operating
environment and you encounter any problem, contact the manufacturer.
We have found that problems may arise from routines supervising the energy use
of the computer. If these routines disconnect the USB, or serial communication port, at
reconnection the initialisation might be not proper. In such cases we recommend to
reconfigure the WINDOWS operating system energy saving functions.
For older (shipped before 2006) measuring heads the power saving purposes
some of the laptop computers, equipped with serial port driver circuits according to the
newer RS232D or RS232E standards, so in some cases (especially at near discharged
batteries) these output circuits could not supply enough energy on the signal lines for
proper operation to the measuring head. Recently, by using new LEDs, the energy
requirements dropped (about 2 mA average current at min. 6 Volts), so the problem
has been solved. We find no problem when using the USB – RS232 adapters with
recent measuring heads.
The content of the computer's screen may be printed by the WINDOWS as usual.
The Comcolor program also saves the graphic results in the usual formats (either .bmp,
or .jpg extension). Printing can be accomplished according the user’s preference (eg.
inserted in text file).
PRINCIPLE OF OPERATION
The measuring head illuminates the sample at an angle of 45° by a set of light
emitting diodes (LEDs), one after another with radiation of different colours, while a
photodiode senses the perpendicularly reflected amount of light. The reflected radiance
factor function is determined by comparing the reflected amounts of radiation at
different wavelengths to that determined by using a reflectance standard.
LEDs do not emit at monochromatic wavelengths, instead in a wavelength range
depending on the used base materials and manufacturing processes. The supplied
program performs sophisticated mathematical computations to correct the
measurement results according to the emission spectra of the used LEDs, and to
approximate the real spectral reflectance factor function of the measured sample (See
Nr. HU 224 581 patent). Computing of the usual colour numbers are from these
functions in accordance to the international (CIE, ISO) recommendations.
As the emitted power of the LEDs depends on temperature, there is a built in
sensor to measure the temperature of LEDs. The supplied program compensates the
temperature dependence of the emitted radiation by correcting factors determined at
instrument quality control for each LED.
The measurements made by the measuring head are performed by the help of a
dedicated microcomputer, according to the commands received from the main
computer. A special program in the microcontroller takes care that measurements
should be taken only after the measuring head has been stabilised on the surface to be
measured.
HOW TO USE IT
The COMCOLOR instrument functions differently to the traditional instruments, it
has no external setting knobs, can be used after set-up without delay, there is no need
for a warm-up period or calibrating measurements. The use of the instrument is very
COMCOLOR SPECTROPHOTOMETER - USER'S MANUAL
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simple, after selecting the desired measurement mode you position the measuring
head on the surface to be measured and strike the "M" key on the keyboard, or
alternatively click on then screen's Measure button by the mouse.
Note: The characteristic values of the measuring head are contained in the
application software supplied with the measuring head. The measuring head must be
used with the software packet supplied with it, otherwise the instrument malfunctions or
measures inaccurately.
The measuring head contains no moving parts, thus there is no wear-out. The
stability of electronic parts used in the head is much better than the measuring
accuracy of the instrument and any change is automatically compensated at calibration.
The most vulnerable parts are the LEDs, for these parts manufacturers provide ageing
information usually time periods longer than 10 hours of continuous use. In case of our
instrument this corresponds to more the 10 million measurements, as the LEDs are
only flashed for some millisecond at measuring.
The calibration interval depends not on the ageing properties of the parts used,
rather on the circumstances you use the instrument. If the instrument is used in a dry
and clean environment at not extreme temperature differences our experience has
shown that calibration to the supplied white standard necessary only after three months
of use. If the environmental conditions are not so good, a more frequent calibration to
the supplied standard (even weekly or daily) can be advised, as calibration is simple
and takes time less than one minute.
Problem may occurs when the LEDs and/or the detector gets contaminated by
dust, or by small particles coming from air or from the measured materials. The effect
of smaller amounts of dust can be compensated by calibration, in case of heavy
contamination the inner part of the measuring head cavity has to be cleaned. For
cleaning we suggest sending the measuring head back to the manufacturer, or
cleaning with a soft brush and parallel draft by an appropriate vacuum cleaner can help,
too. By no means should any solvent be used for inner cleaning. After cleaning the
instrument also has to be calibrated.
Parts used in the instrument function well in the temperature range from 0°C to
+40°C, but due to the fact that the LEDs are highly temperature sensitive devices –
despite the precise temperature compensation – outside the recommended range from
+15°C to +35°C, the accuracy of the instrument might deteriorate or calibration
difficulties might arise. These disappear, however, as soon as the instrument is brought
back into an environment of the recommended temperature range.
Note: If you would like to use the instrument outside the recommended
temperature range, contact the manufacturer.
Accurate results can only be expected if the measuring head is placed on a
uniform, plane surface. In case of not plane surfaces (textiles, textured or brushed
surfaces) we recommend the use of the built-in averaged measurements or averaged
difference measurements operating modes. For best results between measurements
the measuring head should be rotated around its vertical axis.
We can supply an optional measuring cup. By the help of this cup powders or
pulps (butter, tomato pulp, etc.) can also be measured.
In case when the cable supplied with the instrument is not long enough, a normal
computer MODEM cable of maximum 10 m can be used as extension cable. Should
you encounter any problem, please contact the manufacturer.
COMCOLOR SPECTROPHOTOMETER - USER'S MANUAL
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MEASURING ACCURACY
As every other measurement, colour measurement is also a comparison. Our
intention is usually to get information on how a human observer, or at least an average
human observer senses the investigated sample. Human vision however has a number
of characteristics that can not be taken into account in an objective measurement.
Such characteristics of human vision are, e.g. the accommodation to different
light levels and the chromatic adaptation to different lighting situations (colour of the
illuminant). Human colour perception will change when the sample occupies a smaller
or larger part of the visual field, it will also depend on the colour and the illumination
level of the surrounding environment, etc.
International standardisation in the field of colorimetry is done by the International
Commission on Illumination (CIE) that has worked out standardised methods to
characterise the colour stimulus, the angle of observation, the irradiation and
observation geometry. There is, however, no exact method to compare results
measured under different circumstances.
Due to these facts the accuracy of the instrument can only be described in parts.
The measurement of the reflected light is performed by 13 bit resolution (one part in
about 8000). By the help of the dedicated microcomputer used for controlling
measurements in the measuring head we could eliminate the +/-1 LSB (least significant
bit) uncertainty.
The short-term stability of the instrument can easily be checked using the white
standard supplied with the instrument. According to many measurements made by the
manufacturer at laboratory conditions, the repeatability for 10 individual measurements
was found to be on the average of 0.01 E*ab units (the maximum was 0.02 with a
standard uncertainty of +/- 0.01), when the measuring head was not taken off the white
standard between measurements. When the measurements were performed by taking
off and replacing the measuring head, we found the following values: average of 0,03
E*ab units (maximum 0.08 with a standard uncertainty of +/- 0.03), these later results
include the surface irregularities of the white standard, too.
Note: E*ab is the colour difference quantity in the widely used CIELAB
colorimetric system. As a first approximation we can say that for not dark and not too
saturated colours the following correlation can be found between the perceived colour
difference and the measured E*ab value:
E*ab difference perceived colour difference
0.0 … 0.6
not perceived,
0.6 … 1.5
just perceived,
1.5 … 3.0
perceived,
3.0 … 6.0
well visible,
6.0 … 12
large.
With some colorimeters one finds also the description of a quantity, called
absolute measurement inaccuracy. As there is no internationally agreed sample set to
calculate such a value, this is only a relative quality criterion. For most instruments one
does not find such a general descriptor of error, probably to avoid conflicts between the
readings of different instruments produced by the very same manufacturer or
instruments produced by other manufacturers. Differences originating from different
measuring geometries may be, especially for dark samples, much larger than the such
stated absolute errors.
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CALIBRATION
The absolute reference in CIE colorimetry is the perfect reflecting diffuser, but
such a surface can not be realised in practice. For secondary standard the CIE
recommended pressed barium sulphate tablets. These surfaces are very fragile and
can only be used at laboratory circumstances. Recently also reported good results
using pressed or sintered PTFE material.
The instrument is supplied with a housed white ceramic tile standard, which is
calibrated against a working standard made from the same material and has been
officially measured by the Hungarian National Office of Measures (OMH) using 45°:0°
measuring geometry. The stated OMH measuring accuracy was +/- 0.7 %.
The manufacturer has observed a few per cent measuring difference depending
on instrument calibration to data measured by OMH or to a pressed barium sulphate
surface (for which data may be found in the literature). Based on this finding there is a
possibility in the instrument software for the user to select one of these reference
bases. At saving measured results the saved data also includes the used reference
setting.
The simplest way to calibrate the instrument is by the help of the supplied white
standard, but there is the possibility of using barium sulphate or PTFE standards too.
For instrument calibration first select the Calibrate measuring head or Auto-calibrate
measuring head function in the Operating mode combo box. Then place the measuring
head onto the supplied white standard, and click the Measure button for automatic
calibration, or perform some (max 10) measurements manually, turning the head
slightly around its vertical axis between measurement. At he end of the calibration cycle
the new calibration values for the LEDs used in the instrument show up for a few
seconds as small circles. If these values deviate more than 4-5 percent from unity, this
usually means the contamination of LEDs. By calibration approximately 10 per cent
deviation can be corrected.
Note: Unlike other instruments, the COMCOLOR spectrophotometer is calibrated
to only one (white) standard, as this instrument continuously readjusts its dark zero
level between measurements. If the instrument is used in the recommended
temperature range, but the calibration differences are too large for the instrument to
accept the new values, or when it might be necessary to use a different calibrating
standard (e.g. Halon or PTFE) please contact the manufacturer or distributor.
SOFTWARE DESCRIPTION
This variant of the COMCOLOR instrument software has been developed
explicitly for running under WINDOWS operating systems. The selection of instrument
settings are done by the help of the well known combo box and spin edit objects, and
screen buttons are used for controlling instrument measuring and data handling
operations. Stored and/or measured results are displayed in graphic and text fields
according to the selected settings. When the cursor points to a field in the monitor, an
appropriate hint is displayed temporarily to inform the user about the function of that
control. The language of computer hints and messages may be selected as English,
Hungarian or German.
The instrument software at start checks in a few seconds the proper connection
and working of the measuring head, and after changing the appropriate settings if
needed, the instrument is ready for work. Put the measuring head on the sample you
want to measure and start measuring operation by clicking the mouse left switch when
the cursor points to the monitor's Measure button, or alternatively by hitting the "M" key
on the keyboard. For averaged measurements select the appropriate line in the opened
Operating mode combo box window and make some (maximum of 10) measurements,
after that click by the mouse on the Average button or hit the "A" key. Between
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averaged measurements the graphic field is not cleared, that is the results are drawn
over each others, but after averaging only the averaged result is displayed.
In the combo boxes the COMCOLOR program continuously shows the actual
settings and at opening the boxes the user sees all the possibilities for changing the
current selection. Setting the new selection is happens in the very same mode as usual
in WINDOWS. When properly exiting COMCOLOR program (clicking on the Exit button
or hitting the "X" or "ESC" key) the software saves the last selected settings (used
language, colour system, used working file, operating mode, etc.) and at next running
the program automatically starts with these settings.
INSTRUMENT SETTINGS
Here is shortly discussed the selectable typical COMCOLOR instrument setting
possibilities. These possibilities may be changed according to the buyer's requests or
with software enhancement.
Computer language combo box: here you can select the language for the
computer messages and measurement information display.
English - Hungarian - German.
Computer serial port combo box: here you can select that which of the computer's
serial port the measuring head is connected to.
COM1 .. COM8.
Standard illuminant combo box: here you can select that the colour data for what
of the standard CIE illuminants be computed from the measured reflectance data.
CIE A - C - D50 - D55 - D65 - D75 - F1 .. F12.
Standard observer combo box: here you can select that the colour data for what
of the standard CIE observers be computed from the measured reflectance data.
2° (1931) - 10° (1964).
Calibrating device combo box: here you can select that by what device the
instrument is calibrated.
Supplied tile - Barium sulphate standard - Other (according to the user's
request e.g. Halon or PTFE).
Calibrating base combo box: here you can select what calibrating base the
instrument's measurements be referenced to.
Perfect reflector (barium sulphate) - Industry standard.
Operating mode combo box: here you can select the function to be performed by
the instrument's software.
Normal measurements - Difference measurements - Averaged measurements
- Averaged difference measurements - Display from file - Calibrate measuring head
- Auto-calibrate measuring head - Calibrate CMYK conversion.
Graphic results combo box: here you can select in what form the measured
results be displayed in the graphic display field.
Reflectance graph - CIE Y,x,y chromaticity diagram - CIE L*,a*,b* colour
system - Munsell colour order system - Coloroid colour order system - Results in
tables - K/S graph.
When selecting Munsell and Coloroid colour order systems, the standard
illuminant and standard observer is automatically changes to the one used by the
selected system.
Graphic results background spin edit: here you can select the background
brightness for graphic results field.
From 0 (black) to 100 (white) per cents.
Numeric results combo box: here you can select what numeric colour data be
displayed in the numeric results field.
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X, Y, Z, x, y, L* ,a *, b* - X, Y, Z, x, y, dom. wavelength, purity - Y, x, y, L*, a*,
b*, C*, h - X, Y, Z, x, y, H, V, C - L*, a*, b*, C*, h, H, V, C - X, Y, Z, x, y, A, T, V L*, a*, b*, C*, h, A, T, V - Colour differences - Whiteness grades - Opacities Densities, C, M, Y, K - Relative densities.
When selecting Munsell and Coloroid colour order system values, the standard
illuminant and standard observer is automatically changes to the one used by the
selected system. In some cases the Operating mode and the Graphic results selections
also changes according to the selected results.
Colour display field: the program shows here approximately the colour(s)
computed from measured and/or stored reflectance data according to the selected
standard illuminant.
Measured colour - Reference (stored) colour - Background for colour(s)
displayed.
Colour display field background spin edit: here you can select the background
brightness for the colour(s) displayed.
From 0 (black) to 100 (white) per cents.
HANDLING MEASUREMENT DATA
For saving measured results in the memory of the computer the COMCOLOR
program uses special files, marked by "mrd" (measured reflection data) extension. At
opening the Working file combo box all the files in the current directory having "mrd"
extension is displayed and can be easily selected by a click on the line containing it's
name, or new working file can be selected by the keyboard. For naming a new working
file you can use a string formed by maximum of sixteen ASCII alphanumeric
characters, the extension is handled automatically by COMCOLOR. After selecting
existing files for a click on the Open button the program opens that file, in case of
naming not existing file the program creates and opens a new one.
When the opened file contains stored data marked by the identifier number
currently found in the Measurement identifier number spin edit control's window, the
software displays that record from the data file, otherwise displays the record marked
by the smallest identifier number and also accordingly sets the current measurement
identifier number. In the opened file you can easily select for display other stored data
records by clicking one of the First, Next, Prior or Last (or showed by appropriate
arrows) buttons, by clicking on the increment/decrement arrows in the Measurement
identifier spin edit window, or by directly writing the new identifier number from the
keyboard.
The software always displays the data record marked by the current identifier
number according to the current selections in the Graphic results and Numeric results
combo boxes (in cases when there is no stored data marked by the actual identifier
number, reflectance of unity is used). Data no longer needed may be removed from the
file by clicking on the Delete button.
To save the results of the last measurement click on the Save button, or hit the
"S" key. At saving the program automatically saves the measured reflectance spectra
values from 360 to 760 nm in 10 nm steps, together with the date and time of
measurement and the current calibration setting. When already there are data marked
by the current measurement identifier number, the program overwrites the old data set,
otherwise the new data set is inserted into a new place according to the value of the
identifier number in the working file.
Files used for storing measurement data are usual text files, by the help of the
WINDOWS operating system's programs (e.g. Notepad) these files can also be viewed,
edited or deleted manually. The COMCOLOR software automatically sorts the
measurement data sets in increasing order of the values of measurement identifying
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numbers, thus during manual editing (e.g. at typing in data sets determined by other
instrument for comparison) you must not change this order of identifier numbers.
When selecting the Display from file function in the Operating mode combo box
and Data in tables line in the Graphic results combo box, the software displays the
stored data in the current file in a table, according to the selected line in the Numeric
results combo box. At clicking on the now Make file (otherwise Measure) button, the
program creates a new file with the currently selected working file name, but marked by
"txt" extension. This file can easily be viewed or printed by text editing programs, e.g.
the WINDOW'S Notepad accessory program.
FURTHER USAGE
Colourists in the print industry traditionally use density meters to control the
printing processes and check the quality of printed materials. The COMCOLOR
instrument provides much more information about the quality of colour reproduction
than a density meter does, however for the convenience of people used to the readings
of density meters the COMCOLOR software also determines the three colour (filtered)
and visual density values, and by the help of the modified Neugebauer theory computes
the C,M,Y,K values to be used for reproduction of the measured colour.
As these C,M,Y,K values highly depend on the used base materials (paper and
inks) as well as on parameters of the technology used, good results can only be
obtained if the instrument is firs calibrated by the help of a trial print made by the same
process (otherwise the program uses data according to the DIN 16539 standard). For
the calibration process select the Calibrate CMYK conversion function in the Operating
mode combo box and make measurements according to the messages coming from
the computer.
For print industry measurements select the Densities, C,M,Y,K, or Relative
densities lines in the Numeric results combo box. To calculate the reflection density
values from reflectance data, hypothetical filters with 432, 536 and 624 nm peak
wavelength and 20 nm half value bandwidth are mathematically used.
To determine the opacity of papers used for printing select the Opacities line in
the Numeric results combo box, COMCOLOR automatically selects the averaged
difference measurements function and graphic display of reflectance data. First the
reflectance spectrum of the paper sample has to be measured at several locations on
the sample, with several layers of backing from the same material or with a white
backing. The instrument will calculate the average reflectance spectra, these data have
to be saved as usual in the current working file. Then reflectance measurements are
taken again in different locations of the sample, but with black backing. When
averaging the measurements, the instrument calculates numeric opacity values
according to four different national standards used in practice and also draws the
wavelength dependence of opacity over the reflectance graphs.
COMCOLOR SPECTROPHOTOMETER - USER'S MANUAL
TECHNICAL DATA
Measuring geometry: CIE 45°:0° (45° illuminating, 0° viewing).
Measured area: 9 mm diameter circle on plane surface.
Operating modes:
Normal measurements,
Difference measurements,
Averaged measurements,
Averaged difference measurements,
Display from file,
Calibrate measuring head,
Auto-calibrate measuring head,
Calibrate CMYK conversion (option).
Language for computer messages: English, Hungarian, German.
Measurement time: 1.5 second (for standard measurement).
Short time repeatability (E*ab deviation at 10 measurements on white standard):
average: 0.01, maximum: 0.02, standard uncertainty: 0.01.
Standard illuminants: CIE A, C, D50, D55, D65, D75, F1…F12.
Standard observers: CIE 2° (1931), CIE 10° (1964).
Calibration to industry standard or to perfect reflector.
Spectral range: from 380 to 740 nm, extrapolation to 360 and 760 nm.
colour data computing from 360 to 760 nm in 5 nm steps.
Displayed data (all screens are printable):
reflectance factor graph and table, K/S graph,
colour spaces: CIE Y,x,y, CIE L*,a*,b*, Munsell, Coloroid, HunterLab,
colour data: CIE X, Y, Z, x, y, L*, a*, b*, C*ab, hab, W, Tw,
Munsell H, V, C, COLOROID A, T, V, calibrated C, M, Y, K,
Taube W, Berger W, ISO W, Hunter W, Stensby W,
opacities, densities, relative densities,
on request: CIE u’, v’, w’, u*, v*, suv, C*uv, huv, Hunter L, a, b,
OSA L, j, g, AN-40 L, a, b, Glasser L, a, b, etc.
colour differences: L*, a*, b*, C*ab, H*ab,
CMC DE, CIE E*ab, E*94, DE2000,
on request: E*94tx, E*Völz AN-40 E, FMC-II E, OSA C, etc.
Required computer: IBM PC or compatible laptop, notebook,
USB or RS232 port (head is powered from signal lines),
operating systems: Windows from ‘98,
min. 800x600 graphic resolution.
Temperature ranges
operating: 15 to 35 °C,
storage: -15 to 70 °C.
Measuring head mechanical data
size: 40 mm diameter x 80 mm,
weight: approx. 110 g.
Included items (in 200 x 160 x 60 mm wooden case):
Measuring head with RS232 connecting cable
USB – RS232 adapter,
white standard for calibrating,
User's Manual,
software on CD or pen drive.
12
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APPENDIX A
This short overview has been prepared for those who would like to get acquainted
with the use of the COMCOLOR spectrophotometer, but do not have enough
knowledge in the field of colorimetry. We hope that the skilled users of colour
measuring instruments will also find some interesting information in this description.
COLOUR PERCEPTION
Light energy is a sort of electromagnetic energy that spreads out in the form of
radiation. The wavelength of optical radiation is located between that of radio waves
and X-rays. The human visual system uses the visible radiation part of this spectrum,
this part contains also the stimulus that is responsible for colour perception. It is usual
to describe the 360 nm to 760 nm part of optical radiation as visible radiation, it
continues on its short wavelength side in the ultraviolet spectrum, and on its long
wavelength side in the infrared spectrum.
The human eye can function under a very wide dynamic range. We can
distinguish the shapes of objects at about 0.1 lux illumination (stronger moonlight), at
approximately 1 lux we start to look colours, at 100 lux we can read comfortably and in
bright sunlight the illumination can even reach the 100,000 lux level.
The sensitivity of the human eye changes with the wavelength of the radiation.
The average eye is most sensitive in the vicinity of 555 nm. The eye sensitivity curve is
bell shaped, at 510 nm and 610 nm the sensitivity drops to about 50 per cent, at 470
nm and 650 nm to about 10 per cent. Below 360 nm and above 760 nm the eye
sensitivity is practically zero.
The rays of visible radiation penetrate into the eye, they are absorbed in the
retinal layer, where the receptors are located, generating there neural excitations that
are fed to the visual cortex of the brain producing here the light sensation. Part of this
sensation relates to colour and manifests itself in colour perception. When the stimulus
is monochromatic radiation, a 480 nm radiation provokes blue colour perception,
radiation in the vicinity of 530 nm produces the perception of green colour, yellow is
created by radiation having a wavelength of about 580 nm, while radiation above about
630 nm is sensed as red. Colour perception varies continuously with wavelength, for
example the radiation of about 560 nm is called yellowish green and about 600 nm is
called orange (reddish yellow).
In practice the radiation we observe is usually not monochromatic, rather it is
composed of different wavelengths, mostly containing different amount of the entire
visible spectrum. The colour perception depends on the spectral power distribution of
the stimulus in a complicated way (different spectral distributions can provoke the same
colour perception). In case of observing not self-illuminating but light reflecting objects,
the perceived colour depends both on the spectral power distribution of the illuminating
light source and the spectral reflectance properties of the object (in case of translucent
materials on the spectral transmission properties of the object).
Colour perception is influenced by a number of factors. Thus, e.g. the absolute
amount of light reaching the eye, the position of the observed surface related to the
illuminating source and to the eye of the observer, the size of the observed object, the
colour of the surrounding of the observed surface and its illumination. It also depends
on the chromatic adaptation of the eye, etc.
COLOUR MIXTURES
Colorimetric studies have shown that any colour stimulus (any hue, tint and
lightness) can be matched by the additive mixture of three properly selected
(independent) basic stimuli (in some cases negative amount has to be used).
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Additively mixing blue and green monochromatic lights, depending on their
relative amount, will produce bluish-green and greenish-blue hues. By mixing green
and red lights yellow hues can be produced, while the mixture of red and blue lights will
produce purple hues. In colorimetry the unit amounts of red, green and blue stimuli
have been selected in such a way, that equal amounts of them should produce neutral,
or achromatic perception, i.e. depending on their amounts they are perceived as white,
grey or black colour (in a surround of medium strong stimulation).
The monochromatic radiations are perceived as having the highest purity, they
produce the most saturated colours. When the stimulus consists of the mixture of more
than one monochromatic colour stimuli, the resultant perception will show lower
saturation, in case of darker stimulus it produces a more dull, dirty impression.
COLOUR ORDER SYSTEMS
According to colorimetric estimates the human eye can distinguish among about
200 hues and, when the different chroma and lightness values are also considered, it
can distinguish among 10 - 20 million colour stimuli. Colour perception is only partly the
outcome of a physical-chemical-biological process, it depends also on a number of
psychological processes, and these are difficult to measure. This is – at least partly –
the reason why a number of colour order systems have been designed and some of
them are still in use in the different areas of colour technology and colour science.
It is well known that a colour perception can be described by three attributes, thus
e.g. by the hue, saturation and lightness. Nowadays the attribute of colourfulness is
often used instead of saturation, and lightness is measured along a value scale. For
self-luminous objects (light sources) brightness is the correct term instead of lightness.
Hue is usually depicted in a hue circle, saturation or chroma or colourfulness is a
radial co-ordinate so that with hue they form polar co-ordinate systems. Lightness (or
brightness) is then depicted either as parameter, or extends the system to a threedimensional colour solid.
The first internationally accepted standardised colour system was introduced in
1931 by the CIE (Commission Internationale d’Éclairage). Its fundamentals were real
primaries of 700 nm (Red), 546,1 nm (Green) and 435,8 nm (Blue) lights. Based on
these primaries experiments were performed and the rules of additive colour matching
were laid down. Due to practical reasons, mainly to avoid negative lobes in the colour
matching functions, the CIE introduced a transform of the RGB system to the XYZ
system, where the XYZ fundamentals are non-real colours, but from these
fundamentals all the real colours can be produced as additive mixtures.
The CIE defined the characteristics of the average observer, called CIE 1931
standard observer and standardised its colour matching functions. Also some standard
illuminants have been defined by their spectral power distribution. From the original
definition the CIE standard illuminant A is still a standard, it has a spectral power
distribution very near to that of a 100 W general purpose incandescent lamp. CIE
illuminant C was thought to represent the average daylight. Later experiments showed
that different phases of daylight have more complicated spectra. At present one of
these, the D65 with a correlated colour temperature of approximately 6500 K, is the
other CIE standard illuminant. Other phases of daylight, with correlated colour
temperatures of 5000 K, 5500 K and 7500 K are also often used and termed as D50,
D55 and D75. No sources have been standardised for the daylight illuminants, as no
real sources were found for which their spectral power distribution resembled well
enough to the theoretical curves.
For colorimetric measurement of opaque surfaces the original 1931 CIE
recommendation called for an irradiation under 45° and an observation under 0° (with
present day description 45°:0° measuring geometry). The next recommendation in
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1964 permitted the reversed geometry as well, i.e. perpendicular irradiation and
observation under 45°, together with diffuse irradiation and near perpendicular (8°)
observation and its reverse: d:0 and 0:d. Recently there are more standardized diffuse
measuring geometries, with included or excluded specular reflection.
In 1931 the smoked magnesium oxide surface was standardised as reflection
standard. Since 1969 the perfect diffuse reflector is the primary reflectance standard
and pressed barium sulphate is recommended as secondary standard.
To obtain the CIE tristimulus values for a surface measured one first has to
multiply at every wavelength the reflectance spectrum with the spectral power
distribution spectrum of the selected reference illuminant and the three colour matching
functions at 5 nm intervals (or in case of rapidly varying spectra at 1 nm intervals) from
380 nm to 780 nm (more recent instruments usually work in the 360-760 nm. range).
Then for the three colour matching functions one has to sum up these products. Finally
one has to multiply each sum with an appropriate constant to obtain the X, Y, Z
tristimulus values for the sample. The constant above is selected to give Y=100 value
for the perfect reflecting diffuser.
It is usual to characterise the colour stimulus of the surface not by stating its X, Y
and Z values, but its Y tristimulus value and the so-called x, y chromaticity co-ordinates.
These are derived from the tristimulus values dividing X and Y by the sum of the three:
X+Y+Z. The x, y chromaticity co-ordinates can be depicted in the chromaticity diagram,
where the dominant wavelength of the colour of the sample (characteristic for its hue)
and the excitation purity (correlating to the saturation) can be determined. In such a
description the Y value is taken as a correlate of lightness.
The CIE 1931 system is valid for small visual angles (about 2°). For many
practical colour measurements this is not adequate, therefore in 1964 colour matching
functions were defined for a 10° observer. Using the data defined for this observer, the
tristimulus values and chromaticity co-ordinates can be calculate similarly to the 2°
observer. To show that the calculation has been performed with the 10° observer, 10 is
used as subscript for the X, Y, Z, x, y values.
In practice the major drawback of the CIE x, y chromaticity co-ordinate system is
the lack of its linearity, that is distances in the co-ordinate system do not represent
proportional perceived colour differences, not even for the same lightness. During the
decades many transformations were tried, main steps to reach a uniform colour scale
were the introduction of the UCS chromaticity diagram, then the U*, V*, W* colour
space. At present mostly the CIELUV and CIELAB spaces and the CIELCH
representation of the later are used.
The CIELAB colour order system and its CIELCH polar co-ordinate version found
general acceptance in surface colorimetry, thus this system will be introduced here in
some detail. The descriptors in this system are the L* CIE-lightness and the a*, b* coordinates. Their numeric values can be obtained from the tristimulus values of the
sample and illuminant as follows:
L* = 116 (Y / Yn)1/3 - 16,
a* = 500 [ (X / Xn)1/3 - (Y / Yn)1/3 ],
b* = 200 [ (Y / Yn)1/3 - (Z / Zn)1/3 ],
where Xn, Yn and Zn are the tristimulus values of the illuminant. If X/Xn, or Y/Yn or Z/Zn
is smaller than 0.008856, thus for very dark samples, other linear equations are valid,
see the literature.
The polar co-ordinate representation (CIELCH) calculates the CIELAB chroma
and the CIELAB hue as
C*ab = [ (a*)2 + (b*)2 ]1/2,
COMCOLOR SPECTROPHOTOMETER - USER'S MANUAL
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hab = arctan ( b* / a* ) .
For two colour samples with not too large colour difference the CIELAB colour
difference is calculated as
E*ab = [ (L*)2 + (a*)2 + (b*)2 ]1/2,
where L*, a*, b* are the L*, a*, b* co-ordinate differences for the two samples.
Based on the CIELAB colour system, a modified colour difference computing
formula was worked out by the British Colour Measurement Committee (CMC), and this
procedure was also accepted by the International Organization for Standardization as
ISO standard.
In 1995 the CIE recommended a new formula to determine colour difference
values for two adjacent colour stimuli, as it was found to give results in better
accordance to the perceived colour differences than the old version above. The new
formula in its most general version is
E*94 = { (L*)2 + [C*ab / (1 + 0.045 C*ab)]2 + [H*ab / (1 + 0.015 C*ab)]2 }1/2,
where L*, C*ab, H*ab are the CIELCH co-ordinate differences for the two sample
and C*ab is the CIELCH chroma value of the reference colour.
In 2000 the CIE recommended a more sophisticated colour difference computing
method. The lengthy computing procedure is not introduced here, the COMCOLOR
instrument gives this result as E*00.
We should to mention that the COMCOLOR instrument for signalling differences
uses the letter "d" instead of the "" sign, because the computer representation of the
Greek "" depends on the symbol code used on the given computer. Due to the same
reason the instrument display does not use superscripts.
To be able to get a visual impression of the CIELAB co-ordinate system one can
say that if b* is near to zero then a positive a* value describes red colours and a
negative a* value green colours. Yellow colours have an a* near to zero and large b*
values, while blue colours are found in an area of the diagram, where a* is near zero
and b* has large negative values. Saturation is proportional to C*ab.
The usual graphic appearance of the CIELAB (or CIELCH) colour system is
where the a* values are drawn by the horizontal, b* values are drawn by the vertical
axes of a two-dimensional plane, while L* values are given as parameters and hue
angles are measured anticlockwise from the horizontal axis. An alternative appearance
may also be found, where b* values are drawn by the horizontal and a* values by the
vertical axes, so the hue angle is measured clockwise from the vertical axis and now
the places of colours in the hue circle are near to that of Munsell's colour circle.
At the beginning of the 20th century Munsell developed a colour order system that
– in its 1943 version (Munsell re-notation system) – is widely used, especially in the
USA and Japan. In this system colours are represented in the H V/C form, where H is a
hue descriptor, V stands for value that describes lightness and C is for chroma. The
hue circle has been divided into five main hues: R red; Y yellow; G green; B blue and P
purple and five mixtures of the adjacent main hues: YR, GY, BG, PB and RP. The hue
is numerically described by a 10 step scale between the main and mixture hues. Step
sizes have been defined in a form where the steps represent perceptually equal
differences. The value scale describes lightness on a perceptually uniform scale from 0
(black) to 10 (white). The chroma scale is a uniform open scale, from the black-greywhite axis, where the chroma is zero, up to highly saturated colours. As colours with
different hues show different amounts of saturation, the maximum chroma of yellow or
red or blue colours will be highly different.
The Munsell Book of Color is a representation of the Munsell Color Order System,
it contains 1450 coloured cards and 18 grey ones. The coloured samples are arranged
into 40 “leaves” of the Munsell “tree of colours”. In every leave the hue of the samples
COMCOLOR SPECTROPHOTOMETER - USER'S MANUAL
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is constant. Samples are arranged on each leave in such a form that colours with
constant value (from 1 to 9) are arranged in one row, samples with constant chroma
form columns, increasing in steps of 2. The CIE co-ordinates (for illuminant C and the
2° observer) have been determined thus Munsel notations can be converted into CIE
tristimulus values and vice versa.
In Europe the other most widely used colour order system is the Swedish NCS
(Natural Color System), based on six perceptually unique hues: two achromatic: black
and white and two other antagonistic chromatic hue-pairs: red-yellow and green-blue.
As the CIE nominal values of NCS colours have been determined by measurements
using diffuse illumination (specular component included), the COMCOLOR instrument
(as it uses 45/0 measuring geometry and excludes the specular component) can not
directly compute NCS colour notations. Nevertheless the NCS colour shades, as the
samples are available, may be handled by COMCOLOR as any other colour collection.
The COLOROID colour order system has been worked out by Prof. Necsics and
his co-workers at the Budapest Technical University. The system has been defined for
the 2° standard observer and both for standard illuminants C and D65 (recently only
D65 is used). Hue, saturation and lightness are described by the A-T-V co-ordinates.
The system contains 48 basic hues, described by integer numbers, where the first digit
refers to one of the basic colours (10-16: yellow, 20-26: orange, 30-35: red, 40-46:
purple-violet, 50-56: blue, 60-66: cold green, 70-76: warm green). These 48 basic hues
have been defined in the CIE x, y chromaticity diagram by their angle. The COLOROID
saturation is computed by a rather complicated formula, while the lightness scale has
been derived from the CIE Y tristimulus value as
V = 10 Y1/2.
COLOUR COLLECTIONS
Beside of the samples representing the above-described colour order systems a
number of further colour collections have been designed and brought to market. Such
collections are, e.g. the PANTONE and the RAL colour shade collections. These colour
samples were designed neither according to a colour perception model, nor based on
some colorimetric know-how, but to show colour samples in a pleasing form. (However
RAL has a version (RAL Design System), where samples are arranged according to the
CIELCH system.)
If you would like to use these colour collections by COMCOLOR, the best way is
to acquire a fresh sample set, measure the samples and save the results in appropriate
files using COMCOLOR. Remember that the colour shade of the samples might
change by age, but data in the computer remain unchanged. It is often necessary to
refer to a particular set of samples, as sets supplied by the same manufacturer at
different occasions might differ considerably in their colour characteristics.
COLORIMETERS
In this chapter only the most important characteristics of known instruments used
in the investigation of opaque and not self-luminous colours will be enumerated, which
might be of importance for the possible users of COMCOLOR. These instruments can
be classified into two categories: spectrophotometers and tristimulus colorimeters.
Spectrophotometers measure the reflectance factor of the sample in its
wavelength dependence. Usually, either inside or coupled to the instrument, a
computer also calculates colour co-ordinate values for one or more colour order
systems.
The traditional spectrophotometer uses an incandescent lamp as radiation source
and a prism or a so called monochromator device to produce a near monochromatic
light-ray (a light-ray with a bandwidth of only a few nanometers) in an arrangement,
where the peak wavelength of the light-ray can be changed continuously. This nearly
COMCOLOR SPECTROPHOTOMETER - USER'S MANUAL
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monochromatic radiation is directed onto the sample, the wavelength is changed with a
step size of 5, 10 or 20 nm, while the reflected radiation is measured at each
wavelength step.
In some more modern instruments the sample is irradiated with polychromatic
radiation (i.e. non-dispersed white light) and the reflected radiation is dispersed by
prism or grating and sensed by a photodiode array, e.g. a CCD detector. Each detector
of the array measures the reflected light in a narrow wavelength band. This
arrangement, as uses no moving parts, permits a more rapid determination of the
reflectance spectrum relative to the traditional one.
Recent achievements in semiconductor technology, especially in the development
of high performance light emitting diodes (LEDs), enabled a new instrument design as
well. Here the sample is irradiated by LEDs with different emission spectra, one after
another while the reflected radiation is measured by a photodiode.
Problem with this design is that the emission spectra of some LEDs is far from
monochromatic (they have considerable broader emission bandwidth than what can be
produced by monochromator) and therefore rather complicated algorithms are needed
to calculate the real reflectance spectra data from the measured values. Modern
personal computers can easily perform the necessary calculations together with the
visualisation of the results in numerical and/or graphical form. Storing and re-reading
measured results also comfortable by modern computers. This fundamental idea is
realised in the COMCOLOR instrument, where the sample is measured by the
measuring head and all the necessary calculations are performed by the host PC.
Samples showing fluorescence, as e.g. papers or textiles treated with optical
brighteners, produce further problems. When measuring such samples the irradiation
should be a polychromatic light-ray and for good results its spectral distribution should
approximate that of daylight. Thus instruments, where illuminating is done through
monochromator or by a range of LEDs, are not recommended for this purpose.
Tristimulus colorimeters do not measure the spectrally resolved reflectance
spectrum, rather show directly the tristimulus values. In these instruments the spectral
distribution of the irradiating light is modified by sophisticated filter arrangements
according to the selected CIE standard source and observer. There are different filters
for each X, Y and Z values, many times even two filters for the single X value.
CALIBRATION OF INSTRUMENTS
Reflectance factor spectra is measured comparing the reflected radiation to that
of the perfect diffuse reflector. Naturally such a reflector does not exist in reality, there
are only sophisticated measurement methods to obtain data compared to this primary
standard. The pressed barium sulphate, recommended by CIE for secondary standard,
is very fragile and can only be used at laboratory circumstances.
Manufacturers of colorimetric instruments usually supply with their instrument one
or more calibrating standards (ceramic, enamel or plastic plates, etc.). These standards
have usually glossy surfaces, for the purpose of easy cleaning. The variety of CIE
measuring geometries will produce different results for glossy surfaces if not exactly
duplicated in the different instruments, so comparing results from different instruments
is not an unanimous task. To obtain continuity in the measurement results – according
to our observation – most of the major colorimeter manufacturers are still using the
traditional calibration techniques even after the change of the recommended CIE
primary standard some 30 years ago.
For calibration of COMCOLOR instruments ELCOMP uses a ceramic tile primary
working standard, made from the same material and similar to those supplied with each
instrument. This white standard has been calibrated by OMH (the Hungarian Office for
Measures) under 45/0 measuring geometry. When however, the COMCOLOR
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instrument is calibrated to this standard, for a barium sulphate standard, prepared
according to the CIE recommendation, produce readings somewhat above 100%
(similar discrepancy was observed when measurements were taken on some other,
high grade instruments).
Due to this difference the COMCOLOR instrument is supplied with a software that
enables the user to get results either referring to the OMH data (industry standard) or to
the a barium sulphate standard (perfect reflector). The colour difference caused by the
two calibration settings is approximately 1 E*ab.
COMPARISON OF RESULTS
Remember, when you look at an uniformly painted and polished haul of a car in
daylight you can observe many different shades of colour. Similarly when someone
compares the measured data gathered by instruments using different measuring
principles, usually gets different results. If you would like to compare the results of
different instrument types you have to consider a number of factors, such as
- working principle (especially in case of fluorescent samples),
- measuring geometry used,
- specular component included or excluded,
- selected reference illuminant,
- selected standard observer (observation opening angle),
- calibration of the instrument.
If the factors above are the same, the readings of the different instruments should
not differ from each other by more than a few percent. If instruments using different
measuring geometries are compared, then much larger differences can be expected.
Usually in case of light and unsaturated colours the differences are smaller, but with
highly saturated and/or dark samples the differences can be considerable, even when
results obtained by high precision and well used instruments are compared.
RECOMMENDED LITERATURE
Wyszecki, G, Stiles, W. S, Color Science: Concepts and Methods, Quantitative Data
and Formulae. 2nd ed. John Wiley and Sons New York, 1982.
CIE Publication No. 15. Colorimetry. 2nd ed. Central Bureau of the CIE Vienna, 1986.
Nemcsics A: Colour Dynamics, Ellis Horwood Ltd, 1993.
Szabados M: Error-compensated spectrummeter, Nr. HU 224 581 patent, 2001.
CIE Publication No. 15:2004: Colorimetry. Central Bureau of the CIE Vienna, 2004.
Schanda J: Colorimetry: Understanding the CIE System, Wiley, 2007.
Useful internet links:
www.cie.co.at
www.aic-colour.org
www.nist.gov
http://en.wikipedia.org/wiki/RGB_color_space
www.elcomp.hu
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APPENDIX B
COMCOLOR MEASURING ACCURACY
There is no internationally accepted method to unanimously determine the
accuracy of spectrophotometers and instruments converting their results to colour data.
One of the main problems is that the perfect diffuse reflector, presently selected as
reference for reflectance measurements, cannot be realised in practice. The other
source of difficulties is the fact, that instruments using different measuring geometries
give even theoretically different results, depending on the sample measured.
In the class of the COMCOLOR spectrophotometer, where a relatively new
method is implemented (light emitting diodes, shortly LED's are used for sample
illumination) there is not even accepted reference instrument, so for reference we
selected an accurate model of a well known manufacturer, using 8°:de measuring
geometry (specular component excluded), like in the COMCOLOR instrument.
Perhaps short time repeatability is the most significant qualifier at the older type
of spectrophotometers, as not easy to measure accurately the small amount of signal
reflected from the measured sample and weakened in the monochromator or optical
grating. Most recent semiconductor technologies can produce LED's through the whole
visible spectrum with good light transform efficiency, so the electrical uncertainty of a
carefully designed, microcontroller based measuring instrument is smaller, than the
errors caused by surface irregularities of the seemingly even sample surface.
To gather data for the accuracy of the COMCOLOR spectrophotometer, we
measured 15 stable, coloured ceramic samples by the reference instrument, and the
same samples plus the barium sulphate standard used for calibration, by five
COMCOLOR instruments, made not in a single lot. For repeatability data 10-10
measurements were made so, that the measuring head was not moved on the
samples. To determine surface colour unevenness also 10-10 measurements were
made, but the measuring head was lifted off the sample and slightly turned around its
vertical axis between measurements. Measured result were stored by the instruments
and from these stored data comparative results were computed and drawn by a
computer, as can be seen on the attached graphs. Numeric values of colour
differences and averaged colour differences were also computed from these results. As
colour difference values according to the CIE 1976 and 1994 recommendations
significantly differs at some sample, to aid evaluation both data series are shown.
For featuring absolute accuracy the averaged results of the five COMCOLOR
instruments are compared to the results obtained by the reference instrument in Fig. 1.
At samples 1, 3, 4 and 12 the few per cent difference possibly arose from the use of
different calibrating standards. The surface of samples 2 and 7 are not fully flat
(textured to some grade), so the light trap of the reference instrument could not work
perfectly. From the graphs may be seen, that the COMCOLOR resolution versus
wavelength is not so good than that of the much more expensive laboratory instrument,
but the differences in numerical results can be acceptable in most of the applications.
The short time repeatability of one of the COMCOLOR instruments is shown on Fig. 2,
while Fig. 3. shows inaccuracy data for the five instruments, compared to each others
and computed colour difference values to the averaged results. For objective evaluation
you must consider colour unevenness of the samples, shown on Fig. 4.
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