Colorimetry
Silja Holopainen
29.3.2006
Outline
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Introduction to colorimetry
Colorimetry in general
Measuring diffuse reflectance
Measuring fluorescence
Measuring transmittance
Measurement geometry and special cases
Conclusion
Introduction
• Color has always
been important in art,
religion and clothing
• At present color is
also used for signs,
safety cloths, paper
whitening etc.
• It is often important to
be able to measure
color accurately
What is color?
• Electromagnetic radiation
between 380-780 nm
• Color is one aspect of
appearance
• Color = light source +
object properties + eye +
brain
• The human eye is most
sensitive at 555 nm
The three dimensions of color
• Hue distinguishes blue
from green from yellow
etc.
• Lightness distinguishes
light colors from dark
colors
• Chroma describes how
different a color is from
grey
White
Lightness
Chroma
Hue
Black
Hue
Lightness Chroma
Colorimetry
• Two objects may appear the same
when viewed under one light source,
but different under another =
metamerism
• Metamerism is one of the major
industrial problems in color matching
• Colorimetry attempts to quantify the
perception of color
• CIE is a voluntary organization giving
recommendations concerning
modern colorimetry
Sources and illuminants
• Source = physical
entity that produces
radiation
• Illuminant = table of
values of spectral
power distribution
• Illuminant D65
represents average
daylight. D50
represents typical
indoor light
Color perception
• 92 % of men and 99,5 %
of women have “normal”
color vision
• The retina comprises rod
cells (night vision) and
cone cells (color vision)
• Majority of the cells are
rod cells
• There are three types of
cone cells: one has peak
sensitivity to blue light,
one to green light and
one to red light
Tristimulus values
• All colors can be matched by varying amounts of
red, green and blue lights (X, Y and Z)
• The amounts of X, Y and Z that must be mixed
to match a color are called the tristimulus values
• The tristimulus values depend on the reflectance
or transmittance of the object, the illuminant and
the observer
• Pairs of objects are said to match when their
tristimulus values are the same
The CIE Standard Observers
• In the CIE experiment one
half of a circular field is
illuminated with spectrum
color and the other with a
mixture of red, green and
blue
• The observer adjusts the
red, green and blue until it
matches the spectrum color
• The result is a set of color
matching functions used to
calculate the tristimulus
values
Test Side
Matching
Side
Spectral
Light
Red +
Green +
Blue
Color difference
• Color measurements are mostly made to
determine quantitatively whether or not the color
of two objects are the same
• The total color difference ∆E and its coponents:
lightness ∆L, chroma ∆C and hue ∆H can be
numerically calculated
• The color difference is calculated using the
tristimulus values
• Numerical color differences may be used for
setting tolerances for quality control
Objects
• Objects are characterized by the
amount of light they emit and
reflect or transmit at each
wavelength of interest
• When light is incident on an
object a part of it is absorbed, a
part is reflected and a part may
be trasmitted
• The object may also emit light
• All these characteristics
contribute to the observed color
Incident
Light
Reflected
Light
Absorbed
Light
Transmitted
Light
Reflectance
• Specular (regular)
reflectance = mirror like
reflectance
• Diffuse reflectance =
reflectance in all
directions
• Gloss = combination of
specular and diffuse
reflectance
Specular
Diffuse
Glossy
Definitions
• Reflectance ρ is the ratio of the total radiant flux
reflected by the surface to the flux incident on
the surface
• Reflectance factor R is the radiant flux reflected
in the direction delimited by a given cone to that
reflected in the same direction by a perfectly
reflecting diffuser identically irradiated
• If the solid angle of the cone approaches a limit
of 0 or 2π sr, reflectance factor R approaches
radiance factor  or reflectance ρ
Spectrophotometers and
colorimeters
• Spectrophotometers are used to measure an
object’s reflection characteristics
• Colorimeters measure directly tristimulus values
or related color coordinates
• Colorimeters are less expensive and simple to
use but less accurate for determining tristimulus
values
• Colorimeters determine the color difference
between two samples better than tristimulus
values
• Colorimeters can not determine metamerism
Measuring diffuse reflectance
• Instruments measuring the color of reflecting
objects consist of an illuminator, a sample
holder, and a receiver
• The CIE recommends four illuminating and
viewing geometries for making reflectance
measurements: 45/0, 0/45, d/0, and 0/d
• The most common instrument for measuring
diffuse reflectance is the integrating sphere
• Another type of technique, which is getting more
popular, is the angular integration of
gonioreflectometric measurement results
Integrating sphere-based
techniques
• An integrating sphere is
coated from the inside with
uniformly diffusing material
• It has openings for the
sample, light source and
the receiver
• The idea is to either create
a diffuse geometry of
illumination or to collect
light scattered diffusely by
the sample
d/0 geometry
• The light is incident on
the sphere wall and is
reflected in all directions
• As the result of multiple
reflections the sample is
illuminated from all
directions
• The sample is viewed in a
near normal angle
• The specular reflection is
directed back to the
source and is not
measured
Photometer
Specular port
Light
source
Sample
0/d geometry
• The light is incident
on the sample
• The sample scatters
the light and after
multiple reflections it
illuminates the
detector from all
directions
• The 0/d geometry is
equivalent to the d/0
geometry
Photometer
Baffle
Sample
Light
source
Absolute and relative measurement
methods
• Relative measurement methods produce values
that are relative to reference standards
• Absolute measurement methods relate the
reflectance values of a standard to that of the
perfect reflecting diffuser
• The relative methods are commonly used in
industry, whereas the absolute methods are
commonly realized in national standards
laboratories
Example of relative method
• Signals are measured
from the sample, the
reference, and the light
trap (light incident on the
trap)
• The light trap gives the
dark signal which is
subtracted from the
results
• The sample and
reference readings are
compared and corrected
by the known values of
the reference
Sample
Holder
Reference
Entrance
Reference
Holder
Light
Trap
Sample
Entrance
Example of absolute method
• Taylor’s method: Detector
readings when the
sample port is not
covered (a), it is covered
with sphere material (b),
and it is covered by the
sample (c)
• Increase from a to b is
proportional to the
reflectance of the sphere
• The reflectance of the
sample is calculated from
the ratio of a and c
C
B
Sample, Cap
or Light trap
A Light
source
Goniometric techniques
• Gonia = angle
• The idea is to illuminate the sample in a certain
angle and measure reflectance on the surface of
a hemisphere around the sample (or vice versa)
• In practice this can be realized with a two-axis
goniometer or with a one-axis goniometer by
integrating over the polar angles
• Enables bidirectional measurements
Gonioreflectometer at TKK
Rotation
Source and input optics
enclosure
SPM
MD
M
QTH
GT2
GT1
Meas.
L
position
BS

P
A
Iris
OPM
Light-tight enclosure
Double monochromator
Translation
Detector
OSF
• One-axis goniometer
• The idea is to illuminate
the sample in one
direction and measure
reflectance over the
semiarch
• Total diffuse reflectance is
obtained by integrating
the measured values over
the whole hemisphere
Sample Reference
position
Things to be considered
Rotation
Source and input optics
enclosure
SPM
QTH
MD
M2
M1
GT2
GT1
A
Meas.
L
position
DP

OPM
Iris
SPM
Light-tight enclosure
Double monochromator
Translation
Detector
OSF
• The major source of
uncertainty in the system
is isochromatic stray light
• The biggest contribution
is light scattered about
the main beam
• To compensate the effect
a significant correction
factor must be used
• In our previous system
the correction factor was
much greater than today
due to the more
complicated optics
Sample Reference
position
Gonio vs. sphere
• Goniometric technique provides bidirectional
measurements which are not possible with a
sphere
• The scattering of light about the main beam is
clearly a problem for the gonio but not for the
sphere
• Systematic deviations have been reported
earlier between goniometric and sphere-based
techniques
• The scattering of light about the main beam is a
strong candidate for causing these
discrepancies
Fluorescence
• A fluorescent material
absorbs some of the light
incident on it and emits it
on higher wavelengths
• Part of the energy of the
incident photon is lost in
internal vibrations and
heat
• Fluorescence is used e.g.
in paper whitening, safety
signs and textiles
Commercial fluorescent colorants
• Inorganic fluorophors: stable but toxic, used in
security markings and fluorescent lamps
• Optical whiteners: organic compounds, with
excitation at 340-400 nm and emission at 430460 nm, used heavily in textile, paper and plastic
industries to whiten materials
• Daylight fluorescent materials: organic
compounds, emission and excitation in the
visible part of the spectrum, used to color papers
and plastics and especially in safety applications
Measuring fluorescence
• Polychromatic illumination → appearance and
color
• Monochromatic illumination → fluorescence
separated from reflectance
• Often we want to measure fluorescence
quantum yield of a material
• Fluorescence quantum yield = the number of
emitted photons relative to the number of
absorbed photons
• Quantum yield measurements require
monochromatic illumination and viewing
Reference spectrofluorimeter at
NRC
Gonio-fluorometer at TKK
The principle of a CCD
• CCD = charge-coupled
device
• The CCD comprises a
two-dimensional array of
pixels
• Every pixel gathers
radiation from a different
spatial position → large
area of spectrum (~200
nm) measured in one
picture
Problems related to fluorescence
• Stability of the fluorescent standards
• No universally recognized method for
characterization of fluorescent instruments
• Different instruments give different results
• Even the same instrument can give different
results over time
• Comparing different fluorescent samples is
difficult even with the same device
Transmittance measurements
• Similarly to reflectance,
we can have regular,
diffuse or glossy
transmittance
• Transmittance is utilized
e.g. in interference filters
and glass filters
• The most common
measurement geometry
is 0/0
Regular
Diffuse
Glossy
Fabry-Perot filter and interference
filters
• The cavity length
determines the passed
wavelength
• MDI filter: thin partially
transmitting metal layers
• ADI filters: alternating
layers of substances with
differing refractive indices
• Sensitive to temperature
and angle
Fabry-Perot
cavity
Transmitted
waves add
In phase
Input
signal
Reflections
A double-beam transfer standard
spectrometer at TKK
• Used to calibrate
filters
• The idea is to
measure similar
beams through the
filter and through air
• Detector readings
from both sample and
reference are
compared to yield
transmittance
Light
Source
Reference
MC
Sample
Detector
Unit
A single-beam reference
spectrometer at TKK
• Detector readings are
taken through the filter,
through air and dark
reading
• The filter and light trap
can be moved into the
beam by a linear
translator
• The measurement
system can be modified
to measure e.g. diffuse
transmittance
Light
Source
MC
A
OPM
Filter-holder
Unit
Averaging
Sphere
Detector
Choosing measurement geometry
• Bidirectional illuminating and viewing geometries
can be very sensitive to surface texture and
polarization
• Bidirectional geometries are similar to the way a
person evaluates color visually
• Diffuse geometries minimize the effect of a
sample’s texture and gloss
Special cases
• Metallic and
pearlescent samples
• Retroreflecting
samples
• Lamps, light sources
and displays
Conclusion
• Color and appearance are important quantities
in several branches of industry e.g. paper, textile
and plastic industry
• The color and appearance of a material are
effected by the light source, observer and
spectral properties of the material
• Reflectance, transmittance and fluorescence
measurements all require special instruments
• Fluorescence measurements still present severe
problems due to the instability of standards and
lack of universal calibration methods of
instruments