Colour
David Molyneaux
1
What is Light?
• Light is electromagnetic energy
• Light is a wave
• Colour is the way the eye perceives light at different
wavelengths inside the visible spectrum
2
The Visible Spectrum
When energy at these wavelengths reaches
our eye, it causes us to perceive a colour.
The rainbow - ROYGBIV
3
The Eye + How we see colour
The retina of the eye (D) contains millions of
photoreceptors
• Rods
• Number about 120 million, and are very sensitive to light
• Cones
• Number 6 – 7 million, and are sensitive to colour
4
The Colour Wheel
• Understanding how colours interact makes colour
adjustments much easier
5
Describing Colour
• Hue
– Hue refers to the attribute of colors that permits them to be
classed as red, yellow, green, blue, or an intermediate between
any contiguous pair of these colors. Hue differences depend
primarily on variations in the wavelength of light.
• Brightness
– Brightness refers to the relative lightness or darkness of the
color. It is determined by the degree of reflectivity of the physical
surface receiving the light. The higher the brightness, the lighter
the color.
• Saturation
– Saturation refers to how vivid a color appears. It is measured in
terms of the difference of a color from a colorless (neutral) gray
with the same degree of brightness. The lower the saturation, the
grayer the color. When saturation is zero, the color is gray.
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Additive and Subtractive Colour
Objects in the real world
absorb and reflect particular
wavelengths of light – this
is what gives them a colour.
Subtractive – Colour
printers, prints made in
the darkroom
Black (K) is usually added to CMY in printers
Additive – Monitors,
TV, projectors, etc..
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Adding, Subtracting
• The RGB colour wheel demonstrates the relationship
between the colours.
• Two adjacent colours can be mixed to form intermediates
– E.g. Magenta can be made from red and blue
– Yellow and cyan, when mixed together, produce green.
• Colors opposite each other on the color wheel are
complementary.
– For example, if the photograph you've taken has too much
green in it, you can suppress this effect by adding in its
complementary color, magenta (or red and blue in the RGB
model).
– Conversely, you can make red appear brighter by reducing
cyan (or reducing green and blue, in the RGB model).
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Colour in the Wet Darkroom
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How Colour Film Works
• Colour film is composed of several
layers of light sensitive chemicals
• Each layer is sensitive to a different
wavelength of light
– (A) is Blue sensitive
– (C) is Green sesitive
– (D) is Red sensitive
• Mask layers (B) filter out certain
wavelengths, so that lower layers are
not exposed
• Everything sits on the film base (E)
10
Developing Colour Film
C41 Process
• C41 for colour negatives is relatively straightforward
• Two bath processes are available - a developer and a
bleach-fix (BLIX) bath
• Both require constant temperature around 35°C +/- 0.5
E6 Process
• E6 colour reversal chemistry for slides is more
complicated than C41
• You had to develop the negative image, bleach it away,
then develop the positive image…
• Again all steps require constant temperature around
35°C +/- 0.5
11
Colour Paper
•
Both similar process to black and white, but with an additional bleach stage
to form the colours. This may be combined with the fixer to create a bleachfix (BLIX)
• RA4 Process:
– RA4 process for printing from Colour Negatives
– “Chromogenic” printing works by colour developer acting on "colour couplers" in
the paper. Color dyes that are in the chemistry interact with the developer to form
a color image on the paper.
– Typically 3 layers:
• Red sensitive (forms a cyan colour on developed)
• Green sensitive (forms a magenta layer on development)
• Blue sensitve (forms a yellow layer on development)
• Ilfochrome / Cibachrome Process:
– Ilfochrome (formerly Cibachrome) / P3 / P30 process for printing from Colour
Positives (slides)
–
The necessary colors (azo dyes) are built into three emulsion layers of silver salts sensitized
to one of three colors: red, blue, or green. The image is formed by selectively bleaching dyes
already existing within the paper
12
The Colour Enlarger
• LPL 7700 Enlarger Components:
– 3 colour filter wheels (CMY)
– Filter In/Out knob (for focussing)
– Integral Negative Mask
– Height Adjust
– Focus Adjust
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Enlarger Operation
•
Set an initial filtration (e.g. 75M 45Y 0C)
•
Set enlarging lens to f11 or f16
•
Focus your negative using the focus-scope
•
Switch off all lights except the minilux torch
•
Expose a test strip to determine the correct exposure (ignore colours)
•
Expose a test strip for colours – change filters in large increments to start with
•
Important: Generally you should only use one or two of the filters, as using all
three gives you a neutral density (only useful if you need to lengthen the
exposure)
•
Important: Exposure also changes as the colour filters change (increase in
filters = longer exposure required)
•
Viewing Filters can help you decide what filtration changes are required
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Filtering for Colour Correction
•For colour negatives:
Cast
Correction
Green Subtract magenta
Blue
Subtract yellow
Magenta Add magenta
Red
Add yellow and magenta
Yellow Add yellow
Cyan
Subtract yellow and
magenta
10 point cast
•Do the opposite for slides
as this is positive-positive.
20 point cast
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Colour Analysers
• A colour analyser can reduce time spent colour printing
• However, it must be calibrated first!
– Take a photo of the test card using the film type you want to print from
(e.g. Kodak Gold Ultra) outside on an overcast day
– Have this developed and put it in the enlarger
– Manually print a correctly exposed and colour corrected print
– Switch on the enlarger with the correct settings as determined by the
manual print and switch on the analyser
– Zero the dials / centre the LEDs on the analyser
– The analyser is now calibrated to print from that one type of film.
• See analyser instructions for actual usage with other images on
this film type.
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Durst Printo and Jobo CPE2
•
The Durst Printo system is a roller transport process
– The paper travels through the heated chemical baths on rollers
•
For RA4 the system consists of :
– Intro Module – where you introduce the paper
– Two Heated Chemical Baths for Dev and Blix
– Plus an external tray rinse
•
•
•
Paper requires 45 seconds per bath @ 35°C
Uses 2.5L of chemicals per bath, but replenishable
Good for high volume work
•
The Jobo CPE2 is a heated water bath with
drums for paper and film development
Paper requires 45 seconds per chemical @ 35°C
•
– May also need a stop bath (same chemicals as B+W)
•
•
Difficult to get good results due to very short dev times,
so can process for longer at lower temperature if required
Only requires 150-200ml of chemical per paper, but is one-shot
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Colour Quiz
• Guess the Colour cast:
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Colour Quiz
• Magenta!
– So remove magenta, or add green
Uncorrected
Corrected
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Colour Quiz
• Guess the Colour cast:
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Colour Quiz
• Green!
– So remove green, or add magenta
Uncorrected
Corrected
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Colour Managed Workflows
in the Digital Darkroom
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Digital Darkroom Equipment
• Computer, Monitors, Flabed Scanner, Slide Scanner,
Digital Camera, Printer
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RGB
•
Many digital imaging systems use the RGB system to describe colour
•
RGB are three values, specifying the intensity of the Red, Green and Blue
channels, which when mixed will create the correct colour
•
Most digital cameras and computers use 24 bit colour, where each channel
uses 8 bits to describe its intensity (a range of 0 to 255 in decimal)
•
0 is low intensity, 255 is high intensity (bright), so:
–
–
–
–
–
•
•
Red = [ 255, 0, 0]
Green = [ 0, 255, 0 ]
Blue = [ 0, 0, 255 ]
Mid Grey = [ 128, 128, 128 ]
Light pink = [ 236, 152, 234 ]
Higher bits values (e.g. 16 bits per channel) allow a greater range of colours to
be described and more natural looking images, as the possible number of
colours is larger
Unfortunately higher bit images take up a lot more room on your disk…
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Colourspaces and Colour Gamut
• This is the CIE XYZ device
independent model for
describing colour
• This describes a colour
mathematically without
referring to any particular
device
• Difficult to work with directly as
it is an abstract model
• Device gamut's can be
described inside this model
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Device Gamuts
• Each device will have a
slightly different gamut
associated with it,
depending on its capabilities
• Monitors and Televisions
(additive colour) have a
different gamut to printers
(subtractive colour)
• This is why it is difficult to
print exactly what you see
on a monitor in an
uncalibrated system
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Profiles
•
•
To solve this gamut difference
problem, we can create a device
profile
A profile mathematically describes the
size and shape of gamut for the device
we are using
•
With known device profiles, we can
convert images between each device
colour space and what we see on
each device should be roughly the
same
•
To generate these profiles we can use
spectrophotometers (or colorimeters)
to compare the colours our devices
produce against a calibrated standard
•
Profiles have a .icc or .icm extension
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Rendering Intent
•
Perceptural or Relative
Colourmetric mappings are
between profiles are favored
•
Perceptural attempts to
maintain the relationship
between colours, so will
compress out of gamut
colours (saturation decreases)
•
Relative Colourmetric keeps
colours inside destination
gamut unchanged, but maps
colours outside to the nearest
hue. (saturation is clipped)
White is mapped to white.
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Where is all this used in Windows?
• Where are the profiles kept?
– C:\WINDOWS\system32\spool\drivers\color
• How do I add/install a profile?
– Right click on the icc or icm file and choose “Install”
• Where do I set my monitor profile?
•
•
•
•
Right click on the desktop, select “Properties”.
Go to the Settings tab, Click “Advanced”
Go to the Colour Management Tab
You can now select your monitor profile (only useful if you have calibrated
your monitor)
• What about all this perceptual mapping stuff?
– See Colour Management Menus in
• Adobe Photoshop
• Paint Shop Pro
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Colour Correction
• Photoshop, Paint Shop Pro, The GIMP, or other image
editing software can be used to correct any remaining
colour casts.
• Tip: Use the Levels or Colour Balance function to
introduce a large colour shift in the preview, then slowly
return to what you think is correct…
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The Magenta Problem
• Magenta is most likely the colour problem you will have.
• It is a common result of atmospheric scattering of light
from haze or other particulate matter.
• However, our eyes have difficulty seeing it, so most
people may not notice a slight magenta cast.
• Tip: Increasing the saturation of an image to maximum
will clearly show you what colours are present in an
image – a good way to identify colours that shouldn’t be
there.
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Metamerism
• Colours can look different in different
lighting conditions
• Can be beneficial, as it allows us to
produce the same colour with different
inks and pigments
• TIP: Always evaluate a print under the
lighting conditions in which it will be
displayed
5000 Kelvin Proofing Light
Tungsten Light
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The End
One Recommended Book for Colour Management on Computers:
Color Confidence: The Digital Photographer’s Guide to Color Management
By Tim Grey
£21.00 from Amazon.co.uk
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