An Introduction to Analyzing
Colors in a Digital Photograph
Rob Snyder
How do our eyes and cameras detect
different colors?
How does ADI reveal differences between
primary colors of light and primary
pigment colors?
What can students learn about colors and
pixels using ADI?
Human Eyes Have Photoreceptors
Our eyes have two main types of photoreceptors
called rods and cones. These cells are located in a
layer at the back of the eye called the retina.
Cones allow us to see colors. They are not as
sensitive as the rods so they only work in bright light.
Rods are used to see in very dim light and only
show the world to us in black and white. This is why
you see only black and white when you are outside in
the evening or in a dimly lit room.
L cones have a peak detection of greenish-yellow.
M cones have a peak detection of green .
S cones detect principally blue and violet colors.
L, M & S refer to wavelengths in nanometers
Color processing that begins in an eye’s retinas is
transmitted via optic nerves to the brain where
data processing continues.
Our brain employs an “opponent” process
where differences in the responses of cones to
visible light are compared and interpreted as a
specific color.
A digital camera is similar in some respects to
our eyes. Digital cameras have red, green and
blue filters over a pixel array of an image sensor.
The sensors detect the intensities of light
transmitted through the filters.
The ADI Software/Brain Analogy
Our brain detects a
wide range of colors by
analyzing the data it
receives from cones on
the retina.
The ADI software in a
computer analyzes data it
receives from sensors in a
camera.
Part Two
How does ADI reveal differences
between primary colors of light and
primary pigment colors?
In the demonstration we used inexpensive
spotlights to mix Red, Green and Blue light and
produced some very interesting results
A wide variety of devices are available that mix
red, green and blue light.
These are colors produced with
inexpensive Red, Green & Blue spotlights
 Red + Blue = Magenta (fairly well)
 Blue + Green = Cyan (even better)
 Red + Green = Yellow (not very well)
 Red + Green + Blue = White (not very
well)
So why did and mix of red and green light
appear to be yellow?
Yellow light has a wavelength of approximately
580 nanometers.
Red light has a longer wavelength and green
light has a shorter wavelength.
However, yellow light and the combination of
red and green light stimulate cones in our eyes
in the same way, so that we perceive color as
yellow in both cases.
An ADI rectangle tool can analyze the result of shining
inexpensive Red, Green and Blue spotlights onto a screen.
Average Intensities: Red = 61%, Green = 59%, Blue = 59%
An ADI histogram reveals fairly high intensities of red, green and
blue light in the white light produced during the demonstration.
Mixing colors of light is called an additive process.
High quality spotlights could produce the
complementary and white colors very well.
Note: Any 3 colors from three different regions of
the visible spectrum can be primary colors.
ADI also has a “line tool” that reveals which
colors of spotlights created the color cyan.
Red = 1.46%. Green = 66.4%, Blue = 58.3%
Notice the changes in intensities of red, green, and blue
as a line crosses colors of pH indicator paper.
This week, you will learn several ways that your
students can use ADI software to conduct scientific
investigations.
How are primary pigments or paints
different from primary colors of light?
Cyan, Magenta, and Yellow
Elementary school students often learn
pigments are often used to print
that Blue, Red, and Yellow paints can
be used to create many colors.
many different colors.
Different colors of a paint, dye or ink absorb
different colors of light.
 Blue paint absorbs principally red and green
wavelengths of light.
 Red paint absorbs principally blue and green
wavelengths of light.
 Yellow paint absorbs principally blue
wavelengths of light.
Mixing these 3 colors of paint absorbs a set of
primary colors of light. The cones of our eyes are not
stimulated very much and we perceive color as a
black or muddy brown color.
ADI software that can analyze a mix of blue, red
and yellow paints or dyes.
Intensity Levels: Red = 11%, Green = 10%, Blue = 9%
An ADI histogram reveals the lower intensities of red,
green and blue colors of light reflected from a mix of
blue, red and yellow paint.
The mixing of pigments is a subtractive process.
What can students learn about
colors and pixels using ADI?
9 tabs in Color Basics provide information about and
activities relating to color.
An example: The results of mixing colors of light and colors of
pigments can be explored.
Students can explore the “grey scale” by changing
equal intensities of cyan, magenta and yellow.
Students can assess their ability to determine the
intensities of the primary colors that produce a hue.
This is a trimmed image of lilac bush leaf.
The green color appears to be fairly uniform.
Use Color Enhancement tools to visualize
differences in colors and see new patterns.
The Green versus Blue (Normalized) enhancement
tool indicates more variation in color in the image.
Color Enhancement strategies are described in pages 13
to 18 in the pdf ADI Help document.
Digital Image Basics provides activities that build an
understanding of Pixels.
A 64 pixel image makes it difficult to identify the plants
in a photograph.
Now we have enough pixels!
Some photographs have too many pixels and must
be trimmed when using ADI utility “Trim Image”
Click on the
Digital Image Basics Icon or the
Color Basics Icon to
experiment with colors and pixels.
Summary
Eyes and digital cameras detect light in the red, green
and blue portions of the visible spectrum.
 Primary colors of light are different from primary
colors of paints or pigments: additive vs. subtractive.
 ADI
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analyze mages of different mixtures of colors or light
and pigment colors.
provides a suite of resources that provides information
about many aspects of digital images.
transforms a digital camera into a scientific instrument.