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.

How does ADI reveal differences between primary colors of light and

primary pigment colors?

Part Two

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 pigments are often used to print

many different colors.

Elementary school students often learn that Blue, Red, and Yellow paints

can be used to create many 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.

SummaryEyes 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

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.