Detecting and Mixing ColorsSTEM DIGITAL Institute

Rob Snyder

Many instruments can be used to collect visible light data, including

digital cameras.

How do our eyes and digital cameras detect colors of visible light?

Our Eyes Have Two Types of 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 and are often described as red, green and blue cones.

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.

Each type of cone detects a range of wavelengths.“Red” cones have a peak detection of greenish-yellow.

“Green” cones have a peak detection of green . “Blue” cones detect principally blue and violet colors

Which pair of cones might respond when looking at a wall that was painted the color yellow?

Color processing begins in the eye’s cones.Signals are then transmitted through optic nerves to

the brain where data processing continues.

Our brain compares responses of cones to visible light and interprets a color. For example: “Red” and “green” cones both respond to yellow light.

Digital cameras have three types of filters that cover an array of many sensors. The sensors detect the intensities of light of long wavelengths (the red range ) or medium wavelengths (the green range )or short wavelengths (the blue range) that are transmitted through the three different types of filters.

Data is then transmitted to and stored in a memory card in the camera.

Recall that our RGB cones respond to a range of wavelengths of light.

The RGB sensors of a digital camera also respond to ranges of wavelengths of light.

The ADI Software/Brain Analogy Our brain analyzes data it

receives from pairs of “red”, “green”, and “blue” cones in our eyes.

The ADI software program in a computer analyzes the responses of the “red”, “green’ and “blue” sensors in the digital camera.

How can Analyzing Digital Images software reveal the differences between primary colors of light and primary colors of paints or pigments?

Here are two examples of primary paints or pigments that can be mixed.

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.

The ADI “line tool” can reveal how sensors in a digital camera respond to yellow paint.

The average intensities of colors detected along the line are:Red = 62.3%, Green = 45.3%. Blue = 0.16%

ADI can generate a graph of the intensities of red, green and blue intensities along a line.

The ADI Resources Document provides examples of how to use the Line Tool.

The Line Tool: You can draw a line across an image and detect changes in the intensities of red, green, and blue light.Select “Line Tool” and then move cursor to a Starting Point on the PhotoPress down using left click to move the line to an end point and release.You can collect and record data about the start and end point of the line and the average color intensities of the line. Create a graph of colors along a line. Create a graph that shows changes in the intensities of red, green, and blue light along a line that you draw across a photograph.Use the File Menu in the upper left corner.Select “Graph Colors” to generate a graph.You can turn R or G or B or Average intensity on and off.

ADI also has a rectangle tool that can analyze a mix of primary paints or dyes.

The relatively low RGB intensity levels in the rectangle are:

Red = 10.98%, Green = 10.35%, Blue = 9.11%

A histogram of colors in a rectangle reveals low intensities of red, green and blue detected when blue,

red and yellow paints were mixed together.

Mixing primary paints produces a color that absorbs more colors of light so it is called a subtractive process.

The ADI Resources Document provides examples of how to use the Rectangle Tool.

The Rectangle Tool: This feature analyzes average intensities of red, green, and blue light in the rectangle area of a photograph. Select the Rectangle ToolPress down using left click to draw a rectangle on the photograph. Create a color histogram of colors in the rectangle:Use the File Menu.Select “Graph Colors”.You can turn RGB or the Average Line on and off for the entire photograph and/or for the area defined by the rectangle.

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 primary colors of paint absorbs three primary colors of light that our eyes detect. The

cones of our eyes are not stimulated very much and we perceive color as a black or muddy brown color.

Inexpensive spotlights can produce “fairly good” complementary and white colors.

Note: Any 3 colors from three different regions of the visible spectrum can be primary colors.

This is a photograph of a mix of red and green light.

Average Intensities along the line are: R = 69%, G = 57%, B = 38% High quality lights in dark rooms result a much lower intensity of blue.

This graph of colors along the line across yellow light illustrates that “red” and “green” sensors of a digital

camera responded to yellow light. The “blue” sensor did not respond much to yellow light

An ADI rectangle tool can analyze the result of shining inexpensive Red, Green and Blue spotlights onto a screen.

The relatively high intensities of RGB detected in the “white” light are: Red = 62.14%, Green = 59.94%, Blue = 60.36%

An ADI histogram reveals fairly high intensities of red, green and blue light in the white light produced during the demonstration.

Mixing primary colors of light produced a lighter color so mixing colors of light is called an additive process.

Compare the graphs. A mix of primary colors of paint

Subtractive

A mix of primary colors of light

Additive

You can use the Compare Colors Tab in ADI’s Color Basics software experiment with colors of

pigments or light.

Some aspects of color theory results in surprising results.

You can also play with color when word using processing programs.

The Color Spaces tab in Color Basics indicates the use of an intensity scale from 0 to 255.

255 + 1 = 256 = 28. 256 is a special number used in digital computing because it is the maximum number represented by an eight-digit binary number.

A Summary of Color BasicsEyes and digital cameras detect light in the red, green

and blue ranges of the visible spectrum.Primary colors of light are different from primary

colors of paints or pigments.Mixing three primary colors of light is additive

producing a lighter color increases the intensities of the primary colors detected by our eyes or a digital camera.

Mixing three primary colors of paints is subtractive producing a darker color and reduces the intensities of the primary colors detected by our eyes or a digital camera.

Play with colorsPlease take some time to explore colors using Color Basics before

we discuss other features of ADI.