Comparing Additive and Subtractive Color

Research Questions:

• What is additive light?
• What is subtractive light?
• Why does additive light handle differently than subtractive light?

When light strikes an object, some of the wavelengths are absorbed and others are reflected. This means that the red shirt you are wearing is not necessarily red; rather, you see only those wavelengths that are reflected back to your eyes. We do not see color in the dark because there are insufficient wavelengths reaching our eyes. This is subtractive color. Additive color is very different from subtractive color. Additive color refers to light that is given off from a source, rather than specific reflected wavelengths. For example, a red brake light is additive. In this experiment, students will try to reproduce exactly what they see using color from an additive source (a backlit computer screen) and a subtractive source (a glossy magazine). They will be able to reproduce the subtractive color, but not the additive color because these colors function very differently.

Materials:

• Computer and monitor,
• Red, blue, green and yellow poster paint
• Brushes
• Colored pencils
• Paper
• Magnifying lens
• Glossy magazine

Experimental Procedure:

1. Turn on your computer and search the Internet until you find two images, one with a lot of purple, and the other with a lot of orange. Using a high-powered magnifying  glass, examine the individual pixels on the screen closely.
2. Using color pencils reproduce a small swatch of what you saw that. This reproduction need only be roughly 20 pixels by 20 pixels. The point of this is to reproduce what you saw on the screen so you have a larger sense of how the individually colored pixels work together. Do this for both the orange image and the purple image.
3. Examine the orange swatch you made in step 2 and evaluate approximately what percentage each color contributed to the whole. For example, you might decide that orange swatch was roughly 10% cyan, 50% yellow, and 40% magenta. Combine equal percentages of those colors of poster paint and mix. What color do you have?
4. Repeat step #3, using the purple swatch.
5. Find glossy magazine photos in which orange and purple dominate a part of the photo. Examine them with a magnifying lens and make two dot drawings, just like you did in step #2.
6. Examine the orange swatch that you made in step 5. Evaluate what percentage of each color contributes to the whole. Mix poster paint in the same percentages to see if you can achieve the same color ad in the magazine.
7. Find a region on your computer screen that appears pure white. Examine this with the hand magnifier. Did you see something white? Repeat, using a white section of a glossy magazine.                                                       

Terms/Concepts: Additive Color, Subtractive Color, Pixels, Color Spectrum

References:

Books

Stroebel, Leslie, John Compton, Ira Current, and Richard Zakia. Basic Photographic Materials and Processes. Focal Press (2000)

Waldman, Gary. Introduction to Light: The Physics of Light, Vision and Color. Dover Publications (2002)

Wyszecki, Günther and W. S. Stiles. Colour Science Concept and Methods, Quantitative Data and Formulae. Wiley (1982).

Websites

Wikipedia: RGB Color Model

http://en.wikipedia.org/wiki/RGB_color_model

Wikipedia: Additive Color

http://en.wikipedia.org/wiki/Additive_color

Wikipedia: Subtractive Color

http://en.wikipedia.org/wiki/Subtractive_color