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Vision – The Eye

This article explores the mechanisms of human vision, focusing on the three types of color receptors in the eyes. We discuss additive color mixing, where red, green, and blue light combine to produce various colors, and give examples like cyan and magenta. In contrast, subtractive color mixing is explained, which is crucial for color reproduction in hardcopy devices, starting from white light and using colorants like cyan, magenta, and yellow. The differences between these two approaches highlight how colors are perceived and generated in both digital and print mediums.

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Vision – The Eye

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  1. Vision – The Eye • The eyes have three different kinds of color receptors; One kind is most sensitive to short wavelengths, one to middle wavelengths, and one to long wavelengths

  2. Color Mixing There are two ways to control how much red, green, and blue light reaches the eye: “Additive Mixing” Starting with black, the right amount of red, green, and blue light are ‘added’ to an image. “Subtractive Mixing” Starting with white, the right amount of red, green, and blue light are ‘subtracted’ from an image.

  3. Additive Color Mixing By exciting the red, green, and blue sensitive cones, any color can be produced by adding together the three additive primaries (R,G,B). Mixing the three color sources is known as “additive mixing” to distinguish it from mixing paints or dyes (“subtractive mixing”).

  4. Additive Color Mixing For example, when blue and green lights overlap, the blue and green cones are illuminated, and we perceive cyan

  5. Additive Color Mixing red + blue = magenta green + blue = cyan red + green + blue = white red + green = yellow

  6. Additive Color Mixing red + green/2 = orange red + green = yellow red/2 + green = lime red + green + blue = gray red + green + blue = gray red + green + blue = white

  7. R G B Additive Color Reproduction Color video projectors use additive color mixing • Projected red, green, and blue images contribute RGB components to create color images

  8. y x Spatial Mixing (Video Monitor) Because the visual system has limited spatial resolution, small areas of different colors are mixed perceptually. Spatial addressability of typical monitors goes from (640 x 480) to (1600 x 1280) pixels.

  9. Temporal Mixing (Digital Cinema) Because the visual system has limited temporal resolution, rapidly changing colors are mixed perceptually. time time time

  10. Subtractive Color Mixing Color hardcopy devices can’t use additive mixing because they aren’t sources of light; they can’t add Red, Green, or Blue components. Instead, they use subtractive mixing. Starting with white light reflected by the substrate, they subtract the unwanted red, green, and blue components using cyan, magenta, and yellow colorants.

  11. b+r = m White light White light White light g+b = c r+g = y Subtractive Color Mixing • The goal is the same; to control the amount of Red, Green, and Blue light getting to the eyes’ three cone types • Each colorant absorbs 1/3 and transmits 2/3 of white light white substrate cyan colorant“minus red” magenta colorant“minus green” yellow colorant“minus blue”

  12. r+g/2 = orange White light White light White light Subtractive Color Mixing • Other colors are made by varying the amount of colorant in each layer. yellow & magenta = red yellow magenta yellow + magenta/2 orange + cyan black

  13. C Y M K Subtractive Color Reproduction • Color printing uses subtractive color mixing. • Adding black allows more accurate grays, and conserves the more expensive CMY colorants.

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