The Eye and Color Sensation

So far in this chapter we have discussed light and colors measured by optical instrumentation. Color identification as used by the painter depends on the response of the eye. Our perception of color arises from the composition of light—the energy spectrum of photons that enter the eye. The retina, located on the inner surface of the back of the eye (Figure 5.19), contains photosensitive cells. These cells contain pigments that absorb visible light. Of the two classes of photosensitive cells, rods and cones, it is the cones that allow us to distinguish between different colors. The rods are effective in dim light and sense differences in light intensity—the flux of incident photons—not photon energy. So in dim light we perceive colored objects as shades of gray, not shades of color.

Fig. 5.19. A cross-sectional representation of the eye showing light entering through the pupil. The photosensitive cells, cones and rods, are located in the retina: Cones respond to color, and rods respond to light intensity.

Fig. 5.20. The response of the three cones to incident light: Cone R (pigment R) has a maximum sensitivity in the orange-red, cone G (pigment G) in the green-yellow, and cone B (pigment B) in the blue portions of the visible spectrum. The sensitivities of the three cones overlap, and the perceived color is due to the relative response of the three cones.

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Wavelength (nanometers)

Fig. 5.20. The response of the three cones to incident light: Cone R (pigment R) has a maximum sensitivity in the orange-red, cone G (pigment G) in the green-yellow, and cone B (pigment B) in the blue portions of the visible spectrum. The sensitivities of the three cones overlap, and the perceived color is due to the relative response of the three cones.

Color is perceived in the retina by three sets of cones, photorecep-tors with sensitivity to photons whose energy broadly overlaps the blue, green, and red portions of the spectrum. Color vision is possible because the sets of cones differ from each other in their sensitivity to photon energy. The sensitivity of the cones to light of the same intensity (the same photon flux) but different wavelengths (energy) is shown in Figure 5.20. The maximum sensitivity is to yellow light, but cone R has a maximum in the red-orange, G in the green-yellow, and B in the blue. The sensitivities of the three cones overlap. For every color signal, or flux of photons, reaching the eye, some ratio of response within the three types of cones is triggered. It is this ratio that permits the perception of a particular color.

The afterimage phenomenon discussed in Chapter 2 is due to fatigue in the cones. If we stare at a blue object on a white background, the cones in the retina are activated by the photon flux from the blue color. The red, blue, and green pigmented cones respond to the mix of photons of various wavelengths in a proportion representing that color. After a period of time, these cones become fatigued. If the blue object is removed, an image appears on the white background. The cones that are not fatigued signal the image color response, and consequently the afterimage is in the complementary color of the real object.

Freehand Sketching An Introduction

Freehand Sketching An Introduction

Learn to sketch by working through these quick, simple lessons. This Learn to Sketch course will help you learn to draw what you see and develop your skills.

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