These create all the colors we see. •Colors do not exist in the world –they are created by our nervous systems. Other animals can “see” other wavelengths of light. •The colors we see are determined by the combination of light that enters into our eye - either directly from a light source, or reflected off objects. B. Color Detection •Color is not a physical property of things in the world, it emerges from the interaction of a physical stimulus (wavelengths) with our nervous system (types of receptors). •Photopic: Light intensities that are bright enough to stimulate the cone receptors and bright enough to “saturate” the rod receptorsSunlight and bright indoor lighting •Scotopic: Light intensities that are bright enough to stimulate the rod receptors but too dim to stimulate the cone receptors Moonlight and extremely dim indoor lighting The moonlit world appears drained of color because we have only one type of rod photoreceptor transducing light under these scotopic conditions
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5/9/2013 6 B. Color Detection: Photoreceptors in the retina •Wavelengths of light must be detected in the first place. •Three types of cone photoreceptors (plus rods) S-cones detect short wavelengths M-cones detect medium wavelengths L-cones detect long wavelengths B: Color Detection But how do we know the difference between one wavelength and another? A single photoreceptor shows different responses to lights of different wavelengths of the same intensity. C: Color Discrimination: Problem of Univariance If we only had one photoreceptor, many different wavelengths of light would look the same. Lights of 450 and 625 nm each elicit the same response from this photoreceptor so blue and orange would look the same. C: Color Discrimination: The trichromatic solution •The two wavelengths (blue, orange) that produce the same response from one type of cone (M) produce different patterns of responses across the three types of cones (S, M, and L)