1. Physiology of Color Vision Why learn about how eye perceives color? (can help predict success of computer color) Color is physical light Composed of photons Electromagnetic spectrum Objects emit or reflect light General properties of light Color is more than physical light Physiology of color vision 2. Effects of Physiology on Color Vision Effect of the lens physical effect: short wavelengths tend to focus in front of retina perceptual effect: physical effect: constant refocusing needed to see different colors (more marked if colors are pure) perceptual effect: If you want to make something "move forward" on the screen, is red or blue better, and why? Support or negate this statement: Bright, true color combinations are the best if you want to make special shapes (like icons) easy to look at. Why is it a bad idea to pick five related shades of blue to represent, for example, the relative volume of message traffic? Effect of the retina Light receptors translate light into nervous impulses Rods and cones are not evenly distributed physical effect: cones concentrated in central area of retina, which has no rods perceptual effect: physical effect: rods concentrated at edges of retina perceptual effect: physical effect: center of retina has no blue receptor cones perceptual effect: Fluids in eye also change with age physical effect: transmittance is reduced perceptual effect: Support or negate the following statement: A red object is equally visible, regardless of whether it appears at the center or the edge of a display screen. Why do small blue objects sometimes seem to "disappear" from a display? If your bar-chart display will be used by a variety of ages, should you use a palette of subdued colors, bright colors, or a mix? 3. How colored objects are discerned Forms are identified by their edges physical effect: color difference alone is poor focussing guide perceptual effect: physical effect: sensitivity governed by sum of red and green channels perceptual effect: Optic nerve also responds differently to receptor components physical effect: red and green components signal brightness perceptual effect: What is the real color? Since orange stands out and blue recedes, is bright orange on bright blue a good combination for maximum visibility? Magenta and violet are "trendy" colors. Why wouldn't you use a magenta shape on a violet background? Suppose you must choose three people to test your tool color scheme. What characteristics might be important? 4. Principles Based on Physiology [From Murch, "Physiological Principles for the Effective Use of Colors" -- see Web too (1) Avoid the simultaneous display of highly saturated, spectrally extreme colors. frequent refocusing causes visual fatigue (2) Pure blue should be avoided for text, thin lines, and small shapes. eyes can't handle detailed, sharp vision of the shorter wavelengths (3) Avoid adjacent colors that differ only in the amount of blue. (4) Older operators need higher brightness levels to distinguish colors. (5) Colors change in appearance as the ambient light level changes. (6) The magnitude of a detectable change in color varies across the spectrum. (7) It is difficult to focus upon edges created by color alone. (8) Avoid red and green in the periphery of large-scale displays. (9) Opponent colors go well together. (10) For color-deficient observers, avoid single-color distinctions.