Perception of Light and Color Theory and Practice
Trichromacy Three cones types in retina a b G+B +R
Cone sensitivity functions 100 80 60 40 20 400 500 600 700 Wavelength (nm)
Short wavelength sensitive cones Blue text on a black background is to be avoided, we have few blue cones in the retina and they are not very sensitive Blue text on a black background is to be avoided, we have few blue cones in the retina and they are not very sensitive Blue text on a black background is to be avoided, we have few blue cones in the retina and they are not very sensitive Blue text on a black background is to be avoided, we have few blue cones in the retina and they are not very sensitive
Color Channels Long (red) Med (green) Short (blue) Luminance R-G Y-B
Luminance contrast
Luminance channel Visual system extracts surface information Discounts illumination level Discounts color of illumination Mechanisms 1) Adaptation 2) Simultaneous contrast
Channel Properties Luminance Channel Detail Form Shading Motion Stereo Chromatic Channels Surfaces of things Labels Berlin and Kay Categories (about 6-10) Red, green, yellow and blue are special (unique hues)
Color phenomena Chromatic contrast Small field tritanopia b a d c
Chromatic Channels have Low Spatial Resolution Luminance contrast needed to see detail Some Natural philosophers suppose that these colors arise from accidental vapours diffused in the air, which communicate their own hues to the shadows; so that the colours of the shadows are occasioned by the reflection of any given sky colour: the above observations favour this opinion. Text on an isoluminant background is hard to read Some Natural philosophers suppose that these colors arise from accidental vapours diffused in the air, which communicate their own hues to the shadows;
Color blindness A 3D to a 2D space 8 % of males R-G color blindness Can generate color blind acceptable palette Yellow blue variation OK
Implications Color perception is relative We are sensitive to small differences- hence need sixteen million colors Not sensitive to absolute values- hence we can only use < 10 colors for coding
Color great for classification Rapid visual segmentation Color helps us determine type Only about six categories white black red green yellow yellow green blue brown pink purple orange grey
Color Coding Large areas: low saturation Small areas high saturation Break isoluminance with borders
Color Sequences for Maps Color is poor for form and shape Color is naturally classified Luminance is good for form and shape Luminance results in contrast illusions A spiral sequence in color space - a good solution
Multi - Dimensional Scatterplot V1 - xposition V2 - yposition V3 - red V4 - green V5 - blue Can identify clusters and trends Cannot read values back from visualization
Take home messages Use luminance for detail, shape and form Use color for coding - few colors Minimize contrast effects Strong colors for small areas - contrast in luminance with background Subtle colors can be used to segment large areas
Exercise 1: Distinctive Icons Goal: make something that stands out preattentively Use orientation, color, simple shape, size Danger or alert icon No more than 1.5 cm in diameter Evaluation: rapid exposure in the context of other pattern Movement not allowed