Color and Humans Chapter 13 Human color perception
Human Color Perception • how we see color • how color is described • color blindness
Elements of Color illumination perception reflection
Visible Spectrum We perceive electromagnetic energy having wavelengths in the range 400 – 700 nm as visible light. Higher energy Shorter wavelength Lower energy Longer wavelength
Hue • Hue is that psychological dimension of color which roughly corresponds to wavelength. • A typical color wheel has 16 “hues”
Higher Energy Lower Energy
Value • Lightness or darkness of color • Also called tint, shade, tone • Created by adding white or black to a hue • Discrimination depends on background color – dark vs. light
Brightness Colors are also described by their brightness Hue will change with intensity INTENSITY
The photosensitive part of the eye is called the retina The retina is largely composed of two types of cells; rods and cones. Only the cones are responsible for colour perception Colour-blindness results from a deficiency of one cone type
The Retina – Organization When different cones are stimulated, the result is that the brain “mixes” the frequencies to make all the perceived colors, to include white. Red plus blue plus green will mix to make white.
Cones respond to three colors: red, green and blue. Rods detect intensity of light: black, white, shades of gray. Used for night vision. Photoreceptors in the eye
Color and Vision • When all the colors of the rainbow are combined, we do not see any particular color. • We see light without any color. • We call this combination of all the colors of light "white light".
The three color receptors in the eye allow us to see millions of different colors. The additive primary colors are red, green, and blue. All the different shades of color we can see are made by changing the proportions of red, green, and blue. How we see other colors
When we see an object, the light that reaches our eyes can come from two different processes: The light can be emitted directly from the object, like a light bulb or glow stick. The light can come from somewhere else, like the sun, and we see the objects by reflected light. The object absorbs some frequencies and reflects the others. How we see the color of things Red, orange, yellow, green and violet are absorbed. Blue is reflected. We see the blue.
Colored fabrics and paints get color from a subtractive process. Chemicals, known as pigments, in the dyes and paints absorb some colors and allow the color you actually see to be reflected. Magenta, yellow, and cyan are the three subtractive primary pigments. How we see the color of things The subtractive primary pigments
Review The subtractive primary pigments
Color Blindness Affects: About 8 percent of all males About 0.5 percent of all females (Hackman 1992)
Color Blindness Color blindness (weakness) has four basic varieties: green blindness--individuals confuse greens, yellows, and reds (6.39 percent) red blindness--individuals confuse various shades of red (2.04 percent) blue blindness--individuals confuse blues (0.003 percent) total color blindness, which affects no more than 0.005 percent of both sexes.
Testing for Color Blind ess Just how do technicians test a person for color blindness – the inability to distinguish colors or shades of color? Your thoughts?
A quick colour blindness test… Both normal and those with all colour vision deficiencies should read the number 12.
What number do you see in the circle? Normal vision should read the number 29. Red-green deficiencies should read the number 70. Total colour blindness should not read any numeral
What number do you see in the circle? Normal colour vision should read the number 5. Red-Green colour deficiencies should read the number 2. Total colour blindness should not be able to read any numeral.
What number do you see in the circle? Normal colour vision should read the number 6. The majority of those with colour vision deficiencies cannot read this number or will read it incorrectly.
What number do you see in the circle? Normal colour vision and those with total colour blindness should not be able to read any number. The majority of those with red-green deficiencies should read the number 5.
4 Colour Theory • How many colours do you really need?
Questions??????? If not, Vision Correction is Next.
reference • Eye of the beholder- designing for colour-blind users(http://more.btexact.com/people/rigdence/colours/) • also has palette files • Colour vision(http://www.psychology.psych.ndsu.nodak.edu/mccourt/website/htdocs/HomePage/Psy460/Color%20Vision/Color%20Vision.html) • Choosing colours for “Mapping Census 2000 – the Geography of American Diversity”(http://www.personal.psu.edu/faculty/c/a/cab38/CensusAtlasCMYK.html) • Colour Brewer (http://www.personal.psu.edu/faculty/c/a/cab38/ColorBrewerBeta.html)