[Accessibleweb] AccessibleWeb@U minutes, Thursday, Jan 8

Richard Ells rells at u.washington.edu
Thu Jan 8 16:57:39 PST 2004

Attending: Mary Waters, Pat Smith-Major, Kathryn Sharpe, Jennifer
Ward, Kathi Woods, Mamoru Iwabucki, Terri Dobrich, Carmen Cook,
Andre Tan, April Ryan, Dan Comden, Harry Love, Kay Pilcher,
Alexis Raphael, Melody Winkle, Carol Robinson, Rick Ells

Speaker: Matt McMahon
I. Senior Fellow in the Department of Biological Structure
II. Vision operates on light that enters the eye
A. Light that is emitted from the display determines
what a person looking at your page sees.
III. Rods and cones
A. Rods are for night time, very sensitive
B. Cones are for daytime vision
1. Color vision is product of how cone photoreceptors are
sensitive to light
2. Different types of cones have different sensitivity
a. blue cone - short wavelengths
b. green cone - middle wavelengths
c. red cone - longer wavelengths
3. Output = intensity X sensitivity
a. Each type of cone can't differentiate colors
within its sensitivity profile (individual
cones are color blind): more intense light in a
less sensitivity part of its profile looks the
same as less intense light from more sensitive
4. Brain creates color by comparing output from the
three types of cones
5. Chromaticity diagram
a. Monitors create colors by mixing color from three
color primaries, or monitor phosphors
b. www.cs.rit.edu/~ncs/color/a_chroma.html
c. People with normal color vision can see all the
colors that a monitor can create
6. Red/green color blindness
a. Types
1. Protan - red defect
a. Severe - Only have two cones, green and
blue, to sense colors
b. Cannot make discriminations in the red/
green range
c. Reds look very dark
d. Another type (mild) is when red cones
are shifted toward the green range
2. Deutan - green defect
a. Lack green cone - only have red and blue
b. Cannot make discriminations in the red/
green range
c. Another type (mild) is when green cones
are shifted toward the red range
3. Tritan - blue defect
a. Lack blue cone
b. Very rare
b. 8% of males are red/green color blind
1. 2% with serious red/green color blindness
2. 6% with mild red/green color blindness
c. Red/green color blindness is a sex-linked
recessive trait
1. All the color vision genes are carried on the
X Chromosome
2. Color defects are very rare in females
7. Confusion Lines on chromaticity diagram
a. All colors that lie along the confusion lines are
hard to distinguish for color defective people
IV. What this means for Web designers
A. Contrast is the key
1. High contrast is easy to read
2. Medium contrast is hard to read
3. Low contrast is impossible to read
B. Extent the concept of contrast to the color blind person
1. Cranking a color up to its maximum value does not
ensure high contrast
2. For the colors chosen, want greatest difference in
luminance or brightness
C. General Rules
1. Always make sure there is high brightness contrast
2. Ask yourself if you require color to convey more
if the answer is yes, then either
a. Choose colors that have previously been shown to have
high contrast for color blind people (a safe palette)
b. Choose colors that you like and then make sure they are
clearly distinguishable by color blind people
D. Methods
1. General color guidelines
a. Red: avoid pure red. Use vermilion or change to
b. Green: avoid pure green, which is confusing with
red or brown. Use bluish green.
c. Light green and yellow will appear the same to
color blinds. Avoid using colors between yellow
and green (which are in the range that color blind
people cannot distinguish).
2. Color blind safe colors
3. Use http://vischeck.com
a. Can simulate how color displays look to color
deficient observers - Does Deuteranope, Protanope,
and Tritanope simulations
1. Effect on web pages
a. Style sheets, javascript, etc. can mess
up the algorithm
b. Often works better to take a screen shot
of the page and run it through Vischeck
as an image
2. Effect on images
b. Can download a vischeck plugin for ImageJ (a java
based image processing program)
c. Can download a vischeck plugin for Photoshop
also at http://www.vischeck.com/downloads/
4. You can't really guess how colors will be perceived by
color blind people, but its easy to test
5. Do not want to operate at a high level of precision -
keep the colors as distinguishable as possible
a. There are huge differences between display
b. Older monitors with more limited colors cannot
present fine differences between colors
c. Printers may not have the same color range of
E. Top Tools
1. http://vischeck.com
a. Simulate the appearance of web pages or images for
color blind people
2. Barrier-free presentation that is friendly to
colorblind people
a. http://jfly.iam.u-tokyo.ac.jp/html/color_blind/
b. contains a color blind safe web palette
3. http://www.iamcal.com/toys/colors/index.php
a. Simulate how colored text on color backgrounds
appears to people with different color vision
F. Using color
1. How can you use a range of colors, such as in a map?
a. Very hard for red/green color blind
b. Use gray scale: just black, white, and grays
c. Grayscale also works well for printing
d. In RGB, colors are created by ratio between red,
green, blue. Brightness is determined by the
overall values - the higher the brighter
e. Examples of transforming color maps are in the
f. Color blind people can see colors, can identify
most colors. May not be able to discriminate
between colors very well.
g. If you do not need color to convey more
information, do not bother
2. Are some people completely colorblind
a. Yes, usually the result of a head injury or stroke
b. Some people are missing all three cone receptors
c. Some people only have the blue cones
3. Failing night vision
a. May be the result of generally poorer vision as
they get older
b. Some degeneration of cones
4. Brightness versus saturation
a. Brightness is lightness/darkness
b. Saturation is about purity of color
c. HSB color system lets you adjust brightness (B)
and saturation (S) separately
1. Makes it easy to clarify difference between
saturation and brightness
d. Can convert back and forth between RGB, HSB, CMYK
and other color models easily in most graphics
5. Can you test contrast by printing out page on black and
white printer?
a. Not a perfect test, but is a good practice
b. Not a good way to simulate color blindness
c. Can give a sense of page contrast, but some
browsers adjust image for black and white when

|- Rick Ells - 543-2875 - rells at cac.washington.edu - Rm 011S MGH Bldg -|
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