Basic Guidelines
(I) Intensity of Light Source
Amount of electromagnetic energy radiated by a light source
(E) Illuminance
Amount of light falling on a surface
Foot-Candlels or Lux
  • Amount of light falling on surface is same wherever on surface
Inverse Square Law
Illuminance is inversely proportional to squared distance

E = I/(D^2)
Measurement Tools
Illumination Meter
(L) Luminance
Amount of light reflected off by a surface
  • Amount of light reflected depends on location (color, darkness)
Measurement Tools
Calculating Luminance
L = R*E
Luminance = Reflectance * Illuminance
Ratio from 0 to 100%
Difference between the luminance of light and dark areas

C(%) = (L-D)*100/(L+D)
Cobb Moss Curve
Represents quantitative relationship between contrast ratio, luminance, and visual angle
can be measured by grating - determining the spatial frequency (the number of dark-light pairs that occupy 1 degree of VA that someone can detect
Contrast Sensitivity
the reciprocal of the minimum contrast between a lighter and darker spatial area that can just be detected. 
  • Level of contrast
  • Level of illumination
  • Age of eye
  • Motion
Simultaneous Contrast
The tendency for some hues o appear different when viewed adjacent to other hues
Cobb Moss Problems
  • Draw a curve for luminance if it isn't already drawn.
  • Always draw for lighter luminance
  • See equation for visual acuity above
  • If VA is below min VA, an observer canno see it
  • Multiply MVA axis by 2 to get 95% accuracy
  • Use contrast ratio equataion
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