Basic Guidelines
(I) Intensity of Light Source
Amount of electromagnetic energy radiated by a light source
UNITS
Candela
(E) Illuminance
Amount of light falling on a surface
UNITS
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
UNITS
Foot-Lambert
• Amount of light reflected depends on location (color, darkness)
Measurement Tools
Photometer
Calculating Luminance
L = R*E
Luminance = Reflectance * Illuminance
Reflectance
Ratio from 0 to 100%
Contrast
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
Measurement
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.
Influences
• 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
Note
Description
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