Photometric Units

While radiometry applies to all wavelengths of radiant energy, photometry applies to only the visual spectra (roughly 400 nm to 700 nm) portion of radiant energy.

Natural Scene Examples

Approximate Luminance of the sky near the horizon in cd/m2 (Middleton,1952)
EnvironmentLuminance Level (cd/m2)
Clear sky at noon10^4
Cloudy sky at noon10^3
Grey sky at noon10^2
Cloudy sky at sunset10
Clear sky, a quarter hour after sunset1
Clear sky, a half hour after sunset10^-1
Night sky - Full moon10^-2
Night sky - Clear without moon10^-3
Night sky - Cloudy without moon10^-4

Light Levels Important to Vision

Luminance in cd/m2 (Hood and Finkelstein, 1986)
SignificanceLum Level (log cd/m2)Illum Level (log trolands)
Absolute Threshold-6-4.4
Scotopic Start
Cone Threshold-3-2
Scotopic End
Mesopic Begin
Rod Saturation Begins22.6
~Mesopic End
~Photopic Begin
Damage Possible88.5

Conversion from Ft-Lamberts

Conversion from Candelas per Meter^2

Conversion from photopic trolands

Conversion from Scotopic trolands

Conversion from photons per receptor per second

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© 1996 Lance Hahn (lance@retina.anatomy.upenn.edu)