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Light – Measurement Summary
Radiant Flux - energy / time - (Joules/sec) Radiant Flux Density - total flux entering
(irradiance) or leaving (radiant excitance) a point
Radiance - total flux entering or leaving a point or
surface in a given direction - (Watts/m2/ steradian)
Radiant intensity - flux in a given direction for
point light sources - (Watts/steradian)
All measures can vary with wavelength!!!
Light -- How it is measured Photometric Units
Photometry measures visible light according
to the sensitivity of human eye:
Cones: blue – short, green – medium, red – long Rods: low illumination Eye sensitivity varies with wavelength, e.g.., green
light appears brighter than red/blue of same intensity!
So, photometric units are radiometric units
scaled by the luminosity function
Same concepts -> different units
Light – CIE Luminous Efficiency Curve
Created using perception matching brightness of
monochromatic light at different wavelengths
Provides weighting curve/function used to convert from
radiometric to photometric measurements
20 40 60 80 100 120 3 7 5 4 4 2 5 4 5 4 7 5 5 5 2 5 5 5 5 7 5 6 6 2 5 6 5 6 7 5 7 7 2 5 7 5 Wavelength % Efficiency
Light – Photometric Units
Luminous Flux - energy / time - (lumen) Luminous Flux Density - total flux entering or
leaving a point or surface - (lux = lumen/m2)
Luminance - total flux entering or leaving a point or
surface in a given direction - (nit = lumen/m2/steradian)
Luminance intensity - flux in a given direction for
point light sources - (candela = lumen / steridian)
All scaled by CIE Luminous Efficiency Curve
Light -- How it is measured
Example
The luminance at a surface due to a blue
light of a given intensity would be less than the luminance at the same surface due to a yellow light of the same intensity.
Why? Humans perceive yellow light to be
brighter than blue light
Light -- How it behaves
Reflection Absorption Refraction Scattering Diffraction / Interference All can be wavelength dependent!!!
