light-and-reflection-18may10-150dpi-med

# light-and-reflection-18may10-150dpi-med - Light and...

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Light and refection Marc Levoy Computer Science Department StanFord University CS 178, Spring 2010 Updated 5/19/10 and recap slides added.

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! Marc Levoy Outline ! measures of light radiometry versus photometry radiant intensity of a point light radiance leaving an area light radiance arriving on a surface irradiance on a surface ! reFection of light diffuse specular goniometric diagrams ±resnel equations and other effects 2
! Marc Levoy Radiometry versus photometry ! radiometry is the study of light w/o considering humans spectroradiometer - power as a function of wavelength radiometer - total power, integrating over all wavelengths measurements include - radiant intensity, radiance, irradiance ! photometry is the study of light as seen by humans spectrophotometer - power we see as a function of wavelength photometer, a.k.a. photographic light meter measurements include - luminous intensity, luminance, illuminance 3

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! Marc Levoy Relationship to tristimulus theory ! the response of the human visual system to a spectrum is ! the total response can be expressed as ! where ! one could also express luminance as 4 ( ! , " , # ) = L e ( \$ ) 400 nm 700 nm % ( ) d , L e ( ) 400 nm 700 nm % ( ) d , L e ( ) 400 nm 700 nm % ( ) d ' ( ) * + V ( ) = ( ) + ( ) + ( ) L = + + = L e ( ) V ( ) 400 nm 700 nm % d luminance radiance (Stone) V( ! ), or luminous efFciency curve L ( ) = L e ( ) V ( ) S is actually much lower than M or L
! Marc Levoy Radiant intensity of a point light ! power given off by the light per unit solid angle ! i.e. the energy per unit time per unit solid angle 1 watt = 1 joule / second 5 I = P ! watts steradian " # \$ % ' (Reinhard)

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! Marc Levoy Steradian as a measure of solid angle ! 1 steradian (sr) is the solid angle such that the area subtended by that solid angle on the surface of a sphere is equal to the sphere’s radius 2 area of a sphere is 4 ! r 2 , so 1 sr = r 2 / 4 ! r 2 " 1/12 of the sphere’s surface ! examples circular aperture 65° in subtended diameter square aperture 57° on a side a circle 12.7’ in diameter cast by a streetlight 10’ high 6 (http://www.handprint.com/ HP/WCL/color3.html)
! Marc Levoy Radiant intensity of a point light ! power given off by the light per unit solid angle ! example 100W light bulb gives off 100 watts over the sphere ÷ 4 ! sr in a sphere = 8 watts within a 12.7’ circle 10’ feet from the bulb 7 I = P ! watts steradian " # \$ % ' (Reinhard) (http://www.handprint.com/ HP/WCL/color3.html)

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! Marc Levoy Radiant intensity of a point light ! power given off by the light per unit solid angle ! related photometric concept is luminous intensity (measured in candelas) 1 candela = 1 lumen / sr ! examples a standard Bouguer candle gives off 1 candela a 100W light bulb with a luminous efFciency of 2.6% (the other 97.4% we don’t see) gives off 17.6 lumens per watt
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light-and-reflection-18may10-150dpi-med - Light and...

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