Light

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Light

• Visible electromagnetic radiation
• Power spectrum
• Polarization
• Photon (quantum effects)
• Wave (interference, diffraction)

From London and Upton
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Topics

• Light sources and illumination
• Radiometry and photometry
• Quantify spatial energy distribution
• Radiant intensity
• Irradiance
• Inverse square law and cosine law
• Radiance
• Radiant exitance (radiosity)
• Illumination calculations
• Irradiance from environment

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Radiometry and Photometry
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Radiant Energy and Power

• Power Watts vs. Lumens
• Energy efficiency
• Spectral efficacy
• Energy Joules vs. Talbot
• Exposure
• Film response
• Skin - sunburn

Luminance
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Radiometry vs. Photometry

• Radiometry Units Watts
• Physical measurement of electromagnetic energy
• Photometry and Colorimetry Lumen
• Sensation as a function of wavelength
• Relative perceptual measurement
• Brightness Brils
• Sensation at different brightness levels
• Absolute perceptual measurement
• Obeys Stevens Power Law

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Blackbody Radiation
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Tungsten Lamp
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Fluorescent Bulb
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Sunlight
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Radiant Intensity
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Radiant Intensity

• Definition The radiant (luminous) intensity is
  the
• power per unit solid angle emanating from
• a point source.

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Angles and Solid Angles

• Angle
• Solid angle

Þ circle has 2p radians
Þ sphere has 4p steradians
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Differential Solid Angles
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Differential Solid Angles
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Isotropic Point Source
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Warns Spotlight
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Warns Spotlight
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Light Source Goniometric Diagrams
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PIXAR Light Source
UberLight( ) Clip to near/far planes Clip
to shape boundary foreach superelliptical
blocker atten foreach cookie
texture atten foreach slide texture
color foreach noise texture
atten, color foreach shadow map
atten, color Calculate intensity fall-off
Calculate beam distribution
Shadows
Shadow Matte
Projected Slide Texture
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Radiance
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Radiance

• Definition The surface radiance (luminance) is
• the intensity per unit area leaving a surface

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Typical Values of Luminance cd/m2

• Surface of the sun 2,000,000,000 nit
• Sunlight clouds 30,000
• Clear day 3,000
• Overcast day 300
• Moon 0.03

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Irradiance
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The Invention of Photometry

• Bouguers classic experiment
• Compare a light source and a candle
• Intensity is proportional to ratio of distances
  squared
• Definition of a candela
• Originally a standard candle
• Currently 550 nm laser w/ 1/683 W/sr
• 1 of 6 fundamental SI units

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Irradiance

• Definition The irradiance (illuminance) is the
• power per unit area incident on a surface.
• Sometimes referred to as the radiant (luminous)
• incidence.

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Lamberts Cosine Law
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Lamberts Cosine Law
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Irradiance Isotropic Point Source
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Irradiance Isotropic Point Source
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Irradiance Isotropic Point Source
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Irradiance Isotropic Point Source
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Irradiance Isotropic Point Source
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Irradiance Isotropic Point Source
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Directional Power Arriving at a Surface

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Irradiance from the Environment
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Typical Values of Illuminance lm/m2

• Sunlight plus skylight 100,000 lux
• Sunlight plus skylight (overcast) 10,000
• Interior near window (daylight) 1,000
• Artificial light (minimum) 100
• Moonlight (full) 0.02
• Starlight 0.0003

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The Sky Radiance Distribution
From Greenler, Rainbows, halos and glories
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Gazing Ball ? Environment Maps
Miller and Hoffman, 1984

• Photograph of mirror ball
• Maps all spherical directions to a to circle
• Reflection direction indexed by normal
• Resolution function of orientation

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Environment Maps
Interface, Chou and Williams (ca. 1985)
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Irradiance Environment Maps
Radiance Environment Map
Irradiance Environment Map
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Irradiance Map or Light Map
Isolux contours
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Radiant Exitance (Radiosity)
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Radiant Exitance

• Definition The radiant (luminous) exitance is
  the
• energy per unit area leaving a surface.
• In computer graphics, this quantity is often
• referred to as the radiosity (B)

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Directional Power Leaving a Surface

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Uniform Diffuse Emitter
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Projected Solid Angle
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Uniform Diffuse Emitter
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Radiometry and Photometry Summary
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Radiometric and Photometric Terms
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Photometric Units
Thus one nit is one lux per steradian is one
candela per square meter is one lumen per
square meter per steradian. Got it?, James Kajiya