Radiometry and Photometry - Computer Graphics at 2 Radiometry and Photometry CS348B Lecture 4 Pat...

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Transcript of Radiometry and Photometry - Computer Graphics at 2 Radiometry and Photometry CS348B Lecture 4 Pat...

  • Page 1

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Light

    Visible electromagnetic radiation

    Power spectrum

    Polarization

    Photon (quantum effects)

    Wave (interference, diffraction) From London and Upton

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Radiometry and Photometry

    Measuring spatial properties of light

    !! Radiant power

    !! Radiant intensity

    !! Irradiance !! Inverse square law and cosine law

    !! Radiance

    !! Radiant exitance (radiosity)

    Goal is to perform lighting calculations in the physically correct way

  • Page 2

    Radiometry and Photometry

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Radiant Energy and Power

    Power: Watts (radiometry)

    vs. Lumens (photometry)

    !! Spectral efficacy

    !! Energy efficiency

    Energy: Joules vs. Talbot

    !! Exposure !! Film response

    !! Skin - sunburn

    Photometric luminance

  • Page 3

    Radiant Intensity

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Radiant Intensity

    Definition: The radiant (luminous) intensity is the

    power per unit solid angle emanating from

    a point source.

  • Page 4

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Angles and Solid Angles

    !! Angle

    !! !! !! !! !!

    !! Solid angle

    "" circle has 2 ## radians

    "" sphere has 4 ## steradians

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Differential Solid Angles

  • Page 5

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Differential Solid Angles

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Differential Solid Angles

  • Page 6

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Isotropic Point Source

    Sphere

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Warns Spotlight

  • Page 7

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Warns Spotlight

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Warns Spotlight

  • Page 8

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Warns Spotlight

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Light Source Goniometric Diagrams

  • Page 9

    Irradiance

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Irradiance

    Definition: The irradiance (illuminance) is the power per unit area incident on a surface.

    Sometimes referred to as the radiant (luminous) incidence.

  • Page 10

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    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

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Beam Power in Terms of Irradiance

    = EA

    A

  • Page 11

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Beam Power Falling on the Surface

    A = EA

    E =

    A

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Projected Area

    A! A

    A = A cos

  • Page 12

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Conservation of Energy

    A! AA = A cos

    =

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Lamberts Cosine Law

    A! AA = A cos

    =

    E=

    A=

    Acos = E cos

  • Page 13

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Irradiance: Isotropic Point Source

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Irradiance: Isotropic Point Source

  • Page 14

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Irradiance: Isotropic Point Source

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Irradiance: Isotropic Point Source

  • Page 15

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Irradiance: Isotropic Point Source

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    The Invention of Photometry

    Bouguers classic experiment !! Compare a light source and a

    candle !! Move until they both appear

    equally bright !! Intensity is proportional to ratio

    of distances squared

    Definition of a candela !! Originally a standard candle !! Currently 550 nm laser

    with 1/683 W/sr !! 1 of 6 fundamental SI units

  • Page 16

    Radiance

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Definition: The surface radiance (luminance) is

    the intensity per unit area leaving a surface

    Area Lights Surface Radiance

  • Page 17

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Typical Values of Luminance [cd/m2]

    Surface of the sun 2,000,000,000 nit

    Sunlight clouds 30,000

    Clear sky 3,000

    Overcast sky 300

    Moon 0.03

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Directional Power Leaving a Surface

    Same dA for all directions

  • Page 18

    Radiant Exitance

    (Radiosity)

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Radiant Exitance

    Definition: The radiant (luminous) exitance is the energy per unit area leaving a surface.

    In computer graphics, this quantity is usually referred to as radiosity (B)

  • Page 19

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Area Light Source

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Area Light Source

  • Page 20

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Area Light Source

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Area Light Source

    Hemisphere

  • Page 21

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Uniform Diffuse Emitter

    Uniform means Lo is not a function of direction

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Projected Solid Angle

  • Page 22

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Projected Solid Angle

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Uniform Diffuse Emitter

  • Page 23

    Radiometry and Photometry

    Summary

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    Radiometric and Photometric Terms

  • Page 24

    CS348B Lecture 4 Pat Hanrahan, Spring 2012

    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