Logistics About the readings: Intro to Light Undergrads: Due Friday by midnight Grads: Due Mon/Wed by midnight If the dropbox is closed…please e-mail. Logistics Plan for today Reminder Intro to light Project Proposals due Monday Ray tracing basics Checkpoint #2 – Raytracing through a Raytracer Checkpoint 1 due Wednesday. camera model Hey, that’s today! Photography and Light Computer Graphics as Virtual Photography pho•tog•ra•phy, n ., the process or art of real camera photo Photographic Photography: scene (captures processing print producing images of objects by the action of light) light on a sensitized surface, e.g., a film in a camera. processing Photography = writing with light camera Computer 3D tone synthetic model Graphics: models reproduction image (focuses simulated lighting) 1
Light Light -- What it is Why important? (photorealistic Electromagnetic radiation images!) induction radio ultra gamma secondary What it is power infrared x-rays heating waves violet rays cosmic rays 10 10 10 8 10 16 10 14 10 12 10 6 10 4 10 2 10 -2 10 -4 10 -6 10 -8 1 How it is measured Wavelength visible light (nm) Radiometric Red 700 nm orange 650 nm yellow 600 nm Photometric green 550 nm blue 450 nm violet 400 nm How it behaves Light -- How it is measured Light – Radiant Flux Radiometric Units Light is radiant energy Radiant Flux ( Φ -Radiant Power/Watt) Measure in Joules ( Q ) Amount of energy / unit time One joule is the equivalent of one watt Joules ( Q ) per second of power radiated or dissipated for one second. dQ CG uses particle model of light Φ = Light travels in localized particles or wave dt packets . Light – Radiant Flux Density Light -- Irradiance Irradiance ( E ) – radiant flux density coming in Radiant Flux Density (Irradiance/Radiant Exitance) Amount of flux per unit area arriving at or leaving d Φ from a point on the surface E = Measured in Watts / m 2 dA (Remember a Watt is Joules/sec.) dA 2
Light – Radiant Exitance Light -- Radiance Radiant exitance ( M ) - radiant flux leaving Radiance ( L ) the surface Flux arriving at or leaving from a given point or surface in a given direction . d � Measured in Watts / m 2 / steradian M = d 2 Φ dA L = dA dA(d ω cos θ ) steradian Light -- How it is measured Light – Radiant Intensity Steradian (sr) - Standard International unit of Radiant Intensity ( I ) – point source solid angular measure. There are 4 pi Amount of radiant flux in a given direction steradians in a complete sphere – (See Watts / steradian Point light sources http://whatis.techtarget.com/definition/0,289893,sid9_g ci528813,00.html) d Φ = I d ω is the d ω measurement of the cone size Light – Measurement Light -- How it is measured Summary Photometric Units Photometry measures visible light according Radiant Flux - energy / time - ( Joules/sec ) to the sensitivity of human eye: Radiant Flux Density - total flux entering ( irradiance ) or leaving ( radiant excitance ) a point Cones: blue – short, green – medium, red – long or surface - ( Watts/m 2 ) Rods: low illumination Radiance - total flux entering or leaving a point or Eye sensitivity varies with wavelength, e.g.., green surface in a given direction - ( Watts/m 2 / steradian ) light appears brighter than red/blue of same intensity! Radiant intensity - flux in a given direction for So, photometric units are radiometric units point light sources - ( Watts/steradian ) scaled by the luminosity function All measures can vary with wavelength!!! Same concepts -> different units 3
Light – CIE Luminous Efficiency Curve Light – Photometric Units 120 Luminous Flux - energy / time - ( lumen ) % Efficiency 100 Luminous Flux Density - total flux entering or 80 60 leaving a point or surface - ( lux = lumen/m 2 ) 40 Luminance - total flux entering or leaving a point or 20 surface in a given direction - ( nit = 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 7 0 2 5 7 0 2 5 7 0 2 5 7 0 2 5 lumen/m 2 /steradian ) 3 4 4 4 4 5 5 5 5 6 6 6 6 7 7 7 Wavelength Luminance intensity - flux in a given direction for Created using perception matching brightness of point light sources - ( candela = lumen / steridian ) monochromatic light at different wavelengths All scaled by CIE Luminous Efficiency Curve Provides weighting curve/function used to convert from radiometric to photometric measurements Light -- How it is measured Light -- How it behaves Example Reflection Absorption The luminance at a surface due to a blue light of a given intensity would be less than Refraction the luminance at the same surface due to a Scattering yellow light of the same intensity. Diffraction / Interference Why? Humans perceive yellow light to be brighter than blue light All can be wavelength dependent!!! Light - Reflection Light - Absorption What is not reflected, can be absorbed Angle of incidence = Angle of reflectance Perfect mirror surface � � r i 4
Light - Refraction Light interactions Bending of light as it travels through different …at the wavelength level media Scattering η sin θ = η sin θ Diffraction i i t t d � n i � i Interference η t θ t t η Where and are the indices of refraction. � i t ( http://www.physics.northwestern.edu/ugrad/vpl/optics/snell. html ) Light - Scattering Light -- Scattering r r � � Light is scattered by small particles in its path (e.g. r << λ total absorption (no scattering) haze, smoke, etc.) r < λ Rayleigh Scattering Given by fraction of light with respect to direction r ≈ λ Mie scattering from particle light impact. Size of particles are on the order of wavelengths of r >> λ Geometric optics light. Light – Raleigh Scattering Light – Mie Scattering Raleigh scattering (smoke / dust ), the Mie Scattering (haze / fog) probability that the light will scatter in 8 1 cos direction α . � + � � Sparse / hazy P ( ) 1 9 � = + � � 2 � � 3 2 � P ( ) ( 1 cos ) � = + 32 1 cos � + � � Dense / murky 4 P ( ) 1 50 � = + � � 2 � � 5
Atmospheric Light Scattering Mie Scattering Rayleigh scattering preferentially scatters light at the wavelength of blue light... this is what makes Earth skies appear blue Mie scatter caused primarily by water vapor, fumes, smoke, and dust results in red/orange appearance of evening skies www.acclaimimages.com/_gallery/ Light -- How it behaves Light -- How it behaves Diffraction Diffraction Interference Bending of light around objects Contributes to soft shadows, color bleeding Interference Superimposition of two waves Accounts for colors in thin films, bubbles, oil slicks, peacock feathers http://www.physik.tu-muenchen.de/ http://sol.sci.uop.edu/ Light -- How it behaves Light – How it behaves Now that we know how light travels, And of course… can we simulate this with the goal of All can be wavelength dependent!!! image synthesis… Power spectrum of light determines Enter… color. Ray Tracing!!!! Will talk more about color later in the course. 6
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