Light I June 15, 1999 Paper summaries on light Any takers? June - PowerPoint PPT Presentation

Light I June 15, 1999

Paper summaries on light ✔ Any takers? June 15, 1999

Motivational Film ✔ Bingo (1998) – Alias / Wavefront June 15, 1999

Photography and Light pho•tog•ra•phy, n ., the process or art of producing images of objects by the action of light on a sensitized surface, esp, a film in a camera. Photography = writing with light June 15, 1999

Today’s Class ✔ Light – What it is – How we measure it – How it behaves – Light and color June 15, 1999

Light -- What it is ✔ Electromagnetic radiation induction radio ultra gamma secondary 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 1 10 -2 10 -4 10 -6 10 -8 Wavelength visible light (nm) Red 700 nm orange 650 nm yellow 600 nm green 550 nm blue 450 nm violet 400 nm June 15, 1999

Light -- How we measure it ✔ Light is Radiant Energy ✔ Measure in Joules ✔ CG uses particle model of light – Light travels in localized particles or wave packets. June 15, 1999

Light -- How we measure it ✔ Radiant Flux (Radiant Power) – Amount of energy / unit time – Joules per second (Watt) dQ Φ = dt June 15, 1999

Light -- How we measure it ✔ Radiant Flux Density – Amount of flux per unit area arriving at or leaving from a point on the surface – Measured in Watts / m 2 June 15, 1999

Light -- How we measure it ✔ Irradiance - flux density in Φ d = E dA dA June 15, 1999

Light -- How we measure it ✔ Radiant exitance - radiant flux out Φ d = M dA dA June 15, 1999

Light -- How we measure it ✔ Radiance – Flux arriving at or leaving from a given point or surface in a given direction. – Measured in Watts / m 2 / steradian June 15, 1999

Light - how we measure it 2 Φ d = L ( cos ) ω dA θ d June 15, 1999

Light -- How we measure it ✔ Radiant Intensity – Amount of radiant flux in a given direction – Watts / steradian – Point light sources Φ d = I ω d June 15, 1999

Light - how we measure it ✔ Each of the measures can vary with wavelength June 15, 1999

Light - how we measure it ✔ In summary – Radiant Flux - energy / time – Radiant Flux Density - total flux entering or leaving a point or surface – Radiance - total flux entering or leaving a point or surface in a given direction – Radiant intensity - flux in a given direction for point light sources – All measures can vary with wavelength June 15, 1999

Light -- How we measure it ✔ Photometric measures – Accounts for human perception of brightness – Radiometric units scaled by luminosity function. – Same concepts -> different units June 15, 1999

Light -- How we measure it ✔ CIE Luminous Efficiency Curve 120 100 % Efficiency 80 60 40 20 0 375 400 425 450 475 500 525 550 575 600 625 650 675 700 725 750 Wavelength June 15, 1999

Light - how we measure it ✔ Photometric Units – Luminous Flux - energy / time – Luminous Flux Density - total flux entering or leaving a point or surface – Luminance - total flux entering or leaving a point or surface in a given direction – Luminace intensity - flux in a given direction for point light sources – All scaled by CIE Luminous Eff. Curve June 15, 1999

Light -- How we measure it ✔ Photometric units – Luminous Flux (lumen) – Luminous Flux Density (lumen/m 2 = lux) – Luminance (lumen/m 2 /steradian = nit) – Luminous intensity (lumen / steridian = candela) June 15, 1999

Light - how we measure it ✔ 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 June 15, 1999

Light -- How it behaves ✔ Reflection ✔ Absorption ✔ Refraction ✔ Scattering ✔ Diffraction / Interference ✔ All can be wavelength dependent June 15, 1999

Light -- How it behaves ✔ Reflection – Angle of incidence = Angle of reflectance – Perfect mirror surface N θ θ i r June 15, 1999

Light - How it behaves ✔ Absorption – Material can absorb light on a wavelength by wavelength basis – Responsible for object color June 15, 1999

Light - How it behaves ✔ Refraction – bending of light as it travels through different media sin sin η θ = η θ θ i i r r i η i η r θ r June 15, 1999

Light - How it behaves ✔ Total internal reflection θ = angle at c which refracted η i rays lies η perpendicular r to normal θ c June 15, 1999

Light - How it behaves ✔ Scattering – Light is scattered by small particles in its path (e.g. haze, smoke, etc.) – Given by fraction of light with respect to direction from particle light impact. – Size of particles are on the order of wavelengths of light. June 15, 1999

Light - How it behaves ✔ Scattering r α June 15, 1999

Light -- How it behaves ✔ Scattering – r << λ total absorption (no scattering) – r < λ Rayleigh Scattering – r ≈ λ Mie scattering – r >> λ Geometric optics June 15, 1999

Light -- how it behaves ✔ Raleigh scattering (smoke / dust ) 3 ( ) ( 1 cos 2 α ) α = + P 4 June 15, 1999

Light - how it behaves ✔ Mie Scattering (haze / fog) 8 1 cos  + α  Sparse / hazy ( ) 1 9 α = + P   2   32 1 cos  + α  Dense / murky ( ) 1 50 α = + P   2   June 15, 1999

Light -- How it behaves ✔ Diffraction – 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 ✔ Generally not considered in CG. June 15, 1999

Light and Color ✔ Color is the perceptual response to light of wavelengths 400 - 700 nm hitting the retina. ✔ When rendering spectrum must be sampled. ✔ Color vision is inherently trichromatic. June 15, 1999

Light and Color ✔ “Indeed rays, properly expressed, are not colored” ✔ Spectral power distributions exist in the physical world but color exists only in the eye and brain. June 15, 1999

Light and Color ✔ CIE Experiments June 15, 1999

Light and Color ✔ CIE RGB curves 40 30 20 R 10 G B 0 375 405 435 465 495 525 555 585 615 645 675 705 735 -10 -20 Wavelength June 15, 1999

Light and Color ✔ CIE xyz color matching curves 200 150 x 100 y z 50 0 375 405 435 465 495 525 555 585 615 645 675 705 735 Wavelength June 15, 1999

Light and Color ✔ Chromaticity coordinates X Y Z = x = = y z + + X Y Z + + + + X Y Z X Y Z 1 + + = x y z June 15, 1999

Light and Color ✔ Chomaticity coordinates – often given in xyY – xy give the chromaticity – Y gives brightness June 15, 1999

Light and Color ✔ Chromaticity diagram June 15, 1999

Light and Color ✔ RGB (or any primary set) can be determined from XYZ – Need chromaticies of primaries and white point. ✔ Primaries generally determined by device. ✔ RGB values are incomplete without specification of primaries & white point. June 15, 1999

Light and Color ✔ sRGB – Standard proposed by Microsoft and HP – Based on ITU-R 709.BT June 15, 1999

Light and Color 3 . 2410 1 . 5374 0 . 4986 = − − R X Y Z 0 . 9692 1 . 8760 0 . 0416 = − + + G X Y Z 0 . 0556 0 . 2040 1 . 0570 = − + B X Y Z June 15, 1999

Light and Color ✔ sRGB June 15, 1999

Light and Color ✔ Other color spaces – HSV (hue-saturation-value) – CMYK (printing) – CIELAB / CIELUV (perceptual) ✔ Why does CG use RGB – Convienience June 15, 1999

Light and Color ✔ Summary – Color is perceptual not physical – Human vision is inherently trichromatic (so too are most display devices) – Color specification involves chromaticity and brightness. – RGB requires specification of primaries and white point June 15, 1999

Light and Color ✔ Further Reading – Giorgianni / Madden, Digital Color Management – Poynton, Technical Introduction to Digital Video – Hunt, Reproduction of Colour June 15, 1999

For next time ✔ Project Proposals are due next class June 15, 1999

Next Class ✔ Lighting Models June 15, 1999

Remember ✔ Class Web Site: – http://www.cs.rit.edu/~jmg/virtualPhoto ✔ Any questions? June 15, 1999