Light, Color & Perception

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Presentation transcript:

Light, Color & Perception CMSC 435/634

Light Electromagnetic wave Photon wavelength l, frequency f = c/l E & M perpendicular to each other & direction Photon wavelength l, frequency f = c/l Visbile l ≈ 380 nm (blue) to 720 nm (red) Photon energy q = h f = h c/l (in J) h = Planck’s constant Spectral energy Q = J/nm

Radiometric Units Term Symbol Units Spectral Energy Q J Spectral Power  = dQ/dt W = J/s Irradiance E = d/dA W/m2 Radiance L = d2/(d dA) W/(sr m2) Radiant Intensity I = d/d W/sr /nm dropped by graphics convention

Light Infinite-dimensional function vector space spectrum(l) Shine two lights on something adds energies Scale light energy, scales the intensity

Color Perception

Color Perception Cones = function dot product Projects to a 3D subspace metamer = different spectrum, looks the same Can transform to any 3D linear basis

Linear Bases Additive (light) Subtractive (pigment) Tristimulus (SLM cone response) CIE XYZ (from color matching experiments) RGB (different for each device) Yuv, YCrCb, … Subtractive (pigment) CMY = 1-RGB (grade school Blue, Red, Yellow) CMYK

Photometric Units Visual intensity Term Symbol Units Luminous Energy Q talbot Lumens  = dQ/dt lm = talbot/s Illuminance (Lux) E = d/dA lx = lm/m2 Luminance L = d2/(d dA) lm/(sr m2) Candelas I = d/d cd = lm/sr

Nonlinear Color Picking HSV: Cylindrical Coordinates Hue = angle Saturation = distance from central axis Value = distance along axis

Nonlinear Perception Linear colors don’t look uniformly different Nonlinear Luminance Gamma (sRGB), L*uv Nonlinear Luminance & Color L*u*v*, L*a*b* Can measure color distances Nonlinear colors do not add

Adaptation White point Dynamic range (HDR) Measure as equivalent to black body ºK Red-Orange-Yellow-Blue-White Incandescent, Dynamic range (HDR) Dark room vs. Sunny day

Tone Mapping Convert HDR to LDR Film exposure Global intensity Local intensity Time Film exposure Aperture, shutter speed, ISO

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