3.1 Light

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Introduction to Light and Color Perception

  • We perceives color from the complex interacction of multiple factors

Factors Determining Perceived Color

  • Material properties (albedo / reflectnace)
  • Surface Geometry (angles, shadows, highlights)
  • Illumination specturm (color temperature of light source)
  • Viewing conditions (adaptation state, surrounding colors)

albedo: the proportion of the incident light or radiation that is reflected by a surface, typically that a planet or moon

Mathematical Representation:

Iperceieved(x,y)=R(x,y)×L(x,y)×G(x,y)I_{perceieved}(x, y) = R(x, y) \times L(x, y) \times G(x, y)

where:

  • R(x,y)R(x, y): reflectance (material property)
  • L(x,y)L(x, y): illumination intensity
  • G(x,y)G(x, y): geometric fctors (surface, angles)

The Eye

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Eye ComponentCamera EquivalentFunction
IrisAperture diaphragmSpans and contracts pupils to control light amount
PupilAperture openingLight entry point
LensCamera lensFocuses light
RetinaImage sensor/filmDetects light
Optic nerveData cableTransmits signals

Retinal Structure

  • The retina is NOT a simple light-detection surface. Light must pass through multiple layers.

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  1. Ganglion cells
  • First layer light encounters
  • contrast-sensitive
  • control iris response
  • process initial visual information
  1. Bipolar and horizontal cells
  • Intermediate processing layers
  • handle lateral inhibitation (edge enhancement)
  • contribute to automatic responses (blink reflex)
  1. Photoreceptors
  • Rods and cones for actual light detection
  • Convert photons to electrical signals
  • Foundation of all vision

Important

This backwards arrangement means light is partially filtered and processed before reaching phtoreceptors

Phtoreceptors: Rods and Cones

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  • Humans have two separate visual systems operating simultaneously.
FeatureRodsCones
Primary FunctionVision in low light, peripheral vision, motion detectionColor vision, central vision, and detail acuity
Light SensitivityHigh; active in dim lightLow; active in bright light
Color PerceptionResponsible gray-scale visionResponsible color vision
DistributionMore numerous and concentrated in the periphery of the retinaLess numerous and concentrated in the center of the retina
Detail Acuitylowhigh

LMS cone cells

  • OPN1LW
  • (long-wavelength, red)
  • X chromosome
  • OPN1MW
  • (medium-wavelength, green)
  • X chromosome
  • OPN1SW
  • (short-wavelength, blue)
  • chromosome 7

Surface

Lambertian Surface

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  • Some light is absorbed
  • albedo
  • Remaining light is reflected in all directions
  • diffuse reflection

Diffuse reflection

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  • Intensity depends on illumination angle
  • less light comes in at oblique angles

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  • However, perceived intensity does not depend on viewer angle
  • The amount of reflected light are proportional to cos(θ)cos(\theta)
  • Visible solid angle also proportional to cos(θ)cos(\theta)

Specular Reflection

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  • Light reflected in one direction
  • Reflected direction depends on light orientation and surface normal
  • Specularity
  • spot where specular reflection dominates (typically reflects light source)

BRDF

  • Bidirectional reflectance distribution function

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More complicated effects

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Things to remember

  • Light has a spectrum of wavelengths
  • Observed light depends on illumination intensities, surface orientation, material
  • Every object is an indirect light source for every other
  • Shading and shadows are informative about shape and position