Production and Transmission of Light

• Measurement

• Light and color production– Pigments, interference, scattering,

bioluminescence

• Transmission through the environment

• Optimal hue, contrast, shading

Color

Light intensity(analogous to sound amplitude)

• Radiant flux Photons/s• Irradiance Photons/s•m2

– Total light, including scattered and direct

• Radiant intensity Photons/s•sr• Radiance Photons/s•sr•m2

– Only direct light emitted from a specific area

– sr = steradian with 4 steradians in a sphere

– Note that intensity is independent of wavelength

– But wavelength is inversely related to energy

Light intensity variation

Color Spectra

Hue Chroma

Intensity

Color has 3 dimensions

Brightness = radiance

Sources of color

• Pigments– Molecules which selectively absorb photons of some

wavelengths and transmit others– Size of molecule affects wavelength absorption– Short chain molecules require high energy (short

wavelengths) for excitation

• Structural colors– Interference– Scattering

Pigments

Carotene absorbs blue andtransmits green, yellow and red

Benzene absorbs UV

Porphyrins: iron = hemoglobin,magnesium = chlorophyl

Carotenoids

Melanin

• Large protein that absorbs all wavelengths and, therefore, appears black

• Present in skin and hair of mammals, chitin of many insects

• Coat color variants are caused by temporal regulation of melanin production during hair follicle growth, e.g. agouti phenotype: dark-light-dark

Status badges in Harris sparrows

Chromatophores

Fish

Lizards

Cephalopods

Iridiophores containplatelets that reflectsome wavelengths

Chameleons can change color

Cuttlefish have chromatophores

Structural colors

Color depends on the reflection angle from feathers

Interference

Interference in feathers

Beetles coat melanin with wax

Color by Scattering

Color by scattering

Bioluminescence

Bioluminescence is common among marineorganisms, especially deep-sea fishes.

Habitat Transmission

• Amount and spectral composition of available light can differ by habitat

• Color brightness of an object depends on wavelengths of available light

• In terrestrial habitats, light spectra is influenced by– Sun’s angle, weather, vegetation

• Expect animals to utilize colors appropriate for habitats

Color filtering by habitat

Spectra in forest

Colors in whitelight

Colors in forest

Light reflectance

Light attenuation• Light attenuation follows inverse square law

which is independent of wavelength

• Scattering and absorption, however, increases as wavelength decreases

Signal detection

• Must discriminate object from background using brightness, color, pattern or movement

• Can adopt countershading or reverse counter shading for crypsis or conspicuousness

• Visual systems often exaggerate contrast to detect objects in background

Signalling may vary with habitat

Phylloscopus warblers

Counter and reverse shading

Pattern contrast

Photon noise

Eye resolution

Shape enhancement

Optimal signal and background hues

Mammalian Eye

Rods and Cones