CH 9: Perceiving Color

Color Vision

• Signaling Function: Indicates ripe fruit, danger, or other important cues.

• Perceptual Organization: Supports figure/ground separation and object recognition.

Color = global image feature

alters perception of facial expressions

Visible Light

• Stimulus for the visual system — a specific frequency band of electromagnetic radiation.

• Produced by electric charges and radiated as waves.

• Wavelength determines color perception.

Electromagnetic Spectrum

electromagnetic radiation differentiated by wavelength

visible light is only a small portion

• 400-700 nanometers

• perceive different wavelengths as different colors

Spectral Colors

• Colors based on a single wavelength of light.

• Newton’s seven: red, orange, yellow, green, blue, indigo, violet.

• Number seven likely chosen for cultural/symbolic reasons.

Newton on light

• White light = mixture of colors.

• Prism experiments: split light into spectrum; second prism bent colors differently.

• Key finding: prisms don’t add color; they separate existing colors.

Object Color

based wavelengths of light bouncing off of objects & into our eyes

Chromatic Colors (e.g. blue, green, red)

some wavelengths bounce off objects more than others

Achromatic Colors (e.g. white, grey, black)

different wavelengths of light bound equally off of an object

Selective Reflection

an object reflects some wavelengths of light more than others

creates chromatic colors

Reflectance Curves

percentage of light reflected at each wavelength for an object

Basic Physics of Light

• Transmission: Liquids, gases, and glass let light pass through.

• Selective Transmission: Only some wavelengths pass.

• Transmission Curve: Shows percentage of transmitted light at each wavelength.

Color Perception

based on wavelengths of light that reach our eye

recieve a mix of wavelengths vs a single one

Color Mixing

• Subtractive (paints): Pigments absorb certain wavelengths; reflected light determines color.

• Additive (lights): Lights combine wavelengths, creating new colors by addition.

Mixing Paints

based on which wavelengths are absorbed

take white light & subtract out the light absorbed by the pigments 

hence “subtractive color mixing”