VSP Lecture 4 Color Vision Part 3a

the attributes of a color were described as

hue

saturation

brightness

Munsell Color System

provides a system to accurately determine 'what color blue is it'

involves describing colors based on 3 attributes: hue, chroma, value

Hue

related to wavelength

Chroma

similar to solorimetruc purity (saturation)

Value

related to brightness (reflectance)

1. Hue

wavelength of the color

Primary hies

Red

Yellow

Green

blue

Purple

intermediate cues

fall in between primary cues

2. Chroma

saturation

repressented as a radius from the center

Chroma ranges

1-14

14 = highest purity (no white)

1 - lots of white (desaturated)

3. Value

runs along the vertical dimension

10= most bright (most reflectance)

0= least bright (least reflectance)

Munsell Color System Practical Application

- geology

- food industry

- health of animals and pets

- dentistry

The perceptual attribute of color know as saturation is similar to what erm used in the Munsell system

Chroma

CIE Color system stands for

Commission Internaltionale de 'Echlairage

CIE is ______ than the Munsell System

precise

CIE parameters based on

spectral power distribution of light from a colored object

and by sensitivity curve that have been measured from the human eye

CIE considers the amounts of

3 primarues ( R, G, B) needed to match a monochromatic test primary light

(based on trichromatic theory)

CIE Color Matching functions

the actual amounts of each of the three primary colors R. G, B that are needed to match a monochromatic test light

From 450 to 550nm

negative amounts of a primary color are needed to match a wavelength in this region

would need to add red to both sides of bipartise field

To avoid the negative quantities of primaries

mathematical transformation of real primaryes

CIE primaries are not real!

R, G, and B become

3 imagined primaries

X, Y, Z

Imaginary Primaries (Tristimulus Values)

mathematically transformed real primaries that serve to locate primary reference points on a CIE chromaticity digram

Can now match all wavelengths

no negatives!

This creates a 3D diagram where X, Y, and Z

are not actually insid this solid shape because they are imaginary colors

CIE chromaticity Diagram provides

the amount of each imaginary primary (X, Y, and Z) needed to match any real color

R,G,B ---> X, Y, Z --->

Chromaticity coordinates (x, y, z)

Converting Tristimulus Values to Chromaticity values

x = X/ (X + Y + Z)

y = Y / (X + Y + Z)

z = Z / (X + Y + Z)

where x + y + z = 1

If only x and y axes are shown we can find z by

1 - (x+y) = z

The CIE 1931 Chromaticity diagram represents perceptual attributes of hue and saturation separated from luminance

All physically realizable colors are in the CIE diagram but

X, Y, Z are not in the diagram because they are not real !!!!!