# Visual Response to Light

## Metamerism

#### Light Metamerism

Many to one transformation

Evolution says:

• Colour differences that matter are likely to be discriminable
• This is a statement about surfaces
• The colour of an object is the colour of its surface
• Note the difference between transparence/translucence and whiteness/opacity

#### Illuminant Metamerism

Hold the surface constant

#### Surface Matemerism

Hold the illuminant constant

# Geometric Representations of Colour

## Special Properties of the Spectral Colours

Colours in the rainbow/spectrum = Colours of monochromatic lights

• call them spectral colours

Mix a spectral colour and white

• Move along a straight line in colour space
• Why straight?
• As white decreases we get to a point where we can't go any farther

Mix the same spectral colour with any other colour

• Arrive at the same point

Remove a little of the spectral colour

• Do the same.
• Arrive at a point joining the first point to the origin

Choose another spectral colour

• In fact, just use the spectral colours to do this for all spectral colours
• The result is a curve that delimits a cone

## Convex Hull of the Spectral Colours

This is the set of physically realisable colours

• The purple line

## Chromaticity Coordinates

The two meanings of `colour' when we say, `the same colour.'

1. Unique hue, saturation and brightness
• Note ambiguity of `bright'
• Intensity might be better than brightness
2. Unique hue and saturation

Remember (?) projective geometry

• If we treat all points on a line through the origin as a single point,
• on a sphere for example
• we get a projective space of one lower dimension
• Attractive because straight lines go to straight lines

Consider the projection

• x = X / (X + Y + Z)

y = Y / (X + Y + Z)

z = Z / (X + Y + Z)

• Then x + y + z = 1
• We are projecting onto the (1,1,1) plane.
• Plot (x,y)
• We are projecting the (1,1,1) plane to the (1,1,0) plane by dropping the z-coordinate

The resulting planar representation of colour is called `chromaticity coordinates'

• The chromaticity of a colour is its hue and saturation with brightness ignored
• Because additive mixture of two colours in colour space is convex combination
• additive mixture of two colours in chromaticity coordinates is convex combination

It is pretty well impossible to make a true colour picture of the chromaticity diagram

• Why?

### Concepts from chromaticity coordinates

#### Dominant hue

• also dominant wavelength
• relative to a white point

#### Excitation purity

• also relative to a white point

#### Colour Temperature

• the black body curve