# Creating Colour: Self-Luminous Devices

Self-luminous' means producing light without an external illuminant.

## Colour CRT

#### Colour CRT

What is it?

• Three monochrome CRTs
• explicitly true for projectors
• old projectors
• new projectors
• three in one glass envelope for ordinary CRTs
• three is a commitment made very early in the process
• basis of hardware design
• basis of television standards
• shape of a pixel

Usability tests

• monochrome CRT -> colour CRT -> MPEG
• how little we have progressed in seventy years

## Calibrating a Colour CRT

Calibration

• putting a device into a standard configuration

Characterization

• having a reliable mapping from device coordinates to standard colour coordinates (tristimulus coordinates)
• ideally one-to-one
• which allows give me colour X
• could be the basis for broadcast applications

### Modelling a Colour CRT

#### The abstraction

1. Three phosphors' emitting light of constant chromaticity.
2. Three guns', each of which controls the intensity of light emitted by one of the phosphors
3. Pixels are independent of one another
4. Light output from phosphors depends only on the input to a single gun.
5. Display surface has uniform characteristics
6. Light output is stable over time

#### The colour gamut

The colour cube

The colour cube in colour space

The colour cube in chromaticity coordinates

#### Emitted colour

Emitted light

• \Phi(R,G,B, \lambda) = \sum_{i=R,G,B} \phi_i(\lambda) f(i)
• R,G,B are the primaries' of the device

Tristimulus values

• For (j=X,Y,X) j(R,G,B) = \sum_{i=R,G,B} f(i) \sum_\lambda \phi_i(\lambda){\bar x}_j(\lambda)
• j(R,G,B) = \sum_{i=R,G,B} f(i) a_{ij}
• where a_{ij} = \sum_\lambda \phi_i(\lambda){\bar x}_j(\lambda)
• To get the tristimulus values we need to know
1. a_{ij}: the chromaticity coordinates of the phosphors
2. f(i): the non-linear responses of the primary intensities to the input

### Measuring the Colour of CRT Output

Geometry

Problems

• temporal stability
• spatial variation

## Colour LCD

Technological basis

• backlight
• polarizers
• liquid crystal between the plates of a capacitor
• colour filters

What if we had four filters?

• Why? We are surely overdoing the blue

## Colour OLED

Solid state photodetectors and LEDs are very closely related