# Lecture 17 - Colour Devices: Subtractive Colour

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# Subtractive Colour

Subtractive colour is defined accord to its mechanism of production. Two examples.

1. Transmission (If you prefer mathematics that looks like mathematics see this.)
• Transmittance of a translucent filter is defined to be
• T(\lambda) = I(\lambda) / \Phi(\lambda), where
• I(\lambda) is the light entering the medium.
• \Phi(\lambda) is the light exiting the medium
• Density is a useful concept
• Show the relationship between optical density, absorption and transmittance
2. Reflection
• Show the similarity between the mathematics of transmission and the mathematics of reflection

It's called subtractive colour

• because light is removed from the incoming light

### Characteristics of Subtractive Colour

• Secondaries are darker than primaries
• "A mixture of colours is darker than either of the mixed colours," is the generalization.
• Saturated colours are darker than desaturated colours
• the same as subtractive colour

### Calibrating Subtractive Colour

Combined with additive mixture (e.g. half-tone printing) model-based calibration seems possible, but ...

1. detailed models depend strongly on
• ink colour
• paper
• screen pattern
• screen rotation
• lighting
• etc.
2. approximate models, such as
• random coverage

## Half-tone Printing

### Laser Printer

process black versus separate black screen

## Commercial Press

1. Solid colour,
• 96 dots per mm
• compare monitor, 4 dpmm
• laser printer, early, 10 dpmm
• laser printer, modern, 24 dpmm
• serifs lightly curved
2. Originally printed in solid colour,
• three inks: red, yellow, black

Reproduced in half-tone (Why?)

• look at the type through the loupe
• and with the naked eye
3. B&W, half-tone colour
• look at the eyes: through the loupe, naked eye
4. Cloth in half-tone
• competing patterns at comparable scales
5. Compare to 3.
• these half-tones were done by a human, not a camera
• look at the eyes
• look at the edge of the blue jacket

## Continuous-tone Printing

Continuous tone is what you have with most additive devices

• half-tone:
• only colour or no colour
• intermediate colours average colour areas with no colour areas, so they are not uniform
• pseudo-pixels must be bigger than the resolution of the device
• continuous-tone
• `continuous' range of colours
• no averaging needed
• pseudo-pixels need be only as big as the number of colour channels

Examples

• rotogravure
• dye sublimation
• ink-jet