CS781 - Colour for Computer Graphics - Winter 2009

Lecture 12

Measuring Colour

What can be measured?

Power of light emitted from a source in all directions

• integrated with luminous intensity function
• called luminous intensity
• unit is candela

Power of light enitted from a source in a particular direction

• called luminous flux
• unit is lumen = candela per steradian
• need to talk about solid geometry

Luminous flux is a property of a ray of light. Remember

• Maxwell's equations
• wavefronts, ray direction
• the Eikonal approximation
• ray going through two holes

Power of light falling on a surface

• called illuminance
• unit is lumen per square metre

Power of light falling on a surface from a particular direction

• called luminance
• unit is lumen per square metre per steradian

Power of light leaving a surface

• called luminous exitance
• unit is lumen per square metre

Power of light leaving a surface in a particular direction

• called luminance
• Why?

Creating Colour: Self-Luminous Devices

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

Colour CRT

Monochrome CRT

What is it?

• evaluated tube
• electron beam
  • focussed
  • gated
  • directed
  • accelerated
• phosphor
  • back of faceplate
  • constant chromaticity: actually relative spectral power distribution
  • variable intensity

What light is emitted?

• f(V) * relative SPD
  • f(V): possibly non-linear function of voltage
  • voltage normally a linear function of stored value
  • f(V) most often set to V^(1/3). Why?
    BUT, if we measure intensity and plot log intensity against voltage
    • there is an intercept
    • Both slope and intercept are strongly affected by contrast and brightness controls
• artifacts
  • reflected ambient illumination
  • light scattered within faceplate
  • rise and fall time of amplifiers
• tristimulus values
• chromaticity coordinates

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
• Shadowmask
  • shape of a pixel

What light is emitted?

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

Creating Colour: Illuminated Devices

Half-tone Printing

Photography

Continuous-tone Printing

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