cs781 - Colour for Computer Graphics - Winter 2012

Course Notes

Lecture 6 - Colour Measurement

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Geometry of Light

Point sources of light

From far enough away every source of light is a point source

Consider a point source of light,


In any unit of time the same amount of light passes through every sphere

Power density of light

The area of the surface of a sphere is (4 pi ) r^2.

Power passing from one area to another

Consider an emitting area, dA1,as an array of point radiators

The receiving area, dA2, is r^2 dw

Multi-pole expansion

Units in which Light is Measured

Considering wavelength, power emiited by a point source,, P_l, is watts per unit wavelength

From a distance the amount of energy captured depends on the size of the detector

Normally we are measuring an area and want to know the emission per unit area of the surface

We often want to measure the amount of light falling on a surface, which we call illuminance, because the surface is being illuminated.

Similarly, we have a name for the amount of light emitted by the surface, luminous emittance.

Important ideas

  1. Luminous versus radiant
  2. Solid angle, steradian, and its relationship to area
  3. Dualism between incoming and outgoing light.

Colour Measurement

Two types of measurements

  1. Visual measurement
  2. Instrumental measurement.
    There are two things that we can measure
    1. psychophysical response to light
      • reproduce and improve on visual measurement
      • using filters and detectors to do optical integration
    2. physical properties of light
      • use results of psychophysical experiments
        • such as colour matching functions

        to calculate psychophysical response from physical measurements

Measuring physical properties

Almost always energy in the past,

Ultimate calibration is to heat

You now have a detector and a calibration.

Measuring a spectral power distribution

In principle, it is straightforward

  1. Split the light into a spectrum using
  2. Spectrum can be spread out in
  3. Get a stream of measurements

Current technology uses an array of detectors, but

Two aspects of calibration are hard

If an instrument is inexpensive they most likely skimped on calibration.

What can be measured?

Power of light emitted from a source in all directions

Power of light enitted from a source in a particular direction

Power of light falling on a surface

Power of light falling on a surface from a particular direction

Power of light leaving a surface

Power of light leaving a surface in a particular direction

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