Summary Slides

Steps for making a lighting design
- Analyse project
- Function of the space
- Requirements
o Visual (functional, safety brightness, contrast, glare)
o Non-image forming (alertness, biological clock)
o Architectural
o Emotional, psychological (mood, performance)
- Make a lighting concept. Decide on:
o Light sources
Spectrum (CCT, CRI), Colour temperature, Colour rendering index, Luminous flux
(lm), Efficacy (lm/W), Luminous intensity (cd)
o Luminaires
o Controls
o Position of the light sources
o Light distribution
o Reflection factors
o Illuminance (E, lx)
o Luminance (L, cd/m2)

Colorimetry
Colorimetry is to compute whether two colours “are” the same.

Colour matching is a test in which the participant is asked to adjusts intensity of primaries to find a
match with the test light. It shows that humans are trichromats.

For three fixed primaries we can adjust the intensities until we find a match:
C ~ r I r  g I g  b Ib
Sometimes we need less than zero Intensity of a light, say Ir . In that case we can find a match by
adding it to the other half

C  r' I r ~ g I g  b Ib

We can write both equations as one by setting r  r '
Doubling the intensities or switching between sides of the screen does not change the equation. The
colours change but they stay matched.

A point in colour space: C = (R, G, B). The coordinates reflect amounts of light needed to match a
colour C with three primary lights. We can compute the luminance from this L = R+G+B (inaccurate)

R G B
r g b  1 r  g
Chromaticity: RG  B , RG B , RG B
The saturation/chroma is the distance from white, the hue is the angular direction of the line from
white.

We can think of the chromaticities as trilinear coordinates c = r*R + g*G + b*B

, CIE RGB Colour Space
Sensitivity * intensity level at that wavelength = colour value

R   r ( ) I ( )d G   g ( ) I ( )d B   b ( ) I ( )d
Approximate integral:

R   r ( ) I ( )d   r (i ) I (i )
i

Monochromatic beams form a curve in rgb-space. The projection on the rg-plane forms the edge of
the chromaticity diagram. The curve has a gap – the purples. All real colours lie inside the curve.
Colours outside the curve are not physically realisable

CIE XYZ Colour Space
The CIE defined primaries X,Y,Z with imaginary colours (Cr, Cg, Cb in previous slide) such that:

The colour matching functions of X,Y,Z are always positive, X and Z have zero luminance, Y is
luminance. The colour matching function of Y is equal to V(l). The line Y=1-X is tangent to the red
monochromatic colours.

The CIE diagram is peculiar for the following reasons:

- It represents colour matching functions for physically non-existing light beams
- It is a projective space; shape, size and area do not have any meaning, projective invariants
are not used at all.
- The projection is very skewed and prevents understanding for novices

Colour Quality
In general, changing the illuminant changes the colours of objects. Even if two light source are the
same white (usually specified by colour temperature) objects look different due to metamerism.

Physics of light
There at least 3 different models for describing how light interacts with matter (and itself)

- Light as waves: Electro-magnetic radiation
o Intensity -> this gives light brightness
o Wavelength -> related to colour of light
o Interference
o Dispersion
o Absorption -> this gives colour to objects
- Light as rays
o optics
o refraction
o reflection
- Light as particles
o scattering -> how light illuminates and colours the environment
o pressure

Atoms and molecules absorb specific photon energies depending on their electron structure.
Absorption/Transmission spectra are important for analysing the chemical composition of a
substance