Fundamentals of Colour and Light
This section covers the Fundamentals of Colour and Light portion of the Colour Photography vocational syllabus. It tests the physics of the visible spectrum, additive and subtractive colour, the attributes of colour, colour temperature and simple light-fall-off calculations.
Core rules
- Visible light is roughly 380–700 nm; violet is short wavelength, red is long wavelength.
- Additive primaries are red, green and blue. R+G = yellow; G+B = cyan; R+B = magenta; all three = white.
- Subtractive primaries are cyan, magenta and yellow, used in inks, dyes and photographic paper. CMY are the complements of RGB.
- Hue is the colour name; saturation is purity; lightness or value is brightness.
- Colour temperature in kelvin: candle about 1800 K, tungsten about 3200 K, daylight about 5500 K, open shade about 7000–8000 K. Lower kelvin is warmer.
- Inverse-square law: doubling distance from a point source gives one-quarter illuminance, two stops less.
- Metamerism: two samples match under one light and mismatch under another because their spectral power distributions differ.
Worked example
A candidate mixes red and green light and gets yellow, then mixes red and green paint and gets a dark brown. Reasoning: lights add energy at wavelengths the eye combines to yellow. Paints subtract: red paint absorbs most green and blue; green paint absorbs red and blue; what remains is low-intensity yellow-brown. The result is not a brighter yellow because pigments remove light rather than add it.
Traps
- Treating RGB and CMY as the same primaries.
- Saying a white object has no colour; it reflects all visible wavelengths.
- Confusing colour temperature with exposure: 3200 K tungsten is not dimmer than 5500 K daylight.
- Forgetting that the inverse-square law applies to point sources, not large softboxes at close range.