- A spectral colour is a hue evoked in normal (trichromatic) human vision by a single wavelength of visible light, or by a narrow band of adjacent wavelengths. This is how we see colour in normal viewing conditions such as sunlight or artificial light.
- Sunshine is a typical case in so far as it contains nearly all spectral hues.
- Sunshine has a very broad spectral power distribution and as light strikes surfaces, some wavelengths are absorbed, and others are reflected. The mix of reflected wavelengths results in the non-spectral colours we see.
- A non-spectral colour is any hue that cannot be produced by light of a single wavelength or that does not occur in the visible spectrum (e.g. Magenta).
- Spectral hues are commonly associated with rainbows, prisms and diagrams showing the colours of the visible spectrum (red to violet). But any medium that reflects or transmits a single wavelength of light produces a spectral colour.
- Spectral colours can be mimicked by RGB colour. This involves mixing coloured lights tuned to the three spectral primaries, red, green and blue. The resulting colours are not spectral colours because of the resulting mix of wavelengths.
- The intensity, which is to say, the brightness, of a spectral hue viewed by an observer may alter considerably depending on the situation. For example, a low-intensity orange-yellow may appear brown, and a low-intensity yellow-green may appear olive-green.
The human eye and spectral colour
- The human eye, and so human perception, is tuned to the visible spectrum and so to all wavelengths that correspond with spectral colours between red and violet.
- The visible spectrum is the range of wavelengths of the electromagnetic spectrum that correspond with all the different colours we see in the world. We do not see colour when exposed to wavelengths of infrared or ultraviolet light.
- A spectral colour is a hue corresponding with a single wavelength of visible light, or with a narrow band of adjacent wavelengths.
- In other situations, spectral hues are called pure hues or monochromatic hues.
- Spectral colours include all the pure hues associated with a rainbow so are sometimes called rainbow colours.
- Rainbow colours include red, orange, yellow, green, blue and violet but the human eye can distinguish many thousands of other spectral colours between each of these.
- In a continuous spectrum of wavelengths, separate hues are indistinguishable to the human eye.
- The fact that we see the distinct bands of colour in a rainbow is an artefact of human colour vision.
- Spectral colours can be mimicked by RGB colour. This involves mixing coloured lights tuned to the three spectral primaries, red, green and blue.