Light reaching the human eye is called a light stimulus because it stimulates the visual system.
- Sometimes the term colour stimulus is used because the light stimulus produces the experience of colour for an observer. Every light stimulus can be described in terms of the composition and intensity of wavelengths of light that enter the eye.
- Imagine, that at a specific moment, the wavelengths of a stimulus include reds, oranges, yellows, greens, blues and violets. The colour an observer sees depends on the response not only to the mixture of wavelengths that correspond with these colours but also to the intensity of the light at each wavelength.
- In many situations, intensity varies progressively across a range of wavelengths and can be described by the spectral power distribution of the stimulus.