Figure 1.

An overview of the stimulus (top) response (bottom) relationship between light and human responses, with a schematic subdivision of the visual and non-visual responses. At the top level, the temporal pattern relates the timing and duration of exposure to a light stimulus, spatial pattern refers to the spatial distribution of light in the three-dimensional light field, light spectrum refers to the spectral power distribution (SPD) that governs color qualities, and light level refers to the quantity of light in radiometric or photometric units. These four factors contribute to the biological potency of the light stimulus. Designers often vary factors together, though any of the factors can be disengaged from any other. Researchers usually vary just a small number of factors, sometimes only one, to isolate cause and effect. Non-lighting factors not shown such as age and chronotype moderate the effects of light on people and are important in practice. This figure only considers the effects of light through the eyes; the effects of optical radiation on or through the skin are not considered here. Figure inspired by de Kort and Veitch (17).