Light provides humans with stereoscopic images of the world at all distances. This entry considers measuring light as it affects human vision. Humans are most sensitive to electromagnetic radiation in a small window between 400 and 700 nanometer (nm) wavelengths (λ). This response begins with four types of retinal receptors. After sitting in a light-free room for an hour, humans report seeing light with only four to six photons. Snow on a mountaintop sends to the eye 100 million times more photons. Although photographic film has a fixed, unique response to the number of photons/area, human visual appearance has a complex spatial relationship to the light on the retina. Nevertheless, light measurement in psychophysics is important: first, to accurately describe the display presented to observers, ...