Light Measurement

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, ...