- The subjective impression produced by stimulating the retina is sometimes designated as light. Visual sensations are sometimes arbitrarily defined as sensations of light, and in line with this concept it is sometimes said that light cannot exist until an eye has been stimulated. Electrical stimulation of the retina or the visual cortex is described as producing flashes of light. In illuminating engineering, however, light is a physical entity — radiant energy weighted by the luminous efficiency function. (See spectral luminous efficiency of radiant flux and values of spectral luminous efficiency for photopic vision.) It is a physical stimulus that can be applied to the retina.
- Symbols for radiometric terms are identified by the subscript e, and those for photometric (luminous) terms by the subscript v. In the event that no confusion will occur, the subscripts may be omitted. A table summarizing the units, Symbols, and defining equations is given in Table T.2.