nonluminance

Nonluminance is a term used to describe the absence of light emission or light output from a body, surface, or region of space, typically defined as having negligible luminance below perceptual or instrumental thresholds. It is not a standard technical term in formal physics, where nonluminous or dark are more common descriptors, but it appears in discussions of optics, vision, and related fields to denote near-zero brightness.

In optics and vision science, luminance is a photometric quantity that measures perceived brightness per unit area. A nonluminous object would emit little to no light and thus produce near-zero luminance. Real-world objects usually emit or reflect some light, so true nonluminance is often treated as an idealization or a practical approximation in scenarios aiming to minimize brightness, such as in dark-field imaging or night-sky studies.

In astronomy and cosmology, nonluminous objects or phenomena are those that do not emit detectable electromagnetic radiation. Examples include certain theoretical or inaccessible components such as dark matter, or astrophysical bodies like black holes that do not produce light detectable by conventional means. The term helps distinguish between visible (luminous) and nonluminous sources in observations and models.

Applications and usage of the concept typically concern design choices that reduce light output, such as surface treatments that absorb rather than reflect light, or imaging conditions where background luminance must be minimized. The precise definition of nonluminance depends on context, instrument sensitivity, and ambient conditions, making it an approximate rather than universal standard.

See also: luminance, luminescence, albedo, reflectance, darkness, nonluminous.