Lumenradius

Lumenradius is a term that appears occasionally in discussions of optics and photometry, but it is not part of the formal SI terminology. It is not universally defined, and its exact meaning can vary between authors. In general, lumenradius is used informally to refer to the effective radius of a circular light-emitting area or aperture that would produce a specified amount of luminous flux or illuminance under particular emission conditions.

One common informal interpretation ties lumenradius to a circular emitter with uniform luminance. If the emitting surface has a uniform luminance L and radius r, the luminous flux generated by the surface increases with its area as the circle grows. Under this simplifying assumption, larger radii produce proportionally more flux, and the lumenradius for a target flux is the radius that yields the desired output for the given luminance. Practitioners often emphasize that the exact relationship depends on the emission model (for example, whether luminance is uniform, Lambertian, or follows another distribution) and on measurement geometry.

Because lumenradius is not standardized, it is generally preferable to specify the quantities directly—such as the target luminous flux (in lumens), the luminance or radiance (in candelas per square meter), and the emission geometry—rather than relying on an informal radius. When used, lumenradius should be defined explicitly in the context of a particular study or product specification to avoid ambiguity.

See also: lumen, luminous flux, luminance, radiance, photometry, illuminance, aperture radius.