Jim Elwell wrote in USMA 18773: >Engineers who work with human-readable displays are familiar with the unit >"nit", which is one candela per square meter (cd/m^2). It is used to >specify how bright a display is (either directly for some types, or via >backlighting for others). For example, a 150-nit display will be readable >in roomlight, but washed out in sunlight, whereas a 600-nit display will be >readable in sunlight, but too bright for viewing in a darkened room. > >The unit "nit" is used for two reasons: displays are a distributed light >source rather than a point source (so lux is not appropriate), and, used >properly, it is a photonic unit, which means it takes into account the >response of the human eye.
>There clearly is no directly equivalent SI unit here, due to the photonic >nature. But I think engineers use it for another reason: it is a short, one >syllable word. It is a heck of a lot easier than saying "photonic-weighted >candela per square meter." The lux (lx) is not limited to point sources. It measures the light arriving at a unit area of surface from all directions and is called illuminance, equal to lumens per square metre (lx or lm/m2). The intensity of the light emitted by unit projected area of a surface in a certain direction is measured in candela per square metre (cd/m2) and is called luminance. In the metric bible the candela is listed as an SI base unit and the lux is listed as an SI derived unit. Luminance (cd/m2) is listed in the Canadian Metric Practice Guide. Both these units are "photonic". The corresponding radiant energy units are irradiance (W/m2) and radiance (W/sr.m2) Joseph B.Reid 17 Glebe Road West Toronto M5P 1C8 TEL. 416-486-6071
