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." I bring this all up to make two points: (a) SI does *not* cover all necessary uses of measurement systems, so there are going to be new units "invented," although we hope they are based on SI (as nit is) and (b) people inherently like short words, which I think is one thing that turns people off about the metric system (e.g., inch vs. centimeter). I doubt anything can be done about (b), but I bet that words like "klick" (for kilometer) are not going to be eradicated even in all-metric countries. Jim Elwell
