Lighting a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square metre (cd/m). The CGS unit of luminance is the stilb, which is equal to one candela per square centimetre or 10 kcd/m. The amount of useful light emitted from a source or the luminous flux is measured in lumen (lm). The SI unit of illuminance and luminous emittance, being the luminous power per area, is measured in Lux. It is used in photometry as a measure of the intensity, as perceived by the human eye, of light that hits or passes
Jordan measure rational numbers contained in the interval [0,1] is then not Jordan measurable, as its boundary is [0,1] which is not of Jordan measure zero. Intuitively however, the set of rational numbers is a "small" set, as it is countable, and it should have "size" zero. That is indeed true, but only if one replaces the Jordan measure with the Lebesgue measure. The Lebesgue measure of a set is the same as its Jordan measure as long as that set has a Jordan measure. However, the Lebesgue measure is defined for a much wider class of sets, like the set of