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This direct normal illuminance is related to the solar illuminance constant E sc, equal to 128 000 lux (see Sunlight and Solar constant). The illuminance on a surface depends on how the surface is tilted with respect to the source.
The solar illuminance constant (E sc), is equal to 128×10 3 lux. The direct normal illuminance (E dn), corrected for the attenuating effects of the atmosphere is given by: =, where c is the atmospheric extinction and m is the relative optical airmass. The atmospheric extinction brings the number of lux down to around 100,000 lux.
Daylight is the combination of all direct and indirect sunlight during the daytime. This includes direct sunlight, diffuse sky radiation, and (often) both of these reflected by Earth and terrestrial objects, like landforms and buildings. Sunlight scattered or reflected by astronomical objects is generally not considered daylight.
In SI units illuminance is measured in lux (lx), or equivalently in lumens per square metre (lm·m −2). [2] Luminous exitance is measured in lm·m −2 only, not lux. [4] In the CGS system, the unit of illuminance is the phot, which is equal to 10 000 lux. The foot-candle is a non-metric unit of illuminance that is used in photography. [5]
Illuminance. To help compare different orders of magnitude, the following list describes various source in lux, which is a lumen per square metre. Factor
Useful daylight illuminance focuses on the direct sunlight that falls into a space. The useful daylight illuminance calculation is based on three factors—the percentage of time a point is below, between, or above an illuminance value. The range for these factors is typically 100–2,000 lux.
Illuminance. Illuminance is a measure of how much luminous flux is spread over a given area. One can think of luminous flux (with the unit lumen) as a measure of the total "amount" of visible light present, and the illuminance as a measure of the intensity of illumination on a surface. A given amount of light will illuminate a surface more ...
The foot-candle is defined as the illuminance of the inside surface of a one-foot-radius sphere with a point source of one candela at its center. Alternatively, it can be defined as the illuminance of one lumen on a one-square foot surface with a uniform distribution. Given the relation between candela and lumen, the two definitions listed are ...