When a surface is tilted at an angle to a source, the illumination provided on the surface is reduced because the tilted surface subtends a smaller solid angle from the source, and therefore it receives less light. For example, a pocket flashlight aimed at a wall will produce a given level of illumination if aimed perpendicular to the wall, but if the flashlight is aimed at increasing angles to the perpendicular (maintaining the same distance), the illuminated spot becomes larger and so is less highly illuminated. The illuminance on a surface depends on how the surface is tilted with respect to the source.

This direct normal illuminance is related to the solar illuminance constant E sc, equal to 128 000 lux (see Sunlight and Solar constant). The unobscured Sun provides an illumination of up to 100 kilolux (klx) on the Earth's surface, the exact value depending on time of year and atmospheric conditions.

A barely perceptible magnitude 6 star provides 8 nanolux (nlx). For instance, a star of apparent magnitude 0 provides 2.08 microlux (μlx) at the Earth's surface. The illuminance provided by a light source on a surface perpendicular to the direction to the source is a measure of the strength of that source as perceived from that location. Office building hallway/ toilet lighting Moonless clear night sky with airglow ĭark limit of civil twilight under a clear sky įamily living room lights (Australia, 1998) Moonless, overcast night sky ( starlight) Here are some examples of the illuminance provided under various conditions: Thus, lighting a larger area to the same illuminance (lux) requires a greater luminous flux (lumen).Īs with other named SI units, SI prefixes can be used. To light a factory floor with dozens of times the area of the kitchen would require dozens of such fixtures. However, the same 1000 lumens spread out over 10 square metres produces a dimmer illuminance of only 100 lux.Īchieving an illuminance of 500 lx might be possible in a home kitchen with a single fluorescent light fixture with an output of 12 000 lumens. One lux is equal to one lumen per square metre:Ī flux of 1000 lumens, spread uniformly over an area of 1 square metre, lights up that square metre with an illuminance of 1000 lux. A given amount of light will illuminate a surface more dimly if it is spread over a larger area, so illuminance is inversely proportional to area when the luminous flux is held constant. 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.

Illuminance is a measure of how much luminous flux is spread over a given area. The word is derived from the Latin word for 'light', lux.Įxplanation Illuminance In English, 'lux' is used as both the singular and plural form. It is analogous to the radiometric unit watt per square metre, but with the power at each wavelength weighted according to the luminosity function, a model of human visual brightness perception, standardized by the CIE and ISO. In photometry, this is used as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface. It is equal to one lumen per square metre. The lux (symbol: lx) is the unit of illuminance, or luminous flux per unit area, in the International System of Units (SI).

Equation of light intensity