The peak of the luminosity function is at 555 nm (green) the eye's image-forming visual system is more sensitive to light of this wavelength than any other. There is no single conversion factor between lux and W/m 2 there is a different conversion factor for every wavelength, and it is not possible to make a conversion unless one knows the spectral composition of the light. The lux is one lumen per square metre (lm/m 2), and the corresponding radiometric unit, which measures irradiance, is the watt per square metre (W/m 2). The weighting factor is known as the luminosity function. The difference between any photometric unit and its corresponding radiometric unit is that radiometric units are based on physical power, with all wavelengths being weighted equally, while photometric units take into account the fact that the human eye's image-forming visual system is more sensitive to some wavelengths than others, and accordingly every wavelength is given a different weight. Like all photometric units, the lux has a corresponding ' radiometric' unit. Relationship between illuminance and irradiance In practical lighting problems, given information on the way light is emitted from each source and the distance and geometry of the lighted area, a numerical calculation can be made of the illumination on a surface by adding the contributions of every point on every light source. For a point source, the illumination on the tilted surface is reduced by a factor equal to the cosine of the angle between a ray coming from the source and the normal to the surface. 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 000В lux (see Sunlight and Solar constant). The unobscured Sun provides an illumination of up to 100В 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 (Ојlx) 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 Дark limit of civil twilight under a clear sky ДЇamily 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 level of lux requires a greater number of lumens.ДЄs with other SI units, SI prefixes can be used, for example a kilolux (klx) is 1000В lx. To light a factory floor with dozens of times the area of the kitchen would require dozens of such fixtures. However, the same 1000В lumens spread out over 10 square metres produces a dimmer illuminance of only 100В lux.ДЄchieving an illuminance of 500В lx might be possible in a home kitchen with a single fluorescent light fixture with an output of 12 000В lumens. One lux is equal to one lumen per square metre:ДЄ flux of 1000В lumens, spread uniformly over an area of 1 square metre, lights up that square metre with an illuminance of 1000В 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 (measured in lumens) 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. 1.2 Relationship between illuminance and irradianceД®xplanation Illuminance.

12 hours dark moonlight