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Lux (lx) is the International System of Units (SI) derived unit that describes the measurement of illuminance, the total luminous incident on a surface as described in photometry. Lux means lumens per square meter (lm/m2 or cd sr m-2) and is colloquially referred to as brightness, which is then often confused with luminance. Both are similar, as they are both a measure of light, but lux takes into consideration the area while lumen does not.
Lux is a unit of measurement for light output in a given area (generally in square meters), or simply referred to as light intensity. This is a very appropriate unit for measuring the brightness of a beam of light where light is concentrated in a smaller area such as those in lighthouses, searchlights or flashlights. If the light is concentrated in a small area, one would perceive this as bright, but if the same amount is spread over a wide area then it is seen as dim. In this context, a searchlight with a beam concentrated in a small area that is perceived to be very bright and can travel long distances is considered as high lux, while a low-lux device would have light that is perceived to travel shorter distances but have a larger footprint. Even though both devices might be emitting the same amount of light, the beam is perceived to be brighter because it is more concentrated.
In essence, lux is the measure of the concentration of light emitted in an area. An object with a high lux output is considered bright from farther away because the light arriving to the viewer is more concentrated, such as the case with a flashlight, while a low-lux device such as a lightbulb is dimmer and smaller because only a small portion of the light output is reaching the viewer's eyes.