Lux is the base unit of measuring illumination in the International System of Units, whereas a lumen is a unit derived from lux that is used in measuring artificial light. One lux is equal to approximately one lumen per square meter of space. Lux is used to measure the intensity of light, and lumen is used to measure the amount of light. Lumens are roughly analogous to pounds or gallons.
Lux is more accurately described as the amount of luminous flux spread over a certain area. For example, if spread over about 1000 square meters, 1000 lumens hit each square meter with about 1 lux. If that same 1000 lumens is focused to one square meter, its intensity is equal to 1000 lux. In scientific equations and calculations, lux is represented by the lx symbol.
Measuring lumens is accomplished by measuring the amount of light visible to the human eye from a source, such as a lightbulb or the sun. Artificial forms of light, such as lamps or lightbulbs, are labelled by lumens and can have their intensity compared visually. This measurement replaced the measurement of lights using watts, which is a unit that measures energy, particularly in electrical measurements.