A lux meter is a device for measuring brightness, specifically, the intensity with which the brightness appears to the human eye. This is different than measurements of the actual light energy produced by or reflected from an object or light source. The lux is a unit of measurement of brightness, or more accurately, illuminance. It ultimately derives from the candela, the standard unit of measurement for the power of light.
A lux meter works by using a photo cell to capture light. The meter then converts this light to an electrical current, and measuring this current allows the device to calculate the lux value of the light it captured.