Photometry is the measurement of visible light as perceived by the human eye. Photometry is a name formed by combining the words photo and metry. Photo is a prefix meaning light. Metry refers to the process of measurement.
Light measuring devices called lux meters can measure with high precision in a wide measuring range. But when it comes to LEDs things get a little more complicated. Different brands and models of lux meters may take different measurements at the same point.