A lux meter is an instrument that measures illuminance at a surface in lux. For meaningful comparisons, the sensor position, orientation, distance, ambient conditions, and measurement method must be consistent and documented. The meter’s spectral response, cosine correction, range, calibration, and condition influence accuracy. Stray light and shadows from the person taking the measurement can also affect the result. For a machine or site assessment, measurements should be taken at predefined grid points and recorded together with lamp configuration, aiming, supply voltage, and environmental conditions.