A method of sensing magnitude of lumen depreciation not only of a lamp of a lighting fixture, but other factors, by sensing a relative light level external of the lighting fixture to derive not only lamp lumen depreciation but total lumen depreciation from the fixture. An apparatus of monitoring light level of one or more lighting fixtures comprises a sensor positioned external of the lighting fixture(s), a comparator adapted to compare a measured actual lumen output signal from the sensor to a pre-set reference or threshold lumen output value. An error signal is generated by the comparator if the comparison indicates actual lumen output to have dropped below the reference or threshold lumen output. An alarm or communication can be actuated by the error signal to instruct corrective action to ensure relatively constant light levels to be maintained for the lighting system over time, thus providing ongoing assurance to the end user of proper system operation.