Lumen & PAR Meters
Lumens measure the intensity of light to the human eye, but not necessarily the spectrum of the light. Photosynthetically Active Radiation (PAR) meters measure light intensity within the spectrum that plants use most to photosynthesize (excluding UV light) from 400 to 700 nanometers, which covers both the blue and red spectra of light. Use these meters to measure how well your lights are covering your grow footprint and adjust your light setup accordingly.

Lumen meters are great for determining the brightness of visible light reaching your plants but doesn't give an account of all the available light energy that may be useful to your plants for photosynthesis since it gives more weight to yellow light over the other wavelengths. PAR meters are therefore needed to get a more accurate reading of total Photosynthetically Active Radiation (PAR) reaching the plant's canopy, this covers the same 400-to-700-nanometer range which is more or less in the range of visible light. Typically plants use blue wavelengths for vegetative growth while red wavelengths are especially needed during flowering and fruit production phases, but all wavelengths are known to contribute to your plants in different ways.