Lumen meters are inexpensive and great for determining the brightness of visible light reaching your plants. They don't, however, take into account all the available light energy that may promote photosynthesis. That’s because they give more weight to yellow light over other wavelengths, and all wavelengths contribute to your plants in different ways.
PAR meters cost more but present a more accurate reading of total Photosynthetically Active Radiation (PAR) at the plants’ canopy. This includes light in the 400- to 700-nanometer range, more or less in the range of visible light, including blue and red. Plants typically use blue wavelengths for vegetative growth and red during flowering and fruit production. Whatever type of light you use—fluorescent, MH, CMH, or the newer adjustable, more efficient LED lights—having a more precise reading leads to more precise lighting, including supplemental illumination.