Researchers and growers use Photosynthetic Photon Flux Density, better known as PPFD, to measure the photons that are actually hitting the plant. PPFD measures how many photons are hitting an area per second. PPFD is reported in micromoles per meter squared per second, or umol/m2/s, by using a quantum sensor to a spot measurement.
μmol/Joule is the unit of measurement for photon efficacy. It is important because it tells you how efficiently a grow light fixture is converting electrical energy into photons. It is a strong indicator of energy efficiency.