The normalized quantum efficiency (RQE) curve that shows the relative photosynthetic response to light of the average photosynthesizing plant – originally published by Mcree (1972a, 1972b), replicated by Inada (1976, 1978a, 1978b), and subsequently refined by Sager et al. (1982, 1988) — was used as the basis in developing the phytometric system, a new and more flexible concept of light measurement for plants. Based on the convolution of the RQE curve with the spectral power distribution (SPD) of a given light source, the phytometric measurement would yield units of phytoWm−2. The unit phytoW easily provides conversion factors to the radiometric, photometric, and photon flux (quantum) systems within the photosynthetically active radiation (PAR) of 400 to 700 nm or within the extended PAR of 300 to 800 nm. Indeed, a calculated phytometric value would provide a more accurate measure of photosynthetic photon flux (PPF) or yield photon flux (YPF). The use of the phytometric system was demonstrated by applying it to four types of high-intensity discharge (HID) lamp. Pertinent conversion factors were calculated for each lamp type.