Almost all the energy on the earth’s surface comes directly or indirectly from the sun. Plants convert light energy from the sun into biologically-useful forms through photosynthesis. We call this light energy photosynthetically active radiation (PAR). Historically, measuring PAR was somewhat subjective because scientists disagreed about the appropriate wavelength range and measurement units. Also, the only instruments readily available were designed and calibrated to match the spectral sensitivity of the human eye, which is very different from the spectral sensitivity of plant photosynthesis. This means light sources with equal measured intensities but different spectral outputs would drive photosynthesis with different efficiencies, making comparisons difficult when different lighting conditions were used.
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