2018 ASHS Annual Conference

Photosynthesis is driven by photosynthetically active radiation (PAR), which is almost universally defined as photosynthetic photon flux density (PPFD, the integral of photon flux density in units of mmol m^-2 s^-1 from 400 to 700 nm). Accurate measurement of PPFD is essential because of the close relationship between photosynthesis and plant growth. Quantum sensors (so called because a photon is a single quantum of radiation) are the most common device used for PPFD measurement because they are simple and relatively low cost. Quantum sensor accuracy is determined by both spectral response and directional response. Spectral response is the sensitivity to photons at each wavelength, or a measure of how well a sensor matches the defined plant response (equal weighting to photons between 400 and 700 nm, no weight outside this range). Directional response is the sensitivity to radiation incident at different angles, or a measure of how well a sensor matches Lambert’s cosine law (radiation intensity is proportional to the cosine of 90-minus-the-angle between the incident radiation beam and a horizontal surface). Here we compare spectral and directional errors for seven quantum sensors/meters: Apogee original (model SQ-100) and full spectrum (model SQ-500); Kipp & Zonen (model PQS 1); LI-COR original (model LI-190) and improved (model LI-190R); Spectrum LightScout; and Hydrofarm Quantum PAR Meter. Spectral and directional errors were less than 5 % for the Apogee full spectrum, Kipp & Zonen PQS 1, and both LI-COR models. The Apogee original and Spectrum LightScout use photodetectors that are not sensitive to photons with wavelengths greater than about 660 nm, which can cause significant spectral errors under some radiation sources. These errors are partially accounted for by calibrating to specific radiation sources (as is the case for the Apogee SQ-100), but these sensors should be used with caution when measuring deep-red LEDs or other electric lights with significant radiation at wavelengths greater than 660 nm. The Hydrofarm Quantum PAR meter was found to have directional errors of up to 20 % in the diffuse light that is characteristic of growth chambers, with larger errors in low-angle directional light. This meter should be used with caution in commercial and research applications.