ACCURACY OF QUANTUM SENSORS MEASURING YIELD PHOTON FLUX AND PHOTOSYNTHETIC PHOTON FLUX

Photosynthesis is fundamentally driven by photon flux rather than ener gy flux, but not all absorbed photons yield equal amounts of photosynt hesis. Thus, two measures of photosynthetically active radiation have emerged: photosynthetic photon flux (PPF), which values all photons fr om 400 to 700 nm equally, and yield photon flux (YPF), which weights p hotons in the range from 360 to 760 nm according to plant photosynthet ic response. We selected seven common radiation sources and measured Y PF and PPF from each source with a spectroradiometer. We then compared these measurements with measurements from three quantum sensors desig ned to measure YPF, and from six quantum sensors designed to measure P PF. There were few differences among sensors within a group (usually < 5%), but YPF values from sensors were consistently lower (3% to 20%) t han YPF values calculated from spectroradiometric measurements. Quantu m sensor measurements of PPF also were consistently lower than PPF val ues calculated from spectroradiometric measurements, but the differenc es were <7% for all sources, except red-light-emitting diodes. The sen sors were most accurate for broad-band sources and least accurate for narrow-hand sources. According to spectroradiometric measurements, YPF sensors were significantly less accurate (>9% difference) than PPF se nsors under metal halide, high-pressure sodium, and low-pressure sodiu m lamps. Both sensor types were inaccurate (>18% error) under red-ligh t-emitting diodes. Because both YPF and PPF sensors are imperfect inte grators, and because spectroradiometers can measure photosynthetically active radiation much more accurately, researchers should consider de veloping calibration factors from spectroradiometric data for some spe cific radiation sources to improve the accuracy of integrating sensors .