Performance Test of Five-type Photosynthetic Active Radiometers

Photosynthetic active radiometers are used to measure the solar photosynthetic active radiation. Also, it can be turned 180°, facing down to measure the photosynthetic active radiation of the ground. When it is used outdoors, the environmental condition has impacts on the performance, which may increase the measurement error. The performances of five different types of photosynthetic active radiometers are tested, such as sensitivity, cosine response, linearity, temperature dependence, spectral sensitivity and non-stability.The non-linearity errors of five photosynthetic active radiometers are all within 1%. When the irradiance is less than 250 W·m^-2, the differences of the non-linearity errors for different-type radiometers are significant. Cosine response errors gets bigger with the increase of the zenith angle (θ). When the zenith angle changes from-80° to +80°, cosine response errors of PAR LITE, LI-190 and FS-PR type radiometers are within 10%; the error of TRT-5 radiometer is within 10% just in the range of-55° to +55°; and HSC-FPH-1 is within 10% when θ is-45° to +40°. When testing the effects of temperature on photosynthetic active radiometer, the temperature coefficients of PAR LITE, LI-190 and FS-PR type radiometers are within 0.3%/℃, TRT-5 and HSC-FPH-1 type photosynthetic active radiometers are within 0.5%/℃. The wavelength range of photosynthetic active radiometer is from 400 nm to 700 nm. The ideal sensor should have clear cutoffs to light below 400 nm and above 700 nm. The spectral response of PAR LITE, LI-190 type radiometers is approximated to a horizontal line, which is closer to the ideal spectral curve. The response of FS-PR is overestimated within 400—550 nm and underestimated after that. The response of TRT-5 and HSC-FPH-1 photosynthetic active radiometers is lower than the ideal curve in the whole visible waveband. The consistency of five-type photosynthetic active radiometers with the standard value are all within 5% which is tested under solar light outside. At noon, the error is smaller as the zenith angle is small; but when the zenith angle grows bigger, the error is bigger due to the increase of cosine error. The stability for photosynthetic active radiometer in the experiment are within ±3%. Due to the limited time and samples, the stability of homemade radiometer need further test. Zero offset of photosynthetic active radiometer is very small, which could be ignored. Therefore, it is necessary to calibrate the radiometer periodically to minimize the error. The analysis and comparisons of the experiment results provide scientific basis for controlling the radiometer quality, improving the measurement accuracy and selecting instrument for meteorological observations.