The Precision Spectroradiometer (PSR) was developed at PMOD/WRC to upgrade and complement the filter-based Precision Filter Radiometer (PFR). Aerosol Optical Depth (AOD) and trace gas concentrations can be measured over a continuous wavelength range of 320 – 1040 nm.
The PSR is a temperature stabilised grating spectroradiometer with a 1024 pixel Hamamatsu diode-array detector, operated in a hermetically-sealed nitrogen-flushed enclosure. The spectroradiometer is designed to measure the solar spectrum in the 300 – 1020 nm wavelength range with a spectral resolution varying from 1.5 to 5 nm (FWHM). The optical bench made of a carbon alloy is optimised to minimise the temperature dependence of solar measurements to less than 0.1% °C-1 for ambient temperatures in the range -20°C to +40°C.
The optical characterisation of five PSRs has led to the following specifications: i) The temperature dependence is negligibly small, as expected from the mechanical design, ii) the linearity of the detector is better than 1% over the full dynamic range of the instrument, and iii) a correction is made for stray light.
PSRs are absolutely calibrated in the PMOD/WRC laboratory using reference standards traceable to PTB in order to measure direct spectral solar irradiance with an expanded uncertainty of ± 3%.
| Detector | Hamamatsu 1024 pixel diode array |
| Wavelength range | 300 – 1020 nm |
| Spectral resolution | 1.5 to 5 nm FWHM |
| Calibration uncertainty | ±3% |
| Housing | Hermetically-sealed, nitrogen-flushed |
| Change in sensitivity | <0.1% °C-1 for range -20°C to +40°C |
| Wavelength shift | <0.01 nm °C-1 |
| Linearity of data acquisition system | <±0.5% |
| Outer diameter | 190 mm |
| Outer diameter with sun-shield | 196 mm |
| Length (w/o valve) | 441 mm |
| Length with sun-shield (w/o valve) | 465 mm |
| Mass | 6.5 kg |
Figure 1. The PMOD/WRC Precision Spectroradiometer (PSR).