Radiometric characterization of diode-array field spectroradiometers

Abstract

A study was conducted to evaluate the radiometric performance of two commercially available diode-array field spectroradiometers: the Spectron Engineering (SE) Model 590 that has been on the market for some 10 years and the several-year-old Analytical Spectral Devices (ASD) Personal Spectrometer (PS) 2. Both of these instruments provide rapid acquisition (∼1 s) of a spectrum in the visible to near-infrared region (∼0.4–1.1 μm) and are field-portable. These evaluations included: spectral bandwidth, second-order effects suppression, signal to noise ratios, noise equivalent radiances, linearity of response with radiance and with integration time, temperature sensitivity, and polarization sensitivity. In the configurations tested the SE590 instruments had a spectral bandwidth of about 16 nm full width at half-maximum; the PS2 instruments, 3–4 nm. Neither of the instruments produced measurable second-order leakage. When viewing a barium-sulfate-coated integrating source internally illuminated with tungsten lamps, both instruments were able to obtain signal to noise ratios of 1000:1 or better between 500 m and 900 nm over most of the dynamic range provided by the sources when the integration time was optimized for the light level. Signal to noise dropped at shorter wavelengths in part due to the low energy provided by the source and at longer wavelengths due to the decrease in the detector response. In linearity tests all of the instruments generally agreed within 5% of the reference radiometer between 400 nm and 1000 nm; however, results were poorer at lower radiance levels and at wavelengths longer than 1000 nm or shorter than 400 nm. Both of the instruments rely on a shutter to measure their dark current, which is normally subtracted from the signal. However, closing the shutter on all of the instruments altered the dark current readings to some extent such that the dark current that was subtracted was not correct. When the error in dark current is compensated for, the apparent linearity of the system improves considerably with weak signals. The SE590 systems showed significant polarization sensitivity of up to 25%, whereas the PS2 instruments did not. Movement of the fiber optic cable on the PS2 instruments did result in measurable signal changes. Both instruments also showed the temperature sensitivity typical of silicon detectors; i.e., increasing sensitivity at the longer wavelengths with higher temperature.