The Portable Remote Imaging SpectroMeter (PRISM) is a compact, lightweight pushbroom imaging spectrometer compatible with a wide range of airborne platforms (Mouroulis et al., 2014). PRISM measures radiances in the near ultraviolet to near infrared spectrum (350–1050 nm at a spectral sampling of approximately 3 nm). PRISM integrates technologies that as a system achieve high signal-to-noise ratio, high uniformity, excellent dynamic range, low polarization sensitivity, stray light suppression, and calibration stability. See Table 1 for PRISM characteristics.
|Resolution (FWHM)||3.5 nm typ|
|Calibration Uncertainty||<0.1 nm|
|Spatial||Field of view||30.7°|
|Instantaneous FOV sampling||0.882 mrad|
|IFOV resolution (FWHM)||0.97 mrad|
|Cross-track spatial pixels||608|
|Radiometric||Range||0 – 99% R|
|Signal to Noise Ratio *||500 @ 450 nm|
|Uniformity||Spectral cross-track uniformity||>95%|
|Spectral IFOV mixing uniformity||>95%|
The complete sensor comprises the optical head assembly (OHA) and control electronics. At about 40 kg, the OHA comprises the vacuum vessel with the spectrometer and a shortwave infrared (SWIR) radiometer inside, an aircraft interface/mounting plate, an INS/GPS unit attached to the optical head for enabling precise orthorectification and geolocation, as well as the preamplifier and A-D converter for the detector electronics. The vacuum vessel shell is temperature-stabilized to around 26°C with external strip heaters. Required vacuum level for stable operation is < 3 mTorr. The instrument retains this vacuum level through a typical flight without on-board pumping. Three commercial vibration isolators prevent high frequency vibration from transmitting to the optics and detectors.
The PRISM spectrometer incorporates several innovations including a fast and uniform design with a total internal reflection fold, a polarization-insensitive shaped groove concave grating, a black-Si slit for uniformity and stray light reduction, and a fast, wide angle two-mirror telescope. The spectrometer is of the Dyson design form which permits a high throughput (F/1.8) and also provides low angles of incidence for controlling polarization variation. All transmissive elements are made of fused silica with a high-quality broadband anti-reflection coating.