A field-portable thermal infrared grating spectrometer (THIRSPEC)

A thermal infrared grating spectrometer was developed for field studies in the earth sciences. The design is based on a reflection grating and a 60-element HgCdTe detector array. The useful spectral range of the instrument covers 7.9-11.3 pm with a Nyquist limited resolution of 0.16 pm. The instrument averages over a 12" field of view and compares the exitance of the target to that of an internal black body at ambient temperature. The noise equivalent temperature is approximately 0.06"K over the useful spectral range. Background radiance reflected from the surface of the target can seriously impede the determination of emissivity. This effect is removed from the spectra of geological samples by the use of reference samples. TABLE I INSTRUMENT SPECIFICATIONS Size: Optical head is 43 x 23 x 15 cm Weight: 30 kg including battery pack, data logger, optical head, cables, Battery pack: 5 kg weight, 2-3 h of operation Slit dimension: 0.2 x 3.0 mm Grating: 50 LP/mm blazed at 9 pm Lens: germanium triplet 25 mm EFL, f/0.8 Detector array: DT-591A/UA 60 element HgCdTe Military Common Cryocooler: CTI Cryogenics Model CM-5 closed-Stirling cycle cooler. Spectral resolution: Nyquist limited 0.16 pm Noise equivalent temperature difference: <O. 1 K and tripod