Abstract: A differential spectrometry system detects very narrow-band spectral features, while providing much higher optical transmittance and signal-to-noise ratios than prior optical-filter-based spectrometer systems. A plurality of light detectors (10a, 10b) detect light that falls within respective wide wavebands. The wide wavebands have overlapping and non-overlapping portions, one of which is the desired narrow waveband. The detector outputs are operated upon to produce an output signal (22) which includes substantially only the desired narrow waveband. In the preferred embodiment, the light detectors (10a, 10b) are implemented with a pair of optical detectors (30a, 30b) and respective optical interference filters (24a, 24b). The filters have substantially identical cut-off wavelengths (.lambda..sub.2) and cut-on wavelengths that are shifted by .DELTA..lambda. with respect to each other (.lambda..sub.1 and (.lambda..sub.1 +.DELTA..lambda.), respectively).

Abstract: A differential spectrometry system detects very narrow-band spectral features, while providing much higher optical transmittance and signal-to-noise ratios than prior optical-filter-based spectrometer systems. A plurality of light detectors (10a, 10b) detect light that falls within respective wide wavebands. The wide wavebands have overlapping and non-overlapping portions, one of which is the desired narrow waveband. The detector outputs are operated upon to produce an output signal (22) which includes substantially only the desired narrow waveband. In the preferred embodiment, the light detectors (10a, 10b) are implemented with a pair of optical detectors (30a, 30b) and respective optical interference filters (24a, 24b). The filters have substantially identical cut-off wavelengths (.lambda..sub.2) and cut-on wavelengths that are shifted by .DELTA..lambda. with respect to each other (.lambda..sub.1 and (.lambda..sub.1 +.DELTA..lambda.), respectively).

Abstract: An infrared window (26) includes a substrate (40) made of a zinc salt composition and an antireflection coating system (42) deposited on the substrate (40). The coating system (42) comprises a first layer (44) of germanium overlying the substrate (40), a second layer (46) of diamond-like carbon overlying the first layer (44), a third layer (48) of germanium overlying the second layer (46), and a fourth layer (50) of diamond-like carbon overlying the third layer (48). Preferably, the germanium layers (44, 48) are in tension and the diamond-like carbon layers (46, 50) are in compression. The germanium layers (44, 48) are preferably deposited by magnetron sputtering and the diamond-like carbon layers (46, 50) are preferably deposited by plasma activated chemical vapor deposition. Silicon may be used in place of the germanium, and graded compositions can be used for the first layer (44) and the third layer (48).