Patent GB-1464035-A (GB)
Title: Spectroscopic analysis apparatus
IPC: G01J3/28
Filed: 1974-08-29
Abstract: 1464035 Spectroscopic analysis FRANCE ARMED FORCES MINISTER OF 29 Aug 1974 [20 Sept 1973] 37878/74 Heading G1A Spectroscopic analysis apparatus comprises: a source of coherent radiation at several wavelengths: rotatable means for sequentially selecting wavelengths; means for separating radiation into reference and measurement beams; reference and measurement detectors; and a signal processing circuit. A gas laser 1, Fig. 1 containing an active material la, e.g. CO 2  capable of radiating at a band of wavelengths, and having an end reflector 1b and an outlet window 2, may be continuously pumped, or pulsed in synchronism with a motor-driven wavelength selector 3 comprising a grating 4 and a mirror 5 arranged dihedrally. The wavelength selector may alternatively comprise a rotatable prism or a Fabry-Perot system arranged within the laser cavity. Radiation at the selected wavelength is split by a semi-reflective mirror 7 into a reference beam directed at a reference detector 8, and a measuring beam which passes through a collimater 9 and a specimen M, being retro-reflected by a mirror 10 and directed to a measurement detector 11 e.g. a CdTe or HgTe cell. The specimen M may be, e.g., atmosphere suspected of pollution. The effective angular range of the selector 3 may use only 500 out of a possible 20,000 incremental pulses supplied by a position encoder 6, Fig. 2. Coded increments are counted at 13 and decoded at 14, predetermined ones being manually selectable at 14 to occur just before respective wavelengths of interest are scanned by the selector 3. At the beginning of each rotation of the selector 3 a rotation synchronization signal A is fed to a logic circuit 16, and to reference and measurement amplifiers 18, 25 to instruct them each to store a signal representing their own zero drift. As each selected wavelength approaches an indicator signal D causes a generator 15 to supply a gating signal E sufficiently long to outlast the pulse of selected radiation. A reference signal B, amplified and corrected for amplifier drift at 18, is compared with a threshold at 19 and, when of sufficient amplitude, opens an AND gate 17 to start a reference integrator 21 via a line 75. The pulse from the gate 17, after a delay at 26 adjustable to allow time for the measurement beam to complete its return journey, further turns on a measuring integrator 24 receiving the amplified measurement signal L from the detector 11. The integrator 24 output is amplified at 28 with a gain of 1 or 10 selectable by a switch 29. Each one of the integrators 21, 24 is provided with pre-settable means for adjusting the duration of integration. At the end of the measurement integration period the integrator 24 sends a conversion order signal J, Q via an OR gate 23 to two A/D converters 22, 27 which supply respective digital reference and measurement signals to a logic unit 16. When both conversions are complete an AND gate 30 sends a completion signal K to the integrators 21, 24 to reset them and to the logic unit 16 to prepare it to receive the next information. If a reference signal B fails to pass the reference comparator 19 the cycle initiated by the gate 17 will be missed; instead the trailing edge of the gating signal E triggers a restoring circuit 20 which, via the OR gate 23, directly provides a conversion signal Q to the converters 22, 27, which send zero information and a completion signal K to the logic unit 16, thereby ensuring that a blank space is left. In normal operation the restoring circuit is inhibited by the gate 17 output. The logic circuit 16 routes the digital information to a reference store 35, a measurement store 34, a ratio calculator 33, a printer 32 and a tape punch 31.