Patent GB-1292465-A (GB)
Title: Improvements in and relating to spectrometric apparatus
Inventor: GIRARD ANDRE JEAN
IPC: G01J3/28
Filed: 1969-11-27
Abstract: 1292465 Photo-electric spectrometers OFFICE NATIONAL D'ETUDES ET DE RECHERCHES AEROSPATIALES 27 Nov 1969 [11 Dec 1968] 58042/69 Heading G1A [Also in Division G2] In a grid-type spectrometer having a scanning prism or grating 42, 74 and in which a single plate 15, provided with two sets of alternating zones 11, 12, serves as both entrance and exit window such that after dispersion a plurality of mono-chromatic images of the window are focused in the plane of the window but at any one time only one of these images exactly overlaps the window, the optical path through the spectrometer is cyclically variable so as to provide a modulated photo-cell output dependent on the intensity of the wavelength component being instantaneously detected. The zones 11 and 12 may be respectively absorbing and transmitting, or reflecting and transmitting, and the two sets of zones are of equal area. The optical path modulation is achieved by rotating the window or a radiation path object. The photo-cell output is supplied to an amplifier tuned to the modulation frequency which produces a modulated output whose depth of modulation is proportional to the instantaneous intensity of the mono-chromatic radiation. Since the prism or grating is constantly in motion, a wavelength scan of the incident beam is achieved which may be indicated on a recording instrument. To investigate a source 21 as regards its radiation flux the apparatus shown in Fig. 4 may be used. The modification of the radiation path is obtained either by rotating the window 22 or by rotating the double reflector 44, 45 about an axis 46, while the spectral scanning movement is provided by rotation of the prism about an axis 43. The zones of the window are alternately transmitting and reflecting. The spectral scanning may be provided by rotating a reflection grating Figs. 5 and 6 (not shown) in which case the image rotation is provided by rotating a concave collimating mirror (51, 63) about an axis in its own plane. A double beam spectrometer for investigating test objects is shown in Fig. 7. Light from a source 700 is divided and sent along two paths to be incident on opposite sides of the rotating window 72, whose zones are reflecting and transmitting. An absorbing test object is placed in one of these paths while the other path may contain a servo-controlled optical wedge whose position is indicative of sample absorption. The two detector outputs are of identical form except for a relative phase shift of 180 degrees Fig. 8 (not shown), each output being an A. C. signal whose depth of modulation is proportional to the difference in intensity between the two beams incident on the window. To enhance sensitivity, one detector output may be phase-shifted after amplification and added to the other amplified detector output. In an embodiment using the Ebert-Fastie configuration Fig. 9 (not shown) a single photodetector is employed.  To derive a photo-detector response curve any of the above two-beam spectrometers using two photo-detectors, one of which is a standard, may be employed. The two detector outputs are connected to the same amplifier tuned to the modulation frequency. By inter-posing one or more compensating devices in one of the paths, the two detector outputs may be made equal at a single wavelength, shown by an un-modulated amplifier output. The two detector responses may then be compared over an extended spectral range.