Patent GB-849347-A (GB)
Title: Improvements in spectrometers
IPC: G01J3/18
Filed: 1957-05-27
Abstract: 849,347. Photo-electric spectral analysis. JARRELL, R. F., and FASTIE, W. G. May 27, 1957, No. 16767/57. Class 40(3). [Also in Group XX] A spectrometer comprises a first spectrometer having means for dispersing rays from a source of radiant energy to provide a spectrum focused at the entrance slit of a second spectrometer which includes means for collimating the rays entering through said slit, means for dispersing said rays to form a second spectrum and means for recording said second spectrum, the entrance slit and the recording means being located in substantially the same vertical plane and the dispersing means of the second spectrometer being located between said plane and said collimator. In Fig. 3 a prism spectrometer is shown combined with a plane grating spectrograph. The prism spectrometer comprises a source of light 70  line image of which is formed by cross@ cylindrical lenses 26,112 on an entrance slit 120 disposed in front of a lens 34 which collimates the light. The light may then as shown be focused by lens 44, 46 on the entrance slit 146 of the spectrograph or, by rotating the bench 80 carrying the cylindrical lens, slit 120 and collimator, the light may fall on the dispensing prism 40 and be reflected by mirrors 42, 36 through the lens 44, 46 so as to form a spectrum on slit 146. Light of the wavelength 1 passed by slit 146 then passes beneath a plane grating 148 to a concave mirror 160 which reflects it to the grating. The light is again dispersed by the grating and returned to the mirror which forms a focused spectrum on photographic plates 1.68, 170, 172 or photo-multiplier tubes disposed above and in the same plane as entrance slit 146. Mirror 160 is disposed with its focus in the plane of slit 146 and with its optical axis half-way between the centre of the slit 146 and the medial line of plates 168, 170 and 172; the centre point of grating 148 lies on this axis and the grating lines are perpendicular to this axis. The angle between the ruled face of the grating and the mirror axis is dependent upon the spectral region it is required to record and is variable by a lever and cam arrangement. In a modified form, Fig. 4, not shown, all the elements of the prism spectrometer are mounted on a single bench and dispersion is effected by a compound prism made up of an element of fused silica and an element of calcium fluoride and arranged to give zero deviation. This prism is arranged to be moved out of the light path when it is desired to illuminate entrance slit 146 of the spectrograph with undispersed light. To allow different degrees of dispersion, these compound prisms may be mounted on a slide carried by the bench so that the prism may be moved into the light path. A prism consisting of three elements, two of calcium fluoride and one of fused silica, and having two curved surfaces may be used as the dispersion element and collimator, in which case lenses 34 and 44 are omitted. As shown in Fig. 13, the grating spectrograph may also include a photo-electric accessory consisting of an entrance slit 194 and exit slit 196 disposed with their centres in a plane containing the mirror axis, and a photo-cell 198 which is disposed behind slit 196 and is movable therewith by a cam 255 for scanning the spectrum. In a further type of prism spectrometer, Figs. 15 and 16, not shown, a Fery prism is used to collimate and disperse the light before it reaches the slit 146, and the collimated lens 34 is used when it is desired to illuminate slit 146 with undispersed light but is moved out of the light path when the Fery prism is employed. The initial light dispersion may be effected by a transmission or reflection type grating instead of a prism.