Patent GB-918885-A (GB)
Title: Improvements in or relating to spectrometers
IPC: G01J3/12
Filed: 1961-05-09
Abstract: 918,885. Spectrometers. WRIGHT, R. H., and DAYKIN, P. N. May 9, 1961 [May 13, 1960], No. 16863/61. Class 97 (1). [Also in Groups XXXVII and XL (b)] A spectrometer, especially an infra-red spectrometer, comprises a zone plate positioned to produce a series of images of a radiation source, each image corresponding to radiation of a different wavelength and the images being disposed along a line passing through the centre of the zone plate, a radiation detector mounted on this line, and means for effecting relative movement between the images and the detector, so as to cause images of the source to impinge on the detector in succession. The zone plate, which may consist of a flat plate on which a series of concentric circular zones are drawn with radii proportional to the square roots of the natural numbers, may be of the reflecting or transmitting type. The detector (which may be a thermal detector or a photo detector) and the radiation source (which may be an electrically heated solid, a flame or an arc discharge between a pair of electrodes) are preferably mounted in alignment with the axis of the zone plate, in which case they may be mounted for simultaneous movement relative to the zone plate or may be separately mounted so that only one of them is so movable. The radiation source and detector alternatively may be mounted on opposite sides of the zone plate axis. In a further form the radiation source 40, Fig. 17, is mounted on a movable carriage 72 and the converging light reflected from the fixed zone plate 54 is intercepted by an aspherical convex mirror 164, which focuses the light on to the detector 70 and is mounted on the same carriage 72 as the source 40. The detector 70 may be mounted on the other side of the zone plate 54 from that shown in Fig. 17; a central aperture being provided in the zone plate. Alternatively the detector and source with the convex mirror between them may all be mounted on a movable carriage 72. Another variation is described in which the detector 70 and source 40 are fixed on one side of a centrally apertured zone plate 54, Fig. 20, there being a movable carriage 72 on the other side of the zone plate supporting a convex mirror 164 which produces virtual images 168 and 170 of the source 40 and detector 70 respectively. In one particular form of the invention an infra-red spectrometer comprises an I-beam 92 on which are fixed a mount for a radiation source 40, Fig. 6, a sample holder 61 and a reflecting type zone plate 54, this beam also supporting a track 90 along which a carriage 72 mounting a radiation detector 70 is movable by a belt 120 and pulleys 114, 116 of which the pulley 114 is rotated by a servomotor 122. In the path of the radiation is disposed a rotary chopper 50 driven by a motor 60 which, together with the chopper, is mounted on a cross-member supported by the base 97 to which the I-beam is bracketed. This chopper 50 has an even number of holes 56, alternate ones being provided with filters opaque to required wavelengths (for example longer than 25 microns) so that an intermittent signal will be generated by the detector and this signal amplified. A reference voltage of the same frequency is provided for the amplifier by a generator 71 of which the rotor moves with the chopper 50. The continuous signal generated by the detector as a result of the incidence thereon of unwanted wavelengths is not amplified and filters are provided to eliminate unwanted signals caused by the web portions of the chopper disc. The speed of the carriage 72 is adjusted in accordance with a predetermined programme by applying a variable voltage to the servomotor 122. For this purpose the motor is incorporated in one arm of a Wheatstone bridge circuit which includes also a potentiometer 130, Fig. 11, and a resistance wire 146 along which moves a slider 148 attached to the carriage 72 and electrically connected through an amplifier 154 with the motor 122. The potentiometer 130 has a spirally grooved drum 132 connected by a string 136 to a cam 138 formed by a spiral helix so that, as this cam is rotated at constant velocity by a motor 140, the drum 132 rotates at a variable angular velocity resulting in bridge unbalance which movement of the carriage along the track eliminates.