Patent GB-902684-A (GB)
Title: Improvements in or relating to radiation detecting devices
IPC: B61L1/20
Filed: 1959-03-02
Abstract: 902,684. Radiation pyrometers. PHILIPS ELECTRICAL INDUSTRIES Ltd. March 2, 1959 [March 4, 1958; May 30, 1958], No. 7162/59. Class 40 (3). A radiation detector, and more particularly a device for detecting the radiation from overheated bearings having a temperature of the order of 200‹ C. or less where the radiation spectrum has parts in common with the ambient radiation spectrum, includes means deriving two beams from the radiation entering the device, modulating the beams, and directing them to the same detector over paths having different pass bands, wherein a device is provided to adjust the detector output to a constant value when only ambient radiation is incident so that a variable component in the output provides an indication of the radiation to be detected. Fig. 1 shows schematically a diaphragm N through which a radiation beam passes to modulators M1, M2. The outputs of these modulators respectively pass through filters F1 and F2 to a common detector D feeding its electrical output to an indicator B. With a suitable choice of filter pass-bands it can be arranged so that adjustment of the screen C, affecting beam r 1  is possible to give a constant voltage output from the detector for the ambient radiation. If the radiation to be detected passes through only one of the filters then it is possible to measure its intensity from the variable component of the detector output. It is convenient for the modulators to allow the two beams to pass alternately for equal periods. In one device for checking bearings for overheating (Fig. 4), the detector D may be a leadsulphide cell, G is a germanium crystal plate, P a plate of the substance known under the Registered Trade Mark " Plexiglass " and V an optical system of glass. Radiation r 1  must thus lie within the wavelength range 1.9 to 2.1 Á and radiation r 2  in the range 1.9 to 3.0 Á. In daylight the output of the detector, once adjusted to constant voltage, is stated not to require readjustment each time the instrument is used. The radiation to be detected, which lies almost completely in the band 2.1 to 3.0Á, only passes in the beam without plate P and causes a modulated output from the detector of an amplitude dependant on its intensity. A suitable modulator of the rotating disc-type is described with reference to Figs. 6a and 6b (both not shown). In this the modulator disc has two sets of apertures at different radii from its axis which respectively interrupt the two beams. In another embodiment the modulator disc has only a single set of apertures I 1 , I 2 , &c. (Fig. 8), which pass across an aperture E. Immediately behind the disc and covering one half of the aperture is the plate P. The screen C is adjustable over the other half of the aperture. In an embodiment (Fig. 9), in which the inlet aperture N is substantially the same size as the rotating modulator disc M driven by motor E the disc M is divided into equal sectors, alternate ones being opaque and the others allowing the passage of all radiation. Close behind disc M is a stationary disc F having similar sectors which are alternately open and provided with a filter plate. Radiation passing through is directed to a detector D by elliptical mirror A. A screen, or screens, C can be adjusted to cover part of the clear portion of disc F. In variations of this embodiment the modulator disc M may be either rim driven or its drive shaft extended through a small hole in a 45 degree plane mirror (Fig. 12, not shown), to avoid obstruction of the radiation path by motor E.