Patent GB-706355-A (GB)
Title: Improvements in or relating to a method of and arrangement for the comparison of twolight intensities
IPC: G01J1/36
Filed: 1951-08-17
Abstract: 706,355. Photo-electric light comparators. SIEMENS & HALSKE AKT.-GES. Aug. 17, 1951 [Aug. 17, 1950; Aug. 19, 1950; Aug. 23, 1950; June 18, 1951; June 18, 1951], No. 19467/51. Class 40 (3) Apparatus for comparing the intensities of two light beams by passing the beams alternately through two paths, e.g. by an oscillating mirror, comprises means for comparing the received impulses on a photo-electric cell and using the output to vary the transmissivity or duration of one path to equalize the total intensity of the alternate impulses received at the photocell. Light from a source S, Fig. 2, is reflected onto an oscillating mirror 3 sending the light beam alternately through devices F1, F2 to a photocell 5, the path through F1 containing an intensity reducer 7, e.g. a grey wedge or a wedge-shaped diaphragm. For measurement of colour temperature F1 and F2 will be filters of different wavelength ; for comparison of absorption Fl will be a test fluid and F2 a standard fluid, &c. A hexode valve 16 has one grid fed from the amplified, rectified output of the photocell 5, and another grid fed with A.C. from an oscillator 15, the anode current being transformer-coupled to two coils Sp which cause the mirror 3 to oscillate by means of a permanent magnet sheet to which it is fixed. The relative magnitudes of the alternate impulses from the photocell due to the light beam passing through F1 and F2 are compared with the oscillator output in a ring modulator R1 and produces a D.C. bias for the valve 16 which varies the anode current and therefore the energization of the coils Sp until the mean values of alternate impulses are equal. The anode current of valve 16 is thus a measure of the relative intensities of the light beam after passing through Fl and F2. Alternatively, the duration of the impulses may be varied by superposed D.C. on the A.C. energization of coils Sp, derived' from the amplifier, and the wedge 7 is not required. In a modification, Fig. 4, the amplitude of the oscillations of mirror 3 is constant, but a feedback connection from valve 16 through a half-wave rectifier to the amplifier 13 alters the amplification of alternate impulses until the impulses are of equal intensity when they reach the modulator R. The feedback current, indicated by a meter 9 is then a measure of the relative intensity. In another modification, Fig. 5, an image of the ray-dividing apparatus Fl, F2, after reflection by the oscillating mirror 3, is produced on a diaphragm containing a small aperture O in front of the photocell, so that the cell is affected alternatively by light through F1 and F2, the impulses from the cell being amplified at 8 and feeding the oscillator coils of mirror 3. The total intensity of alternate impulses are made equal by altering the amplitude of oscillation of mirror 3, or altering its zero position, by varying the measured output from amplifier 8. The intensity reducer 7 and filters Fl, F2 may alternatively be assembled directly on mirror 3. In another arrangement, Fig. 7 (not shown), the mirror 3 is stationary, but the diaphragm is made to oscillate to produce pulses in the photocell proportional to the light intensity through Fl, F2. The amplitude of oscillation or the zero position of the diaphragm is adjusted to equalise the total intensity of alternate impulses. The aperture may be symmetrical, e.g. square or circular, or wedge shaped. In an embodiment for comparing the light intensities from two sources S, x, Fig. 10, the light is reflected by the oscillating mirror 3 onto a photocell 5 through a screw B with a central aperture allowing light from each source to pass alternately through the aperture to the photocell, the intensity ratio being determined by the output from an amplifier 8 which is used to alter the amplitude or zero position of the oscillating mirror 3, or to vary the intensity of lamp x. In another arrangement, Fig. 11 (not shown), the light from the lamp x is automatically cut off for alternate half-cycles by feeding it with A.C. through a half-wave rectifier, the light from source S being chopped at the same frequency. The colour temperature of a body may be determined by comparing the intensity of light of two wavelengths in accordance with the above methods. If the intensity reduction due to the wedge 7 is proportional to the logarithm of the displacement of the wedge, it is shown that the black body temperature may be determined from the colour temperature measurement by the addition of a constant difference in displacement.