Patent GB-796661-A (GB)
Title: Optical arrangement for comparing the transmission ratio of a sample with a standard
IPC: G01N21/21
Filed: 1956-06-01
Abstract: 796,661. Optical apparatus. ZEISSSTIFTUNG. C.. [trading as JENAER GLASWERK SCHOTT & GEN.]. June 1, 1956 [June 2, 1955], No. 17004/56. Class 97(1) [Also in Group XL(b)] In apparatus for determining the light transmission ratio of a sample with respect to a standard, a beam of light is split into a measuring beam and a comparison beam and the beams recombined after passing through the sample and standard respectively by one or more interference polarizers. An optical device is used to determine the degree of polarization of the recombined beam, preferably by a null method. In a simple embodiment light from a source 10 passes through a condenser 11 and interchangeable filter 12 to an interference polarizer 13 provided with a deflecting prism. The resulting twin beams of polarized light are passed through the specimen 14 and standard 15 respectively and are then recombined by interference polarizer and prism 16. The photocell 18 associated with a suitable indicator views the final beam through analyzer 17. Comparison of the indications of photo-cell current when the analyzer is parallel and at right angles to the incidence plane of the polarizers may be used to determine the transmission ratio. Alternatively the analyzer may be rotated and the A.C. output of the cell measured. To convert this. embodiment to a null method a polarizer is placed in front of interference polarizer 13 and the angle through which it must be rotated to once again give zero output polarization on insertion of the specimen measured to give the transmission ratio. It is also possible to compensate the light beam energizing from the system to achieve zero polarization or to attenuate the comparison beam in a measurable manner for the same purpose. The system of Fig. 1 may be re-arranged so that the measuring and comparison beams are each transmitted at one interference polarizer and reflected at the other to give a symmetrical path arrangement. In this arrangement it is necessary to insert an optical element rotating the plane of polarization in each beam (Fig. 2, not shown). If a #/2 plate is used in the comparison beam for this it may be rotated to attenuate the beam to achieve zero output polarization as referred to above. The angle of rotation will be a function of the transmission ratio. In a further embodiment the paths followed by the measuring and comparison beams are again made symmetrical and the prisms associated with the interference polarizers so arranged that the measuring beam travels parallel to and to one side of a continuation of the axis of the initial light beam whilst the comparison beam travels parallel to and below it (Fig. 3A and B, not shown). As described a rotatable polarizer in the path of the comparison beam is used for obtaining an output of zero polarization. Fig. 4 shows an embodiment in which only one interference polarizer 40 is used and the beams pass through the specimen 41 and comparison member 42 twice because of the use of totally reflecting prisms 43 and 44. Elements 45 and 46 rotate the plane of polarization of the returning beams so that they pass out of the polarizer 40 towards the photo-cell. For the determination of the transmission ratio a rotatable analyzer 47 or any of the means previously described may be used. If modifications are made to the apparatus of Fig. 4 it can be arranged for measuring the reflection coefficient, the proportion of light reflected and transmitted, or the dispersion losses, in a specimen. As described with reference to Fig. 5 (not shown) this may be done by replacing the prisms 43 and 44 by a mirror and placing the specimen under test in the path of the measuring beam. The interference polarizers used, if they have a layer of uniform thickness, only operate within a limited frequency band and it may be necessary to combine a plurality of optical devices having different layer thicknesses in one polarizer if it is necessary to work with a wide frequency band. Specification 768,727 is referred to.