Patent GB-1316677-A (GB)
Title: Method and apparatus for the production of holograms
IPC: G03H1/04
Filed: 1970-05-14
Abstract: 1316677 Holography; photographic exposure control K BIEDERMANN 14 May 1970 [14 May 1969] 23470/70 Headings G2A and G2J [ Also in Division G1] Holograms are recorded on photographic material with the aid of exposure control apparatus including photo-electric detecting device(s) monitoring the intensity or modulation (fringe visibility) in the recording plane and feeding a control arrangement, programmed with sensitometric data and the modulation transfer function (resolution) of the photographic material, etc, and desired values for the intensity and modulation to provide optimum operating conditions and holographic efficiency, which control arrangement accordingly adjusts the relative intensities of the object and/or reference beam. The arrangement of Fig.1 shows photoelectric apparatus for measurement of the modulation in the recording plane 4. The reference beam 1 and the beam 3 reflected from the object 2 interfere in this plane 4, the image at which is magnified by objective 5 to form an image in the plane 6 of a photo-resistor 6a, and also, by means of beam-splitter 10, of photoresistor 8. Since cell 8 is provided with a diffuser plate 9, it measures, on meter 18, the general intensity in the recording plane, while bridge meter 13 measures the modulation (fringe visibility or resolution). A cylindrical lens 7 is arranged to aid this measurement. Signals from the bridge photo-resistors are supplied to a control device, which has been programmed with data corresponding to the settings of path objects, sensitometric andprocessing details of the recording medium etc., which thereby controls the relative intensities of the object and reference beams and the exposure time to be used. Adjustment of grey wedge 15 forms part of a nulling method used in the measurement of the modulation. Oscillating disc 12 for destroying the interference pattern is used in bridge calibration. In the arrangement of Fig. 2, for measuring intensity in the recording plane the control unit is indicated schematically at 17 and includes means to control and monitor movement or settings of a variety of optical path elements to optimise the exposure of the recording material 33. The initial power of the laser source is monitored at 16. To form the object and reference beams, a beam splitter 19, movable under control of the device 17 to vary the beam intensity ratio, is provided, the reference beam reaching the recording medium via mirror 30 and projection means 31, 32. The object 29 is illuminated from two directions by means of adjustable beam splitter 22 and the beams matched to the object size by projection means 25, 27 and 26, 28. In order to measure the intensity and modulation of the object field, a blind 100 is drawn in front of the medium 33 and a partially reflective or pivoted mirror 34 used to direct the light from the object via lens 35 to detectors 36. Measurement of the reference beam intensity is not required for each exposure and is calculated by the control device. The modulation in the recording plane may be brought to the required value in a number of ways; the polarisation or transmission filter 47 may be adjusted (object beam); attenuations of the reference field, using filters 48; adjustment of beam splitter 19 which has a continuous transmission/reflection ratio. When necessary, an auxiliary pre-exposure using flash 49 is made. The control device may be programmed to carry out a series of exposures, e.g. different views of the object, on one layer 33 and calculates the individual exposures accordingly. Various compensating optical elements are provided to deal with specific problems in calculating the required exposure. In order to avoid the "intermodulation noise" caused by the non-interfering (with the reference field) component of the object field, polarizing filter 37 is used. This is rotated parallel to the electric vector of the reference field during measurement of the object field intensity and at right angles to it when measuring the modulation. Alternatively, this component may be removed completely if a polarizing filter 38 is used. Further polarization difficulties arising with mixed metallic and non- metallic surfaces on objects, which are avoided by the use of unpolarized light provided by:  quarter-wave plate 39 with diffuser 40, using an unpolarized gas laser source (one without a Brewster window), the reference field being polarized by filter 46; a Nicol prism as divider 19 (to avoid losses). To provide optimum contrast, the "hard" object beam (provided by point source 28) and the "soft" beam (provided by diffuser 40) may be altered in intensity by masks 41, 42 or movement of splitter 22, or by use of mask 43 affecting one beam only. Inverse square lens compensation is provided in the reference beam by mask 44. Intensity variations in the cross section of the laser beam are corrected by mask 45. Deviations from linear polarization arising due to double refraction effects are eliminated by filter 46. In the focused-image producing arrangement of Figs. 3, 3a, (not shown), in which the object field illuminates the recording medium from one side and the reference field illuminates it from the other, in order to compress the intensity range in the object field and avoid exposure errors arising due to high spots, a transmission mask is prepared in photo-chromic material for superposition on the recording medium.