Patent GB-970369-A (GB)
Title: Improvements in or relating to methods and apparatus for testing optical systems, lenses and the like
Inventor: BAKER LIONEL RICHARD
IPC: G01M11/02
Filed: 1961-09-07
Abstract: 970,369. Optical testing apparatus. BRITISH SCIENTIFIC INSTRUMENT RESEARCH ASSOCIATION. Dec. 7, 1962 [Sept. 7, 1961], No. 32206/61. Heading G2J. [Also in Divisions G1 and H4] General. An optical system, lens or other optical component is tested with the aid of a test object having definite spatial frequency, i.e. a line pattern having a known number of lines per millimetre, an image of the object which is formed by the system being scanned photo-electrically to derive an electrical signal which is a measure of the spatial frequency response of the system, and in terms of its amplitude and phase indicates respectively the loss of contrast and amount of transverse shift of the image. In one way of carrying out the test two images of the test object are formed, one by the optical system under test and the other by a reference optical system, and the two images are scanned to derive electrical signals which are compared by a null method in which the reference system signal is adjusted to balance the test system signal, the amount of adjustment providing a measure of the spatial frequency response of the system under test. In another way of carrying out the test, the spatial frequency of the test object is varied cyclically so as to permit measurement of the system response to different spatial frequencies and the resulting electrical signal is applied to one set of plates of a cathode-ray oscilloscope, the other plates being supplied by a signal proportional to the instantaneous spatial frequency of the test object whereby a trace is drawn of the spatial frequency response of the system. Optical details, Fig. 1. The test object comprises a slit 22-upon which is formed an image of a part 13 of a rotating disc 14 formed with a radial grating, the optical path including an adjustable zoom lens 16 and an image rotating assembly 15 consisting of reflectors 19, 20, 21 which may be rotated as a unit about the optical axis of the system. Figs. 7-9 indicate the relationships between the disc image and slit 22 for different angular positions of assembly 15, the arrangement permitting the spatial frequency (i. e. lines/mm) of the image formed in the slit in the direction of the length of the slit to be varied from a maximum value, Fig. 8, to zero, Fig. 7, through intermediate values, Fig. 9. The rate at which the lines traverse the slit however remains constant for all values of spatial frequency and is determined solely by the rotation rate of disc 14. Slit 22 may be adjusted in width and orientation. Light passing the slit is directed via a reducing lens 25 to a beam splitting cube 26 whereby part passes to the optical system 35 under test and another part forms a reference image 28 which is located in the plane of a pin hole 40. The reducing lens is one of a series with differing magnifications. By choice of appropriate lens in conjunction with adjustment of zoom lens 16 it is possible to arrange that the spatial frequency of the test pattern produced in the focal plane 41 of system 35 may be varied between 0 and 100 lines/ mm. by rotation of assembly 15. Lens 25 forms images of slit 22 at 28 and 29. System 35 views image 28 through a collimating lens 33, whilst adjustable micrometer plate 24 is provided to permit image 29 to be positioned accurately on pin hole 40. A second micro-meter plate 32 is employed to make adjustments of the position of the image due to system 35 when setting up. System 35 forms its image on one of a series of slits 44 which are engraved in a thin metal film on the surface of a glass block 318. The slits are arranged alternately in two directions at right-angles so that by appropriate orientation of slit 22 system 35 may be tested in two perpendicular planes. A particular slit is selected by positioning a hole in an adjustable mask, Fig. 13 (not shown), in front of it. System 35 is mounted in a camera type test bench 30, Figs. 13 and 14 (not shown), so that it may be pivoted about the entrance pupil to permit tests to be made with light rays entering at any angle. Integrating cavities 10 and 47 with matt white interior surfaces are arranged behind pin hole 40 and system 35 respectively, the transmitted light being detected by photo-multipliers 39 and 38. As the lines forming the test object move the corresponding lines in image 29 and the image formed by system 35 move with respect to the pin hole 40 and. the selected slit 44. The images are thereby scanned and the resulting electrical signals 42 and 45, which are in the form of alternating voltages of constant frequency determined by the rate of rotation of disc 14, indicate by their amplitude and phase the spatial response of the reference optical system (i.e. the optical elements which form image 29 on pine hole 40) and the optical system under test. In the first method of testing the signals are compared in a circuit 48 (see below) and signal 42 is adjusted in amplitude and phase until equality is achieved. The necessary adjustments which are indicated by controls 92 and 66 then provide a measure of the response of system 35. In the second method of test, switch 80 is operated to its lower position, where the output of photomultiplier 38 (after demodulation &c.) is applied to the Y-plates of cathode-ray tube 54. Assembly 15 is rotated or rocked back and forth by a motor 36 so as to cause the test object spatial frequency to vary cyclically over the range 0-100 lines/mm. and a voltage proportional to the instantaneous frequency is derived by a sine/cosine potentiometer 52 and applied to the X-plates of tube 54. For use in the first method of test a meter 53 is provided which may be calibrated in terms of lines/mm. Assembly 15 may use a prism in place of reflectors 19-21. In a modification of the part of the apparatus shown between lines P and Q, Fig. 16 (not shown), assembly 15 is dispensed with and disc 14 is carried on an arm so that its axis of rotation 9 may be positioned along a circular path concentric with the optical axis of the apparatus. By this means the angle at which grating lines traverse the optical axis may be varied to control the spatial frequency in a similar manner to the control effected by assembly 15. A colour filter assembly is included to permit the spectral characteristic of the light used for the test to be varied. Reference is made (without giving details) to the use in place of disc 14 of a cathode-ray tube controlled to display a pattern of moving lines or dots. Electrical details ; measurement of phase and amplitude, Fig. 4. The output 45 from photomultiplier 38, which is responsive to the light from the optical system under test, passes through an amplifier 58, band-pass filter 60 and amplifier 62, whilst the output 42 from photomultiplier 39, which is responsive to the light from the reference optical system (pin hole 40), passes through similar units 59, 61, 63. Signal 45 is then applied through an amplifier 72 with automatic gain control to one input of a phase sensitive demodulator 70, and signal 42 is applied through a driver stage 64 to a resolver and phase shifter 65 which produces two outputs 42a and 42b phase displaced by 90 degrees. Output 42a is limited and squared in stage 68 and applied through a driver stage 69 to the second input of demodulator 70. A meter indicates the output of the demodulator which functions so as to produce no output when the two input signals differ in phase by 90 degrees or 270 degrees. In operation, the control 66 of phase shifter 65 is adjusted to produce zero indication on meter 71. The signal 42b should then be 180 degrees out of phase with the test signal 45. Since however demodulator 70 is independent of phase sense, a 180 degrees phase ambiguity may occur. This is detected by adding signal 42b to the signal at the output of amplifier 72 in adder circuit 75 and applying the result which should be zero through switch 80 and rectifier 81 to meter 82. When correct phase balance is achieved, the setting of control 66 indicates the phase component (i.e. lateral image shift) of the spatial frequency response of the system under test. The signals are then balanced for amplitude with the aid of a second similar phase sensitive demodulator 84. The signal 42b is applied through an amplitude limiter 143 and driven to one input of the demodulator and is also applied through an adjustable attenuator 90 and buffer stage 91 to an adding stage 86 where it combines with test signal 45. The output of the adder is then amplified in stage 85 and applied to the other input of the demodulator 84. The control 92 of attenuator 90 is adjusted until meter 87 indicating the output of demodulator 84 reads zero, the setting of the control indicating the amplitude component (loss of contrast) of the system under test. Electrical details; cathode-ray oscilloscope, Fig. 5. The output 42 from photo-multiplier 38 obtained through circuits 59, 60, 62 (see above) is applied through a driver stage 101 to an amplitude demodulator 102 and the resulting signal is applied through a low-pass filter 103 and D. C. amplifier 106 to the Y-plates of cathode-ray tube 54. The signal to control the X-plates in accordance with the instantaneous spatial frequency of the test object is obtained from sine/cosine potentiometer 52 which is driven by assembly 15 (see above), and applied to the plates via D.C. amplifier 114. Either the sine or cosine output from the potentiometer may be selected by switch 112 to allow for positioning of the test object slit in mutually perpendicular directions. A graticule formed of lines at known voltage intervals may be periodically drawn on the tube screen under the control of a timing unit 115 and uniselector switch mechanism 121 which applies the output of a voltage step source 122 and triangular wave generator 119 to the X- and Y-plates and periodically transposes the connections