Patent GB-1280313-A (GB)
Title: Optical bench and method for testing optical systems
Inventor: BURDICK BRUCE CARLETON
IPC: G01M11/04
Filed: 1969-07-22
Abstract: 1280313 Testing optical systems EASTMAN KODAK CO 22 July 1969 [22 July 1968] 36755/69 Heading G1A [Also in Division G2] In apparatus for testing an optical system or component e.g. a lens, means are provided to compensate for measurement errors when for testing purposes, the object viewed by the optical system lies in a plane angularly oriented with respect to the principal plane of the optical system or lies at a (known) distance off the optical axis of the system. Optical path apparatus, Fig. 6, comprises e.g. the lens 7 under test, carried by V mounts, a collimater lens 5, used in tests when the light incident on lens 7 is to be parallel, a slit 67 at which the image is formed and which is horizontally oriented, a detector 9, which may be a photomultiplier, a photo-voltaic device, a photo-resistor or a photo-transistor, and the object generator arrangement 3. This comprises a tungsten-halogen lamp and a condenser lens illuminating a rotatable radial grating disc 17. The disc 17 is mounted on a shaft fixed eccentrically to motor driven rotatable disc 45. By this means, and as illustrated in Figs. 4-5 (not shown), the spatial frequency of the lines crossing vertical output slit 23 varies in a cyclical manner. Also provided in generator 3 are zoom lens 19 for focusing the grating lines in the plane of slit 23, and a filter wheel for selecting the spectral composition of the light. Characteristic evaluated as representative of the lens quality is the modulation transfer function, MTF, calculated as a/b, where a is the peak amplitude of the spatial frequency components of the image produced by the lens under test and b is the average value of the amplitudes. This function is evaluated electrically by the circuit of Fig.10, from the detector output, and is applied as the Y-co-ordinate to a graphical recorder or a cathode ray tube. The X-co-ordinate is the spatial frequency of the test object and is obtained by monitoring the position of the target disc 17 by means of a gear-wheel meshing with gear 43, and controlling a sine or cosine potentiometer 49. The waveform of a cycle of changes in the spatial frequency at object slit 23 is shown in Fig. 3. Compensations. When the object generator is located at a position off the optical axis of the system and the collimater lens 5 is being used, magnification error is produced in the image provided by lens 7 under test. When the object generator is angled (and may also be off-axis) with respect to the image plane, perspective error is produced. Both these errors are a function of the cosine of the respective angles involved and so the output of potentiometer 49 (representing the spatial frequency) is modified by neither, one or both of cosine potentiometers 95, 97, associated with the mounting arrangement for the devices 3-7, depending on the mode of operation and the errors which will be involved. M. T. F. calculation circuit, Fig. 10. The detector output is fed to pre-amplifier 202 whose output is partially directed to resistor-capacitor combination 203, 204 to obtain the average or D.C. level and partially on via band pass filter 205 to pass A.C. only and remove noise &c. The lost D.C. level is replaced by inverter 208 and the signal reinverted by inverter 209 before feeding a peak detector comprising diode 210 and capacitor 211. The peak signal is amplified and divided by the average signal from amplifier 207, in divider circuit 213 to provide the M. T. F. signal which is amplified at 214.