Patent GB-1384382-A (GB)
Title: Apparatus for evaluating optical performance parameters of  optical components
IPC: G02B27/42
Filed: 1971-08-23
Abstract: 1384382 Testing optical components photoelectrically EALING BECK Ltd 2 June 1972 [23 Aug 1971] 39479/71 Heading G1A An optical component is evaluated by arranging the component to receive radiation from an illuminated aperture giving a known angular intensity distribution and to form an image on a radiation sensitive surface which is electronically scanned to produce an electrical signal representative of the energy distribution, the signal being processed to evaluate a selected optical performance parameter. As shown, Fig.1, a lamp 10 and condenser 11 uniformly illuminate a slit 12 to form the object. the.image of which is projected on to the target of an image dissector tube (IDT) 14 by means of the optical component 13 under test. The IDT 14 scans the image transversely to the length of the slit to produce a signal representing the line spread function. A processor 18 may perform a Fourier transform at a number of discrete points and the resulting information displayed. The optical transfer function (OTF) may be determined by multiplying the line spread function by a cosine function at a given spatial frequency, integrating the product, normalising with respect to the integral of the product for zero spatial frequency to produce an output value and then repeating the process for an incremented spatial frequency. The OFT may also be produced using a switching integrator. The phase transfer function may be obtained by using a sine function instead of the cosine function. In a second embodiment, the object comprises an IR source illuminating a slit via a chopper which modulates the radiation at a frequency of about 10 Hz to produce correct operation of the detector which is a pyroelectric vidicon tube. The vidicon is provided with a main objective lens and a supplementary lens may be provided which can be moved into the radiation path to alter the position of the effective plane of the vidicon target e.g. from the image plane to the plane of the exit pupil of the component under test. The energy distribution may be displayed directly or the calculated transfer function may be displayed for which purpose a subsidiary display in the form of a long persistance CRT, a storage oscilloscope, a pen recorder, or an X-Y plotter may be provided. The detector may be pyroelectric or conventional vidicon, an image dissector tube or an IC photodiode array and multiplexer. The optical component under test may be a lens system, a mirror system, or a fibre optic face plate. The calculations may be performed by a general purpose digital processor, a hard-wired or software controlled special digital processor or by analogue circuitry. The transform output may be used to control a servo adjusting the position of a component in the system under test or to control the grinding and/or polishing of a component. If the system is a telescope the source object may include a collimator. Optical filters may be included in the source or a monochromatic source e.g. a laser may be used. The radiation may be IR, UV, or visible light. A pinhole object aperture may be utilised to evaluate the encircled energy function. A plurality of source slits may be utilised to produce off axis OTFs.