GB1480289A - Measuring the sharpness of images - Google Patents

Abstract

1480289 Photo-electric measurement of image sharpness RESEARCH CORP 25 Sept 1974 [17 Jan 1974] 41675/74 Heading G1A [Also in Division G2] To detect the sharpness of an image it is projected on to a transducer comprising a medium whose electrical properties vary with the incident image and also with a strain wave in the medium, and the modulation of the electrical properties at the strain wave frequency is observed. The basic transducer is described in Specification 1445119 (see Division H3-5). In Fig. 1 the medium is a CdS film 22 on a quartz substrate 20; a generator 26 establishes a strain wave 28 on the substrate, the wavelength being greater than the desired sharpness resolution but less than the size of the image projected from source 44. The wave frequency is for example 3-8 MHz. Contacts 24a, 24b allow the film resistance to be monitored by applying a constant bias voltage to it from supply 32. The D.C. current measured at 34 corresponds to a conventional measure of image intensity, while the modulation at the strain wave frequency measured at 42 is a function of image sharpness. In variations of this basic arrangement the film electrodes may be inter-digitated or curved, and the film may be thicker at its central region than elsewhere Figs. 8, 9 (not shown); these latter two arrangements allow the sharpness of images of low intensity or of low contrast to be detected. Two dimensional measurements can be made by using two generators whose waves intersect at the film, or by using two films angled with respect to one another, Figs. 7, 13 (not shown). It is also possible to replace the film with a tunnel barrier formed by a thin oxide layer between two aluminium films on a glass substrate, Fig. 10 (not shown); the film may also be replaced by a degenerate semi-conductor or a metal, or a photoemissive cathode and a collector plate. In all these arrangements the detected modulation is a maximum when the image is at its sharpest, Figs. 5, 6 (not shown). This can be used to automatically focus a slide projector, either by projecting the image on to a screen behind which is a focusing system and a Fig. 1 detector, Figs. 3, 4 (not shown), or by having a beamsplitter in the projector such that the image on the screen is reflected back into the projector and thence via the splitter to a Fig. 1 detector, Fig. 12 (not shown). The modulation can also be used to focus an SLR camera as in Fig. 11, where the SLR mirror directs the image on to the viewing screen 72 which carries a CdS film 22 and receives a strain wave from 26; a meter 34 acts as a conventional exposure meter while the tuned detector output can be displayed at 81 for manual focusing or can drive a lens setting motor 84.