Optics: motion pictures – With focus control
Patent
1991-02-05
1992-06-23
Hayes, Monroe H.
Optics: motion pictures
With focus control
355 44, 355 45, 355 77, G03B 300
Patent
active
051237270
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Stationary mounted film projectors in cinemas are always located at a predetermined distance from the projection screen in the theater. Focusing the projected image, i.e. adjsuting the optics of the projector, should, in theory, be a non-recurring task, but experience has shown that in practice re-focusing has to be carried out from time to time. For example, deposits from the surface of the film can build up in the film path of the projector which results directly in blurred images, and furthermore the layer structure of the film can vary.
The development within the cinema business has lead to that an operator nowadays has to supervise 8-10 projectors in different auditoria. Quite naturally he is not able then to pay much time and attention to the focusing of the various projectors, which could be inconvenient to the audience, since sharpness is a determining factor when it comes to impression and experience of the picture shown. Also, it can be difficult to find swiftly the correct setting of focus and, in addition, in certain picture sequences it can be hard to judge the proper sharpness of the picture from the projection room. It is often so that the projectionist focuses with the guidance of the translated subtitles occurring in foreign film along the lower edge of the picture. This means that sharpness becomes poorer where best needed, namely, within that part of the screen where the actor's faces mostly appear, i.e. the central area of the screen, at a distance from the top edge of the screen corresponding to about one third of the height of the screen.
In practice there is thus a need for simple and correct focusing means, and earlier several solutions to the problem of focusing have bene proposed. For example, in the Swiss Patent Specification No. 484 443 an apparatus for focusing projected images is described. The basis of this solution is the fact that a sharp image exhibits greater contrasts between light and dark portions, and that a blurred image includes grey tones, i.e. the contrast effect is eliminated. However, it is difficult in practice, in particular when cinematographic pictures are concerned, to utilize grey tone conditions for providing focusing parameters. Several specific steps have to be taken in order that normal fluctuations in image intensity be eliminated and in order that the two measuring signals be obtained which are to form the basis of a correcting signal.
SUMMARY OF THE INVENTION
In black and white film the colloidal silver is collected in tiny grains, and in color film the color substances form in a corresponding manner flocks, whereby in both cases the film obtains a granulated surface. In correct focusing this granularity can be distinctly seen in the projected image, although not from normal viewing distance. The present invention utilizes the occurrence of the image grains, or the like discontinuities, and generally it is proposed that a section of the projected image be scanned by means of an image sensor with respect to individual image elements, that is, projected grain structures, the signal obtained by the image sensor being fed in the form of a frequency spectrum to means for separating one high and one low frequency band, which in turn are treated in order to provide a measurement value indicative of focusing.
In order to transfer the grain structure of the projected image, a scanning image sensor such as a video camera or the like is preferably used, which delivers an electrical signal related to the grain structure and including a frequency spectrum. The size of the test section, which is suitably rectangular, is adjusted such that the granularity falls within the resolving capacity of the image sensor. Thus the signal obtained by the scanning will contain a frequency spectrum related to the grain density. The spectrum contains higher and lower frequencies, and according to the invention one higher and one lower frequency band are selected, a voltage being created on the basis of these bands by e.g. comparison, which voltage
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