GB2289388A

GB2289388A - Automatic focussing system

Info

Publication number: GB2289388A
Application number: GB9409663A
Authority: GB
Inventors: William Nevil Heaton Johnson
Original assignee: Durand Ltd
Current assignee: Durand Ltd
Priority date: 1994-05-13
Filing date: 1994-05-13
Publication date: 1995-11-15
Also published as: GB9409663D0

Abstract

An automatic focussing system for a camera or the like includes a light diffusing or scrambling focussing screen 18 disposed in a housing 10 and a first lens 14 for forming an image of an object on the focussing screen 18, the first lens including at least a part which is adjustable in position in order to allow focussing of an image of said object, formed by the first lens 14, to be focussed onto the focussing screen 18. The system further includes a further lens or optical system 20 arranged to focus the image on the focussing screen onto an electronic imaging device such as a charge coupled device 22, also disposed in the housing, and electronic processing means 24, 26 arranged to receive signals from the charge coupled device 22 and to adjust the first lens 14 until the signal from the charge coupled device 22 indicates a sharp focus. Screen 18 may be formed by selective polymerization of a photopolymerizable monomer to form an array of microlenses. <IMAGE>

Description

DESCRIPTION OF INVENTION Title: "Automatic Focussing System" THIS INVENTION relates to an automatic focussing system for cameras or other applications in which analogous focussing requirements arise.

It is an object of the invention to provide an improved automatic focussing system for a camera or the like.

According to the invention there is provided an automatic focussing system for a camera or the like which includes housing means a light diffusing or scrambling focussing screen disposed in said housing means, a lens for forming an image of an object on the focussing screen, the lens including at least a part which is adjustable in position in order to vary the position or effective focal length of the lens for image-forming purposes, a further imaging system arranged to focus an image of said screen onto a charged coupled device imager or other electronic imaging device, signal processing means arranged to receive a signal or signals from said imager or imaging device and to process said signal or signals to determine, from the characteristics of said signal, whether the image perceived by the imaging device is sharp or blurred, said processing device being arranged to provide an input signal to an automatic means for adjusting the focus of the camera lens.

Preferably the light-diffusing or scrambling screen is formed by selective polymerisation of a sheet of a light-transmitting photopolymerisable monomer by exposing the monomer to a pattern of illumination with localised variations in intensity of illumination. Such a screen is hereinafter referred to as a "microlens screen" although whilst, as noted below, such a screen may bear an array of microlenses, formed by exposing the monomer to a polymerising light pattern comprising a corresponding array of spots of light, the term as used herein is also intended to cover screens formed by exposing such a monomer to a polymerising light pattern comprising irregular patterns of irregularly shaped spots of light, for example, so that the resulting optical features lack the rotational symmetry, etc. normally associated with "lenses". European Patent No. EP0294122 and Published International Application WO91/18304 disclose types of microlens screens within the definition set out above.

An embodiment of the invention is described below by way of example with reference to the accompanying drawing which is a schematic diagram of a camera having a focussing system embodying the invention.

Referring to the drawing, a camera is shown as having a housing 10 with an aperture 12 occupied by an image-forming lens 14. The lens 14 is arranged to focus an image of a scene, outside the camera, and to which the aperture 12 and lens 14 are directed, onto a focussing screen 18.

The focussing screen 18 is preferably a lighttransmitting screen which has been formed by selective polymerisation of a light-transmitting photopolymerisable monomer by exposing the monomer to a pattern of illumination comprising an array of spots or dots of light to form, by polymerisation of the monomer, a corresponding array of microlenses, each having its optical power partly provided by graded refractive index effects within the photopolymer and partly by corresponding surface relief effects induced by the polymerisation. EP0294122 and W091/18304 describes such screen and methods of forming the same in more detail, to which reference should be had.

A further lens or optical system, represented at 20, is arranged to focus the image on the focussing screen 18 onto the imaging surface of an electronic imaging device such as a charge coupled device imager 22. The imaging device 22 is arranged, for example by a known scanning technique, to provide an electrical signal representing the image focussed onto the device 22. This electrical signal is processed by processing means 24, including a digital binary transducer, which determines, from the signal, the sharpness of the image focussed onto the device 22, for example by determining the abruptness of transitions between levels of illumination at adjoining points on the image surface of the device 22. The processing means 24, in conjunction with a servo-control device 26, controls a motor 28 which drives a focussing mechanism, illustrated schematically at 30, which moves the lens 14 towards or away from device 22 or adjusts the position of an element of the lens 14 or the effective focal length of the lens.

The system 24 is arranged to follow a focussing algorithm in which, for example, the lens 14 is traversed through all possible positions thereof to determine the position in which the image on device 22 is sharpest and is then returned to that position before an exposure is made.

The camera may, for example, be a photographic camera with a separate main lens (not shown) being used to form an image on a photographic film and with the focussing mechanism for that lens being coupled with that for the lens 14, the arrangement being such that when the image directed by lens 14 onto screen 18 is sharply focussed, the corresponding image directed onto the photographic film by the main camera lens is also in focus.

Alternatively, the screen 18 may be arranged to receive light which would otherwise have struck a photographic image plane of the camera but which has been reflected onto screen 18 by an inclined mirror, which may, for example, be retracted automatically just before an exposure is made.

It will be understood that a similar focussing system might be incorporated in such products as television cameras and camcorders, surveyors' instruments etc.

The use of very fine micro-lens arrays which can readily be produced in photopolymer sheets by the techniques referred to above allows image resolution as low as 5 microns to be achieved.

Other variations will be evident to those skilled in the art.

Claims

1. An automatic focussing system which includes housing means, a light diffusing or scrambling focussing screen disposed in said housing means, a lens for forming an image of an object on the focussing screen, the lens including at least a part which is adjustable in position in order to vary the position or effective focal length of the lens for image-forming purposes, a further imaging system arranged to focus an image of said screen onto a charged coupled device imager or other electronic imaging device, signal processing means arranged to receive a signal or signals from said imager or imaging device and to process said signal or signals to determine, from the characteristics of said signal, whether the image perceived by the imaging device is sharp or blurred, said processing device being arranged to provide an input signal to an automatic means for adjusting the focus of the camera lens.

2. An automatic focussing system according to claim 1 wherein said imaging device is a charge-coupled device.

3. An automatic focussing system according to claim 1 wherein the light-diffusing or scrambling screen is formed by selective polymerisation of a sheet of a lighttransmitting photopolymerisable monomer by exposing the monomer to a pattern of illumination with localised variations in intensity of illumination.

4. An automatic focussing system substantially as hereinbefore described with reference to, and as shown in, the accompanying drawing.

5. Any novel feature or combination of features disclosed herein.