GB2191301A

GB2191301A - Optical processing apparatus

Info

Publication number: GB2191301A
Application number: GB08608319A
Authority: GB
Inventors: Ian Robert Cooper
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1986-04-04
Filing date: 1986-04-04
Publication date: 1987-12-09
Also published as: GB8608319D0; GB2191301B

Abstract

Spatial light modulators 21 in an image processing apparatus are each preceded by an array 27 of areas of different reflectivity or transmissivity and means 29 for projecting the input image on to each area, in order to provide parallel addressing of the modulator 21 over different greyscales. The areas 27 may be semi-reflective mirrors. The modulator 21 is a ferroelectric smectic liquid-crystal cell 23 controlled by light reaching a silicon photoreceptor 25. Two such modulators are included in a four-wave mixing apparatus (Fig. 1) for forming the correlation product of two images. <IMAGE>

Description

SPECIFICATION Optical processing apparatus This invention relates to optical processing apparatus. In particular the invention relates to optical image processing apparatus.

Modern optical image processing apparatus often include programmable optical light modulators, for example liquid crystal light valves. It is a desirable feature of programmable spatial light modulators that they exhibit a greyscale capability, i.e. the capability of spatially modulating an output beam with a representation of an input image containing various shades of grey, the representation containing information relating to the shades of grey.

Whilst such programmable spatial light modulators are available, they are necessarily relatively slow.

It is an object of the present invention to provide an optical image processing apparatus incorporating a programmable spatial light modulator, the apparatus having a greyscale capability, but capable of faster operation than is normally possible.

According to the present invention an optical image processing apparatus includes: a programmable spatial light modulator; a component having a plurality of areas of different optical transmission characteristics located such that light passing through the areas addresses the modulator; and means for projecting a representation of an input image onto each of said areas.

In one particular apparatus in accordance with the invention the component comprises an array of mirrors, each of different reflectivity.

In another particular apparatus in accordance with the invention, the component comprises a liquid crystal cell, and means for varying the optical transmission characteristics in different areas of the cell.

Such another particular apparatus may suitably include a photodetector arranged to detect the range of intensities within the input image, and means for adjusting the transmission characteristics of said areas in accordance with said range.

The optical image processing apparatus is suitably an optical correlation apparatus in which the correlation product of said input image and a second image is produced.

One optical image processing apparatus in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic view of the apparatus; Figure 2 is a view of the parts of the apparatus of Fig. 1 labelled 7 and 9; Figure 3 is a representation of an input image applied to the part 7 of the apparatus; Figure 4 illustrates the equivalent binary image to that of Fig. 3; Figure 5 is a representation of an input image applied to the part 9 of the apparatus; Figure 6 illustrates the equivalent binary image to that of Fig. 5; Figure 7 illustrates the crosscorrelation peaks obtained by the apparatus from the images illustrated in Figs. 3 and 5;; Figure 8 illustrates the crosscorrelation peaks which would have been obtained from the images illustrated in Figs. 3 and 5 by an apparatus not in accordance with the invention; Figure 9 illustrates the autocorrelation peaks obtained by the apparatus from the image illustrated in Fig. 3; and Figure 10 illustrates the autocorrelation peaks obtained that would have been obtained by an apparatus not in accordance with the invention from the image illustrated in Fig. 3.

Referring firstly to Fig. 1, the apparatus is an apparatus for obtaining an image representative of the correlation product of two input images represented as 1 and 3. The apparatus includes a single crystal of bismuth silicon oxide 5 and an argon ion laser (not shown) arranged, via a system of beam splitters and other optical components (also not shown) to direct two coherent light beams, indicated as B1, B2 onto the crystal 1. The beams B1, B2 are each arranged to be spatially modulated with respective representations of the input images 1, 3 by respective parts 7, 9 of the apparatus as further described hereafter. Respective Fourier transform lenses 11, 13 are provided in the paths of the beams B1,B2 such that the Fourier transforms of the beams B1, B2 are produced in the crystal 5.A semireflective mirror 15 is provided between the lens 13 and the part 9, the function of the mirror being described hereafter.

In use of the apparatus the beams B1, 82 interact in the crystal 5 to encode an effective grating structure 6 within the crystal by which a further beam B3 counterpropagating with respect to the beam B1 may be diffracted to produce a fourth beam B4 counterpropagating with respect to the beam B2. The beam B4 will contain a representation of the product of the two Fourier transforms produced in the crystal, by the process known as four wave mixing.As the beam B4 passes through the lens 13, the lens will perform an inverse Fourier transform of the beam, the mirror 15 diverting the beam B4 towards a detector array 17, such that the light falling on the array will be representative of the correlation product of the images 1 and 3.

As so far described the apparatus is of conventional form. The apparatus is however distinguished from conventional apparatus by the form of the parts 7, 9 which are arranged to spatially modulate the beams B1, B2 with the images 1, 3. Referring now also to Fig. 2 the parts 7, 9 each include a spatial light modula tor in the form of a liquid crystal light valve 21 comprising a ferro-electric smectic liquid crystal cell 23, the optical transmission characteristics of which are varied by light falling on a silicon photoreceptor 25, a dielectric mirror and light blocking layer 22 being interposed between the cell 23 and the photoreceptor 25. Such a spatial light modulator is of limited dynamic range, i.e. is limited to binary operation, or a small number of grey levels.

On the light receiving surface of the photoreceptor 25, there is formed a two by two array of mirrors, each mirror 27 within the array having a different reflectivity and thus a different optical transmission. Each part 7, 9 further includes a multiple lens array, indicated at 29, arranged to project an image of the input image 1 or 3 onto each mirror 27 within the array.

Referring now to Fig. 3, the image 1 may be plotted in the form shown in Fig. 3, the figure showing the intensities of the various grey levels in the image as a distribution in an apparently vertical plane. The various optical transmissions of the mirrors 27 within the array, and the thresholding effect of the light valve transforms the image 1 into four intensity distributions across the array where each distribution represents a limited intensity range, the distributions thus being spread spatially over the input plane of the liquid crystal light valve 21 as indicated in Fig. 4.

The corresponding representation of the image 3 and the actual "binary" input to the liquid crystal light valve 21 incorporated in the part 9 of the apparatus are shown in Figs. 5 and 6.

The detector array 17 within the apparatus will thus measure a light intensity distribution representative of the crosscorrelation peaks of the "binary" images illustrated in Figs. 4 and 6 of the form shown in Fig. 7, where the intensity distribution of the grey levels is again shown in the "vertical" plane. This may be compared with the equivalent distribution shown in Fig. 8 for a conventional apparatus where the parts 7, 9 are replaced in their entirity by spatial light modulators of equivalent dynamic range to the liquid crystal light valve 21.

It will be appreciated that as the liquid crystal light valves 21 within the parts 7, 9 are effectively addressed "in parallel" by light passing through the nine mirrors 27 within each array, the processing speed of each of the two light valves is greatly increased over that which would be required for a conventional apparatus having an equivalent greyscale capability.

A further illustration of the operation of the apparatus is provided in Fig. 9 which shows a representation of the autocorrelation peaks monitored by the detector array 17, where the beams B1 and 82 are both spatiaily modulated with a representation of the image 1 by means of the part 7 and an appropriate beam splitting system. The distribution shown in Fig. 9 may be compared with the corresponding representation of the autocorrelated peaks measured in a conventional apparatus.

It will be appreciated that whilst a two by two mirror array in each part 7, 9 has been described in the apparatus in accordance with the invention described herebefore by way of example, each array may have a different number of mirrors dependent on the required greyscale resolution required.

It will also be appreciate that whilst a mirror array is a particularly convenient way of producing a component having a plurality of area 3 of different optical transmission characteristics. many other such components are possible. These include a filter having a spatially varying transmission coefficient, or a liquid crystal cell, which is switched in different areas of the cell to achieve the required varying transmission coefficient. Where such a cell is used, the apparatus may conveniently include a photodetector arranged to detect the range of grey shades within an input image, and to vary the transmission characteristics accordingly.

It will also be appreciated that whilst the invention has particular use in an apparatus for obtaining an image representative of the correlation product of two input images using a four wave mixing process, the invention is relavent to either optical image processing apparatus, for example correlation apparatus using "wet-processed" holographic filters where for example updateable operation is not required.

It will further be appreciated that the invention has application in other optical image processing apparatus, other than "correlation" apparatus.

Claims

1. An optical image processing apparatus including: a programmable spatial light modulator; a component having a plurality of areas of different optical transmission characteristics located such that light passing through the areas addresses the modulator; and means for projecting a representation of an input image onto each of said areas.

2. An apparatus according to Claim 1 in which the component comprises an array of mirrors, each of different reflectivity.

3. An apparatus according to Claim 1 in which the component comprises a liquid crystal cell, and means for varying the optical transmission characteristics in different areas of the cell.

4. An apparatus according to Claim 3 including a photodetector arranged to detect the range of intensities within the input image, and means for adjusting the transmission characteristics of said areas in accordance with said range.

5. An apparatus in accordance with anyone of the preceding claims in which the apparatus is an optical correlation apparatus in which the correlation product of said input image and a second image is produced.

6. An optical image processing apparatus substantially as hereinbefore described with reference to the accompanying drawings.