GB2158602A - Optical processing apparatus - Google Patents

Abstract

An optical image processing apparatus in which first (B2) and second (B3) coherent light beams are directed into a quantity of an optically non-linear material (7) so as to form an effective grating (13) in the quantity (7), one of the beams being spatially modulated with a representation of an image. A read-out beam (B1) is directed onto the grating so as to produce an output beam (B4), fifth beam (B5) also being directed onto the grating (13) so as to modify the output beam. <IMAGE>

Description

SPECIFICATION Optical processing apparatus.

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

In optical image processing it is often necessary to perform image modification, for example subtracting of a first image from a second image, or spatial filtering of an image. It is particualrly desirable to perform this operation in a read-time mode to enable for example the subtraction of a pair of images in which at least one of the images is rapidly changing with time.

It is an object of the present invention to provide an optical image processing apparatus capable of real- time image subtraction.

According to the present invention an optical image processing apparatus comprises: means for directing first and second coherent light beams onto a quantity of an optically non-linear material so as to form an effective grating structure in the quantity, at-least one of the beams being spatially modulated with a representation of a first image, means for directing a read-out light beam onto said grating so as to generate an output beam; and means for directing a fifth light beam onto said grating so as to modify said output beam.

A first particular apparatus in accordance with the invention includes means for spatially modulating said fifth beam with a second image. In such an apparatus the output beam will be spatially modulated with an image representative of the second image subtracted from the first image.

A second particular apparatus in accordance with the invention includes means for varying the intensity of said fifth beam. In such an apparatus, the intensity of the output beam will vary as the intensity of the fifth beam varies.

A third particular apparatus in accordance with the invention includes means for spatially modulating said fifth beam with a second image, and means for producing the Fourier Transform of said first image in the quantity. In such an apparatus the fifth beam will then modify the spatial frequency content of the image in the output beam.

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(a) is a view of a first transparency for use in the apparatus; Figure 2(b) is a view of a second transparency for use in the apparatus; and Figure 2fcJ is a view of the image contained in the output beam of the apparatus.

Referring firstly to Figure 1, the apparatus includes a laser (not shown), and a system of beam splitters (not shown) arranged to produce three coherent input beams B1, 82 and 83 as indicated in the figure. The beams B1 and B2 are expanded in relation to the original laser beam, and are arranged to be counter propagating and to converge in a single crystal of bismuth germanium oxide 7.

The beam B3 is arranged to pass through a transparency 9 and then through a lens 11, to be focussed in the crystal 7.

As so far described the apparatus is of known form and in use of the apparatus the beams 82 and 83 interact within the crystal 7 to form an effective grating structure 13, the beam B1 forming a read-out beam which diffracts from the grating to give an output beam 84. This is the phenomenom known as wave mixing in an optically non-linear material, and with particular configuration of beams B1, 82, 83 shown the beam 84 propagates in the opposite direction to the beam B3, to be split off from the beam 83 by a beam splitter 15 so as to be capable of inspection. The beam B4 is the phase conjugate of the beam B3, and will thus be spatially modulated with an image corresponding to the pattern on the transparency 9.

In the apparatus in accordance with the invention, however, the laser is arranged to provide a further beam B5 as indicated in Figure 1, the intensity of the beam B5 being appreciably higher than that of beam 83. This beam is arranged to pass through a transparency 17, and then be deflected by a mirror 19 such that it passes through the lens 11 to produce an image of the pattern on the transparency in the crystal 7 which overlaps the image produced by the beam B3. The beam B5 has two effects on the grating structure 13, and thus the output beam, 84. The first effect is a 'grating erasure' effect in which the beam B5 is effective to wash-out the grating, this effect occuring with either coherent or incoherent light.The second effect is a 'grating erasure' effect, and occurs only with coherent light such as the beam B5. This grating coupling effect can be considered as an interaction between two similar photoinduced gratings in the crystal 7, one formed from the interaction of beams B1 and 83, and the other from the interaction of beams B1 and B5. With the particular apparatus described in which the beam B5 is spatially modulated with an image corresponding to the pattern on the transparency 17, the effect of the beam B5 will be to cause the output beam 84 to be spatially modulated with an image corresponding to the image contained in the beam B5 subtracted from the image contained in the beam 83.Thus if the pattern on the transparency 9 is of the form shown in Figure 2(a), and the pattern on the transparency 17 is of the form shown in Figure 2(b), the image contained in the output beam 84 will be of the form shown in Figure 2(c).

It will be appreciated that by altering the position of the beam B5 such that the image of the pattern on the transparency 17 moves with respect to the image of the pattern on the transparency 9 within the crystal 7, the image of the pattern on the transparency 17 will subtract from a different part of the image of the pattern on the transparency 17.

It will be also appreciated that one or bothof the transparencies 9, 17 may be substituted for by a programmable spatial light modulator to enable real time image subtraction where the images produced by the or each spatial light modulator is changing with time.

Furthermore the transparency 17 may be replaced by a means for varying the intensity of the beam B5. Thus by varying the intensity of the beam B5 relative to that of the beam B3, grey-scale image subtraction from the image in the beam 83 can be achieved.

Whilst the apparatus described herebefore performs subtraction of two images, it will be appreciated that the apparatus can also be used in a Fourier Transform geometry. If instead of imaging the pattern on the trnasparency 9 into the crystal 7, the Fourier Transform of the pattern is produced by placing the transparency 9 in the back focal plane of the lens 11, and the crystal 7 in the front focal plane, then the beam B5 may be used to modify the spatial frequency content of the image contained in the output beam 84. For example if the image in the beam B3 has a range of spatial frequencies from zero to a maximum value the resulting Fourier Transform will have a bright central order, due to the zero and low spatial frequency components, and dimmer high orders due to the higher spatial frequencies present.Thus if the transparency 17 is imaged onto the crystal 7 some portions of the Fourier Transform will be removed resulting in a modified spatial frequency distribution in the output beam B4. In particular if the transparency 17 is in the form of a pin hole, it will be possible to remove the central order of the Fourier Transform, thus enhancing the high spatial frequencies in the output beam B4. Thus real-time and 'programmable' spatial filtering may be achieved. This is particularly useful when the apparatus is adapted for use as an image correlator by spatially modulating the beam 82 with a reference image, and forming the Fourier Transform of this reference image in the crystal 7. The beam B4 will then be spatially modulated with the correlation product of the image in the beam 82 and the image produced by the transparency 9 in the beam 83.The beam B5 may thus be used to enhance the effects of the higher spatial frequency components in the correlation product, and thus enhance the discrimination over cross-correlation signals in the beam B4.

It will be appreciated that further modifying beams equivalent to beam B5 may be provided in the apparatus to achieve multiple image subtraction. This could be performed either simultaneously or sequentially.

It will also be appreciated that whilst in the apparatus described hereinbefore by way of example, the readout beam B1 is coherent with the beams B2 and 83 forming the grating, the invention is equally applicable to apparatus in which the readout beam is not coherent with the beams producing the grating, and indeed is not the same wavelength.

It will also be appreciated that the modifying beam is not necessarily coherent with the beams producing the grating, as long as the intensity of the modifying beam is substantially greater than the intensity of the beams forming the grating.

Claims (6)

1. An optical image processing apparatus comprising: means for directing first and second coherent light beams onto a quantity of an optically non-linear material so as to form an effective grating structure in the quantity at least one of the beams being spatially modulated with a representation of a first image, means for directing a readout light beam onto said grating so as to generate an output beam; and means for directing a fifth light beam onto said grating so as to modify said output beam.

2. An apparatus according to Claim 1 including means for varying the intensity of the fifth beam.

3. An apparatus according to either of the preceding claims including means for spatially modulating the fifth beam with a second image.

4. An apparatus according to Claim 3 in which the representation is the Fourier Transform of the first image.

5. An apparatus according to any one of the preceding claims in which the fifth beam is coherent with the first and second beams.

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