GB1595587A - Optical sensors - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO OPTICAL SENSORS (71) 1, THE SECRETARY OF STATE FOR DEFENCE, Whitehall, London, SWIM, 2HB, a British Corporation Sole, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to optical sensors, and more particularly to optical sensors adapted for preferential sensitivity to objects in their field of view which have preselected spatial characteristics.

A major problem in the design of an optical sensor consists of desensitising the sensor to unwanted signals or clutter whilst retaining sensitivity to objects of interest. In particular, great difficulties have been experienced in locating spatially small sources or radiation in the presence of a background of direct or reflected sunlight, and in distinguishing between the spatial characteristics of differently shaped objects.

The present invention provides a means for distinguishing between objects and radiation sources on the basis of differences between their spatial characteristics.

According to the present invention, an optical sensor for detecting an object of given spatial characteristics in a field of view includes a first reticle, optical focussing means arranged to produce a Fourier transform of the first reticle, the transform being superimposed on a second reticle located at an image position of the optical sensor with respect to the field of view, wherein the first and second reticles form a mutually complementary pair with respect to sensitising the optical sensor to objects of given spatial characteristics, and an eyepiece or detector means positioned to receive light after traversal of the focussing means and the reticles. Very conveniently, the second reticle is located at a focal plane of the optical focussing means.

It will be apparent to those familiar with Fourier transform optics that it is possible to choose reticle pairings to achieve the required sensitivity.

In a preferred embodiment of the invention, the second reticle comprises a central opaque disc within an annular translucent section. The translucent section may conveniently be an optical filter. The first reticle is preferably a two-dimensional lattice reticle.

In an alternative preferred embodiment, the invention further comprises means to introduce relative motion between the first and second reticles. The relative motion is arranged to be appropriate to the complementary nature of the two reticles, and may be nutatory, oscillatory or a cycloidal scanning motion. Further alternatives include changing the separation or inclination between the reticles. Very conveniently however, the relative motion comprises rotation of one reticle with respect to the other. The relative motion is preferably produced by means acting on the first reticle, the second reticle either being stationary with respect to the field of view or modulating the field of view at a much slower rate than the first reticle.

Conveniently, the focussing means is a tele-photo lens. Alternatively, the focussing means may be a Cassegrainion, Newtonian or other telescope arrangement such as a catadioptric system. Conveniently also the optical sensor includes a field lens to focus light from the second reticle onto the detector means.

In an optical sensor adapted to detect a small target source of electro-magnetic radiation against a background, the first reticle preferably comprises periodic grid, where periodic grid as herein used is defined as a one dimensional grid having parallel equispaced bars. The second reticle may comprise opaque spokes having translucent regions therebetween and radiating from a common centre or may comprise a sectored disc in which the sectors are alternately opaque and translucent. The second reticle may alternatively comprise opaque spiral arms diverging from a common centre and defining translucent regions therebetween.

The translucent regions and the opaque spokes, sectors or arms may be replaced by optical filter materials giving each a respective wavelength-dependent optical transmissivity.

In a further embodiment, the invention comprises an optical sensor according to the last preceding paragraph adapted to distinguish between two spatially small light sources of similar geometry but differing spectral characteristics, the said sensor including a third reticle positioned in the path of light from the scene when passing through the optical sensor, the third reticle being located with respect to the focussing means such that a Fourier transform thereof is superimposed on the Fourier transform of the first reticle, the third reticle comprising a one-dimensional grid arrangement offset by rotation from parallelism with the first reticle, the said grid comprising strips transparent to both light sources disposed alternately with optical filter strips having neutral, non-zero transmissivity with respect to the spectrum of the first light source and wavelength-sensitive transmissivity with respect to the spectrum of the second light source, and the third reticle being movable either synchronously or asynchronously with respect to the first reticle. Preferably the third reticle is rotated in synchronism with the first reticle.

An optical sensor according to the last preceding paragraph is of major importance for the detection of a monochromatic or narrow band first light source, such as for example a laser, in the presence of a broadband second light source such as a reflection of sunlight from a small object.

The first and third reticles produce a twodimensional Fourier transform of the broadband second light source but only a one-dimensional Fourier transform of the narrow band first light source. Synchronous rotation of the first and third reticles produces modulation of the second light source by the second reticle at twice the corresponding frequency of modulation of the first light source, provided either that the second reticle has an odd number of transparent spaces or that the bars of the first and third reticle are not mutually perpendicular. Asynchronous relative rotation or other motion of the first and third reticles produces a changing pattern on the second reticle.

Thus in accordance with a further embodiment of the invention, the detector means is connected to an electronic filter circuit adapted to discriminate between the modulation frequencies produced by motion of the first and third reticles relative to the second reticle, the said motion being preferably synchronous rotation.

In accordance with a still further embodiment, the invention comprises a first reticle, optical focussing means arranged to produce a Fourier transform of the first reticle upon a second reticle located at a focal plane of the focussing means, means to rotate the first reticle on its axis, a cascaded arrangement of optical filters positioned to receive light from the second reticle, each filter being arranged to transmit a respective narrow wavelength band of the light incident upon it and to reflect the non-transmitted light, and a detector positioned to receive light transmitted by at least one of the filters. In one form a detector may be located behind each of the filters in the cascade so that each detector responds only to the narrow band of optical frequencies transmitted by its respective filter.In an alternative form having only one detector and using two filters, the second reticle can be formed as a reflective coating on the first of the two cascaded filters with a mirror being provided to direct reflected light to the second filter, the light transmitted through the two filters being focussed onto the detector. In this latter form the sensor can conveniently be adapted to discriminate against sun glints by providing an optical wedge attenuator in the optical path, the attenuator being adjustable so as to balance the transmission of light through the two filters to thereby desensitise the sensor to sun-light.

In order that the invention might be more fully understood, embodiments thereof will now be described, by way of example only, with reference to the drawings accompanying the provisional specification, in which: Figures 1 and 2 illustrate the production of optical Fourier transforms, Figure 3 shows a side elevation of an optical sensor of the invention comprising two reticles, Figure 4 shows front views of the reticles of Figure 3, Figure 5 shows an optical sensor of the invention comprising three reticles, Figures 6 (a) to 6 (c) illustrate optical filter characteristics appropriate for reticles for use in optical sensors of the invention, Figure 7 illustrates a means for filtering out unwanted broad-band light sources, Figure 8 shows a multi-channel optical sensor, and Figures 9 (a) to 9 (d) illustrate a means for distinguishing objects in the presence of dazzling light sources.

Referring to Figure 1, a biconvex lens 10 is shown in the x-y plane disposed symmetrically about a z-axis 11, x, y and z directions defining a Cartesian coordinate system. An object in the form of a square lattice reticle 12 is illuminated by a coherent light source (not shown) and produces an image 13 at the image plane of the lens 10.

The object reticle 12 and its image 13 lie in respective x-y planes xiy, and x3y3. A Fourier transform 14 of the object reticle 12 is produced at the focal plane x2y2 of the lens 10, the focal plane intersecting the zaxis at right angles at the point 15. The Fourier transform 14, appropriate for the present case of a square lattice illuminated by a coherent source, is a square array of dots 17, the relative sizes of the dots 17 indicating the size of the region illuminated.

The Fourier transform is produced by the action of the lens 10 as a two-dimensional Fourier transformer of any object or the projection thereof at the aperture plane of the lens 10. The line 18 of dots along the y2 axis corresponds to the complex exponential components of the periodic structure of the lattice reticle 12 along the xl direction, the horizontal bars of the lattice 12. Similarly, the line 19 of dots along the x2 axis corresponds to the vertical bars of the lattice 12 parallel to the y, direction. The dots 17, away from the axes x2 and y2, represent the spatial periodicity of the lattice reticle 12 along axes within its plane inclined to both the x2 and y2 axes.

In Figure 2, the use of the Fourier transforming properties of a lens is demonstrated. A single dimensional grid reticle 20 comprising parallel equispaced bars 21 lies in the plane x,yl disposed symmetrically about the z-axis 22. Figure 2 shows the grid reticle 20 rotated into the plane of the figure to demonstrate its geometry, making the x and z axes coincident. However, this rotation is merely to aid clarity and in operation the system of Figure 2 comprises the reticle 20 side-on, i.e. rotated into the plane of the figure. A converging lens 23 is arranged symmetrically about the z axis 22. A field of view (to the left of Figure 2, not shown) illuminates the lens 23 through the reticle 20, the field of view being a large distance, effectively infinite, from the lens. The reticle 20 is arranged to be close to the aperture of the lens 20.This arrangement gives rise to the Fourier transform of the grid reticle 20 being superimposed on the image of the field of view, both the transform and the image being located at the focal plane x2y2. The image is represented by a circle 24, and both the circle 24 and the transform 25 are rotated into the plane of the drawing for clarity.

If the arrangement of Figure 2 is illuminated by a point monochromatic source of light within the field of view at infinity, then the energy distribution in the Fourier transform 25 in the focal plane x2y2 is a linear array of spots of light. If the bars 21 of the reticle grid 20 intersect the xz plane at an angle 0, then the linear array of spots of the transform 25 is inclined at an angle of (90+0) to the x2z plane, this of course being the same plane. The spacing between the spots of the transform 25 is a function of the wavelength of the illuminating point source. If the point source is not monochromatic but broadened out slightly, a number of overlapping arrays of dots are produced which appear as a line of light with superimposed intense spots. If the point source gives a broad-band emission spectrum, the bright spots disappear leaving the line of light.

The illumination of the arrangement of Figure 2 by a background field of view does not give either an array of intense spots or a line of light since the background field of view corresponds to illumination composed of an infinite number of adjacent widely distributed radiators. The effect of the reticle grid 20 on the light from the scene is that of an aperture stop reducing the intensity reaching the focal plane x2y2.

A point source of monochromatic light may be described mathematically as a Dirac delta function in both space and wavelength. In two spatial dimensions x and y appropriate to an optical field of view, such a function would be 6(x, y;A), where A is the wavelength. Point broad-band sources may correspondingly be represented by S(x, y); f (A), whereas F(x, y;A) represents a typical background which is broad spectrally and spatially. As illustrated by Figures 1 and 2, the present invention distinguishes between light sources on the basis of the Fourier transform pattern of a reticle illuminated by the source. The Fourier transform process immediately distinguishes between the various spatial characteristics.

In Figures 3 and 4, a practical optical sensor of the invention is illustrated. A telephoto lens 31 is disposed about an axis 32 and a first reticle in the form of a periodic grid 33 is located in the input aperture plane 34 of the lens 31. The aperture plane 34 is perpendicular to the plane of Figure 3. A second reticle in the form of a spoked wheel 35 is located in the back focal plane 36 of the telephoto lens 31, and a biconvex field lens 37 images the second reticle 35 onto a detector 38. The reticles 33 and 35 are illustrated in side elevation in Figure 3 and in front elevation in Figure 4. The reticles 33 and 35 consist respectively of linearly and radially equispaced opaque bars defining translucent spaces therebetween.The bars 39 of the first reticle 33 are inclined at an angle 0 to the horizontal plane, this being the plane perpendicular to the plane of the drawing. The Fourier transform of the reticle 33, with respect to illumination by a point source of light, is a line 40 inclined at an angle (90+0) to the horizontal, and this transform is superimposed on the spoked wheel reticle 35 in the back focal plane 36.

A field of view containing a point source of light illuminates the reticle 33 from the left of Figure 3. A motor drive (not shown) is employed to rotate the grid reticle 33 about its central axis, this being the axis 32 of the arrangement of Figure 3. The rotation of the reticle 33 produces a corresponding rotation of the Fourier transform line array 40. The line array 40, is a line of light or an array of dots according to whether the point source is broad-band or monochromatic. The light from the rotating line array is modulated by the stationary spoked wheel reticle 35 before reaching the detector 38, and the detector generates an ac electrical signal at the frequency of the modulation. A sectored disc reticle having alternate opaque and transparent sectors is an alternative to the spoked wheel reticle 35 (see e.g. Figures 5(a) and 7).

The image of the field of view is stationary with respect to the stationary spoked wheel reticle 35, and therefore light from the field of view is not modulated by this reticle as the grid reticle 33 rotates. The arrangement of Figure 3 may in some applications be scanned across a field of view, in the manner of a searching telescope or radar aerial. In this case there will be some relative motion of the reticle 35 and the field of view which will produce an image having some corresponding modulation. The criterion appropriate for the rotational rate of the first reticle 33 is that it be sufficiently fast that any corresponding motion of the second reticle 35 is negligible. When this criterion is met, electronic filtering of the detector output signal can be used to isolate the modulation attributable to the first reticle alone.This provision ensures that the arrangement of Figures 1, 3 and 4 is sensitised to small or point sources in a scene but insensitive to the scene itself, and the sensitivity is broadband as regards the spectrum of the light emitted by the source.

The Figure 3 arrangement described above consists of two reticles 33 and 35 which are mutually complementary with respect to producing modulation of a point source of light. Other reticle pairings may also be employed which are mutually complementary with respect to the spatial properties of other light emitting or illuminated objects in the field of view of the optical sensor. To sensitise the optical sensor preferentially to objects of a particular geometry, the first reticle is arranged to correspond to the Fourier transform of the corresponding real image of the object at the focal plane. The second reticle is then chosen to be suitably complementary to the first reticle, in accordance with the laws of Fourier optics.

Although simple rotation of the first reticle with respect to the second was described above, other forms of relative motion may also be used. The relative motion may be nutatory, oscillatory or cycloidally scanned. Further alternatives include changing the relative separation or inclination between the recticles. Each form of relative motion produces a respective type of modulation; for most purposes however simple rotation is the most convenient.

A version of the apparatus illustrated in Figure 3 was constructed with a periodic grid reticle 33 having three opaque bars per millimetre. The stationary reticle was a sectored disc (see later, Figure 5(a)) of alternate opaque and transparent sectors with a mark-space ratio of one-to-three (opaque to transparent). The focal length of the telephoto lens 31 was 250 mm which gave a field view of 6" by 6". It was found that this arrangement produced an easily detected linear array of intense light spots from the emission from a one milliwatt helium-neon laser situated at a range of + kilometre. The laser was monochromatic and appeared as a point source by virtue of its small size and location remote from the telephoto lens.

As has been mentioned, the periodic grid reticle produces either a linear array of light spots or a line of light at the spoked wheel reticle according to whether the point light source is narrow-band or broad-band. To achieve discrimination between narrow and broad-band sources, the spoked wheel reticle is replaced by a reticle having a number of opaque spiral arms diverging from a common centre, a multi-start spiral arrangement. This produces selective modulation of the linear array of light spots without a corresponding modulation o the uninterrupted line of light. However, in typical outdoor environments, it may prove difficult to provide apparatus sufficiently sensitive to resolve the spots of light adequately, and, while quite feasible, such an arrangement may not give an acceptable degree of modulation of the detector signal as the grid reticle rotates. The use of optical filter techniques is hereinafter described as an alternative means of distinguishing between light sources of differing spectral characteristics.

Figure 5(a) illustrates the use of a third reticle to sensitise the optical sensor preferentially to monochromatic or narrow bandwidth light sources. As in Figure 4, the three reticles shown are rotated into the plane of the drawing for clarity. A first reticle 51 is employed in the form of a periodic grid of opaque bars, as in Figures 3 and 4. A second reticle 52 comprises a sectored disc having alternate opaque and transparent sections, and produces similar modulation of a rotating line of light as the spoked wheel reticle of Figures 3 and 4, although the mark-space ratio of the resulting modulations in the two cases is different.

A third reticle 53 is positioned close to the first reticle 51 so that both these reticles lie effectively for optical purposes in the aperture plane of the telephoto lens system (not shown, see Figure 3). It will be appreciated by those skilled in the art of optics that a third reticle can easily be positioned to satisfy this condition. The reticle 53 is a one-dimensional parallel grid array of transparent bars and optical filter bars 54. The filter bars 54 are transparent to the monochromatic or narrow-band radiation emitted by the source to be detected, such as a laser or an incandescent gas. The filter bars 54 are highly opaque in at least a part of the spectrum of broadband point sources of light, such as reflectors of sunlight.The detailed optical characteristics of the filter bars 54, i.e. the variation of transmission as a function of wavelength, are not very critical provided that the third reticle 53 appears as a onedimensional periodic grid to broad-band radiation, and as a transparent sheet to the narrow-band radiation of interest. A detector 55 is located to receive light which has passed through the second reticle 52.

The combination of the first and third reticles 51 and 53 acts as a two-dimensional grid or lattice to broad-band sources of light, and as a one-dimensional grid to narrow-band sources of light of the wavelength to which the filter bars 54 are transparent. The Fourier transform patterns of light produced by small sources of light illuminating this combination of reticles are shown in Figure 5(b). A small narrow-band source produces a line array 56 of intense spots, and a small broad-band source produces a cross pattern 57 corresponding to a two-dimensional Fourier transform.

The first and third reticles 51 and 53 are now rotated in synchronism, thereby rotating the line array 56 and cross pattern 57 against the sectored disc of the second reticle 52. Radiation reaching the detector 55 from the narrow-band source is modulated at the frequency NR, where N is the number of opaque sectors in the second reticle 52, and R is the rotational rate of the first and third reticles in revolutions per second. Radiation reaching the detector 55 from the broad-band source however is modulated at the frequency 2NR, twice the frequency of modulation of the narrowband source. To distinguish between narrow-band and broad-band light sources therefore, the detector output is filtered electronically to separate the frequencies NR and 2NR.It is to be noted that to achieve the broad-band modulation frequency of 2NR, the contributions from the longitudinal and transverse arms of the cross pattern 57 must necessarily be at or near antiphase; otherwise, these contributions will merely add to give an ac signal of 2rosin NRt, instead of the required Rosin 2NRt, where To is a voltage amplitude coefficient and t is the time in seconds. The antiphase criterion can be satisfied by the sectored disc reticle 52 having an odd number of opaque sectors. Alternatively, if the number of opaque sectors is even, then the bars of reticle 53 must be offset from the perpendicular to the bars of reticle 51.

Preferably, the number of opaque sectors in the reticle 52 is odd.

The first and third reticles need not, as described above, rotate synchonously if other forms of modulation of the detector output are required. For example, if these two reticles are rotated in opposite directions, changing patterns will be produced at the second reticle, and a correspondingly varying modulation would be achieved. It is also possible to employ linear or rotational oscillatory motion of the first and third reticles with respect to one another, nutatory motion, cycloidal scanning and so on. Each form of relative motion produces a respective modulation pattern which might be appropriate to particular circumstances. In addition, reticles producing more complex complementary Fourier transforms may be employed, these being chosen to give particular modulation waveforms to objects of preselected spatial characteristics.

However, the foregoing arrangement of equispaced grid reticles 51 and 53 together with synchronous rotation thereof provides, a simple and effective means for achieving preferential sensitivity to small light sources of particular spectral properties.

The previously mentioned optical arrangement of a 250 mm telephoto lens and a first grid reticle (e.g. 33 and 51) of three bars per millimetre was used to test the ability of the invention to discriminate between narrow-band and broad-band light sources. A third grid reticle 53 having optical filter bars 54 was not available for test purposes, so a simulated comparison was made between the images produced by the arrangement of Figure 3 with no third reticle present and the arrangement of Figure 5 with the third reticle having totally opaque bars 54 and being identical to the first reticle. This comparison gives data equivalent to a Figure 5 arrangement using a third reticle 53 with optical filter bars 54 opaque to the broad-band source but transparent to the narrow-band source.

It was found that the two reticle arrangement produced a one-dimensional line array of well-resolved intense spots from a laser light source, whereas the three reticle arrangement gave a very distinctly different two-dimensional array of spots for a heliograph acting as a broad-band point light source. Clearly, these two types of image are easily distinguished from each other by the electronic filtering technique described above, and the operation of this embodiment of the invention is demonstrated by this example.

It will be appreciated by those familiar with optical sensors employed in surveillance or target tracking roles that the ability to discriminate between narrow and broad-band sources is a very important feature of the invention. In particular, small broad-band sources such as sun-glints (solar reflections from small objects) are distinguishable from laser sources. Sunglints normally correspond to unwanted or "false" targets in an optical sensor used for laser detection and systems arranged to filter out sun-glints necessarily exhibit much lower false alarm rates than systems lacking this filtering.

The light energy from a target is modulated in the Figure 5(a) apparatus by the relative movement between the Fourier transform of a reticle illuminated by the target and the stationary reticle, as has been described. However, the modulation waveform depends on the position of the target within the field-of-view of the system.

In the arrangement of Figure 5(a) a pure sinusoidal frequency is obtained only for targets which are accurately aligned with the axis of the optical system illustrated.

Targets off this axis generate asymmetric waveforms at the detector. However, the asymmetry merely adds overtone harmonics to the fundamental frequency NR generated by a point source of light; these harmonics can be removed by a highfrequency reject filter designed for example to suppress the 2N R frequency characteristic of broad-band sources.

Alternatively, the frequency spectrum of the detector signal may be extracted by the use of an electronic spectrum analyser, instead of merely filtering out all but the fundamental frequency. It is then possible to detect and discriminate among many different sources by inspecting the spectra of the corresponding detector signals.

The waveform asymmetry mentioned in the last paragraph makes it possible to design combinations of rotating and stationary reticles so that the electrical phase and the frequency spectrum of the resultant modulation waveform indicates the position of the target with respect to the central axis of the optical system. It is then possible to feed back the electrical phase, frequency and signal magnitude to a servo motor controlling the viewing direction of the optical sensor. Such a system could be arranged so that all frequencies except the fundamental NR disappear. The system would then seek and lock-on to a detected object or target.

The distribution of energy along the line array 56 shown in Figure 5(b) is a function of the spatial frequencies of the first reticle (e.g. reticles 33 and 51) and the spectral content of the source. The reticles 33 and 51 each have a single spatial frequency since their opaque bars are equispaced, but it is by no means essential to use a reticle having only a single spatial frequency. To produce a modulated signal, the reticle 51 is rotated on its axis to rotate the linear array of spots of light over the surface of the stationary reticle 52. It is apparent that the central intense spot of the linear array is not modulated by the rotation, since this spot lies on the rotational axis. It is therefore evident that the magnitude of modulated signal produced by the detector is directly related to the energy distribution in the linear array.In particular, it is advantageous to design the reticle 51 so that as high a proportion as possible of the energy from a light source falls outside the central spot of the linear array. Moreover, mathematical considerations indicate that a fairly rapid truncation of the higher terms in the spatial Fourier series will prevent a significant amount of energy being lost outside the field of view of the detector.

Figure 6 (a), (b) and (c), illustrates optical filter characteristics suitable for the filter bars 54 of the third reticle 53 shown in Figure 5(a). Figures 6(a), 6(b) and 6(c) are plots of relative transmission coefficient S(A) plotted against wavelength A for filters suitable for particular surveillance applications.

In Figure 6(a), the characteristic shown represents filter bars 54 which are opaque to radiation in the range A, to 2, as indicated by the absorption curve 61. A working range 62 comprises the range A2 to A3, and the spectrum of a broad-band light source is indicated by the curve 63. For an optical sensor employed as a conventional surveillance system, the broad-band light source will be the sun emitting the broadband solar spectrum. Sun-glints and point reflectors in the spectral range i, to A2 will appear at twice the modulation frequency of point or small sources in the spectral range i2 to 3 for a Figure 5(a) arrangement with corresponding filter bars 54.

Figure 6(b) shows optical filter characteristics appropriate for filter bars 54 in a narrow-band surveillance version of the arrangement of Figure 5(a). The filter bars 54 transmit radiation in the passband 64 defined by the wavelength interval A, to and are opaque to radiation outside this interval. This filter characteristic makes the Figure 5(a) arrangement sensitive only to point or small sources of light emitting in the region A, to A2. All other sources will give a cross pattern of light as shown in Figure 5(b), whereas a point source emitting light restricted to the wavelength interval A to i2 gives a line pattern of spots of light.

Figure 6(c) shows an optical filter characteristic for the filter bars 54 suitable to produce sensitivity to light in three particular wavelength regions. The filter characteristic has four bands 66, to which the filter bars 54 are transparent, and three bands 67 to which the bars 54 are opaque.

Accordingly, a line pattern (see Figure 5(b)) is generated by the first and third reticles 51 and 53 of Figure 5(a) for sources emitting light at wavelengths in the bands 66 whereas cross array patterns are generated for corresponding wavelengths in the bands 67. A laser search system corresponding to Figure 5(a) and the filter characteristic of Figure 6(c) may accordingly be devised to monitor a scene occupying its field of view for the presence of three separate laser or other narrow-bandwidth light sources.

Figure 7 shows a modification which can be added to the apparatus shown in Figures 3 and 4 for the purpose of eliminating any residual imbalance in the broad-band source (e.g. sun-glint) producing a spurious contribution to the signal from the narrowband source. The stationary second reticle 71, a sectored disc similar to the second reticle 35 in Figure 3, is in the form of a reflecting coating on the surface of a filter 72 as indicated by the arrow 73. The reticle 71 consists of alternate reflecting and transparent sectors; the filter 72 is transparent to the narrow band light source to be detected, but transmits a first section only of the emission spectrum of the broadband source.The light reflected from the reflecting sectors of the reticle 71 is directed to a mirror 74, and thence through a second filter 75 which transmits a second section of the emission spectrum of the broad-band source. The second filter 75 is necessarily opaque to the narrow band light source. The light then passes through an attenuator wedge 76 of neutral or wavelength-independent optical density, and the light from both filters 72 and 75 is focussed by a field lens 77 onto a detector 78.

The wedge attenuator 76 is employed to balance the light energy received by the detector 78 from the two different sections of the emission spectrum selected by the filters 72 and 75. Accordingly, irrespective of the position of the rotating reticle 33 in Figure 3, the detector receives a fraction of the broad-band radiation throughout each cycle of rotation, either via the filter 72 or via the filter 75. Since the wedge 76 balances the intensities transmitted by the two filters, there is no detected variation in the broad-band source intensity during each rotational cycle. Consequently, the electrical signal produced by the detector 78 in response to the broad-band source is unmodulated. The detector however receives light from the narrow-band source only through the filter 72, the filter 75 being opaque to this source.The light from the narrow-band source is therefore modulated by the stationary reticle 71 during rotation of the first reticle 33 of Figure 3. The detector 78 accordingly produces an ac signal in response to light from the narrowband source whereas the corresponding signal from the broad-band source is dc.

Use of the Figure 7 arrangement in combination with the apparatus illustrated in Figures 3 and 4 is an alternative or an addition to the use of a third reticle 53; the broad-band light source is intended to be unmodulated by the stationary reticle 71 irrespective of the type of pattern produced by light from the broad-band source after transmission through the reticles 51 and 53.

However, a slight modulation may occur in practice due to minor design in accuracies, and the Figure 7 arrangement provides supplementary or alternative means to reduce residual modulation to zero.

The arrangement illustrated in Figure 7 is capable of adaption to compensate for changes in the spectrum of the broad-band source. In one particularly important application of the invention, it is required to detect narrow-band light sources such as lasers against varying solar background spectra. For example, the light received by looking towards the setting sun is predominantly red or shifted to longer wavelengths, while away from the setting sun the light received is predominantly blue or short wavelength, a consequence of the Rayleigh scattering law. As a result, broadband intensities which are balanced in the arrangement of Figure 7 for one viewing direction will not necessarily be balanced when the viewing direction is altered.An adjustment to the balance may be provided by detecting the two intensities separately, and by using the difference between the detected intensities to drive a servo. The servo is employed to adjust the position of an attenuator (e.g. the wedge 76 in Figure 7) in one of the light paths so that the balance may be re-established. The servo therefore provides for continuous adjustment of the null balance, so that the laser search system performance remains independent of viewing direction.

Figure 8 shows a further embodiment of the invention adapted to detect several narrow-band light sources independently.

Fourier transformed light from the grid reticle -- telephoto lens -- spoked wheel reticle combination shown in Figure 3 is arranged to fall on a cascaded arrangement of optical filters 81 to 85, each of the filters being a narrow-band interference filter etalon. The filter 81 transmits light in a first narrow-band wavelength interval. This interval is arranged to contain the emission wavelength of one only of the narrow band sources to be detected, and the nontransmitted light incident on the filter 81 is reflected to a second filter 82.Similarly, the filter 82 transmits a wavelength interval corresponding to a second narrow bandwidth source, and successive filters 83, 84 and 85 receive light reflected from the respective previous filter, transmit a respective narrow wavelength interval appropriate to a respective light source and reflect the remaining light to the next stage.

An absorbing termination 86 absorbs light not transmitted by any of the filters 81 to 85.

Detectors 91 to 95 are positioned to receive light by transmission through the filters 81 to 85 respectively. The detectors 91 to 95 are electronically tuned to the frequency defined by the rotation of the grid reticle 33 and the number of spokes of the spoked wheel reticle 35 (see Figure 3). Each filter and the corresponding detector in combination define a signal channel tuned to a wavelength interval defined by the respective filter. Figure 8 therefore displays a five-channel arrangement, and clearly the number of channels may easily be altered by varying the number of filter-detector combinations.

Each of the filters 81 to 85 will transmit that part of any broad-band light falling within its respective transmission band. The applications of the Figure 8 arrangement when in combination with the Figure 3 apparatus include identifying a variety of laser sources in the presence of sunlight or sun-glint reflections from small objects. In each narrow filter transmission band therefore. the corresponding laser energy will be much greater than the broad-band energy which can be largely ignored.

Optical sensors are frequently employed in tracking roles in which it is not uncommon for an operator to be dazzled or a detector saturated by excessive light intensity. Figures 9(a) to (d) illustrate schematically the use of a further modification to apparatus of the invention for the purpose of reducing the effects of "dazzle".

Figure 9 (a) shows a field of view comprising a scene 101 in which a shaded area 102 represents a dazzling target producing saturation of an optical sensor used in a search role. Figure 9 (b) shows an image 103 of a scene at the focal plane of an optical sensor of the invention, the system consisting of a modified version of that shown in Figures 3 and 4. The arrangement of Figures 3 and 4 is modified by firstly replacing the one-dimensional grid reticle 33 by a two-dimensional lattice reticle (see square lattice 12 in Figure 1); secondly, the spoked wheel reticle 35 is replaced by a reticle comprising a central opaque disc or blanking mask surrounded by an annular transparent region.The Fourier transform of the lattice reticle with respect to the dazzling target of Figure 9(a) produces a marker cross 104 at the focal plane of the system (see the focal plane x2y2 in Figure 1), the outer sections of the arms of the cross 104 being visible through the transparent annular region of the second reticle. The blanking mask located centrally of the second reticle obscures the real image of the dazzling target which would otherwise saturate the detector. However, the location (indicated by an arrow 105) of the target can still be inferred by visually extrapolating the arms of the marker cross 104. In this embodiment of the invention, the first reticle is necessarily kept fixed in position, and not rotated as in the foregoing embodiments of the invention.As an alternative to the use of a two-dimensional lattice for the first reticle, a onedimensional periodic grid may be employed if necessary, this reticle giving rise to a line instead of a marker cross. Clearly, however, a cross pattern is more useful for positional definition purposes.

Figures 9(c) and 9(d) illustrate the use of optical filters in combination with the apparatus described in the last preceding paragraph. In Figure 9(c) an intense light source 106 and a weak light source 107, having different emission spectra are shown adjacent to one another and occupying very similar sight lines. With such closely adjacent light sources, it can be desirable to suppress the unwanted light from the intense source 106 whilst retaining the capability to locate the weak source 107.

This is achieved by the use of a twodimensional lattice reticle and a blanking mask reticle, as described in the preceding paragraph, and by exploiting the difference between the emission spectra of the two sources. The central opaque disc of the blanking mask is replaced by an optical filter opaque to the emission spectrum of the intense source 106, but transparent to the corresponding emission from the weak source 107. Figure 9(d) shows the resulting image at the focal plane of the optical sensor (see the focal plane x2y2 of Figure 1).

The intense real image of the intense source 106 has been replaced by a space 108, but the location of the source 106 can still be inferred by extrapolation of the arms of a cross 109, as in Figure 9(b). The space 108 is occupied by a cross pattern 110 at the centre of which is the weak source 107. The cross patterns 109 and 110 are simply the Fourier transforms of the lattice reticle with respect to illumination by the intense and weak sources 106 and 107 respectively. This arrangement therefore makes it possible to locate simultaneously a weak source and a comparatively dazzling, closely adjacent source. A further optional modification is to make the annular transparent region of the second reticle an optical filter, the filter being transparent to both weak and intense source emission spectra, but at least partially opaque to broad-band light such as the solar spectrum.Such a filter enhance the contrast of each source with respect to the background light.

Within the scope of the present invention many combinations of reticles may be employed each appropriate to a particular task. Reticles may be linear, rectangular or circular producing corresponding images. If a particular application dictates the production of a complicated image at the focal plane of a system, the corresponding aperture plane reticle required may be produced by photographing the image through a Fourier transforming lens; this is because the reticle is itself the Fourier transform of the required image.

It will be appreciated that there are very many complementary pairs of reticles which are suitable for particular spatial filtering applications; and may accordingly be used for modulation purposes in accordance with the invention, i.e. each pair may be made either specific to a target of a particular geometry, or insensitive to the spatial characteristics of a scene. Two standard works are available. An Introduction to Fourier Optics J. W. Goodman (McGraw Hill), and An Atlas of Optical Transforms, Harburn, Taylor and Welberry (Bell), which are useful to suggest reticle pairings appropriate to a variety of spatial filtering applications. It is therefore to be understood that the scope of the present invention is by no means restricted to the periodic grid, spoked wheel, sectored disc and other reticles hereinbefore described.

Furthermore, the embodiments described above envisaged the use of a telephoto lens to produce the Fourier transform, and it will be evident that many kinds of focussing means or telescope arrangement may be used for this purpose. Particular alternative examples of appropriate optical focussing means include Cassegrainian or Newtonian reflecting telescopes and catadioptric systems.

WHAT I CLAIM IS: 1. An optical sensor for detecting an object of given spatial characteristics in a field of view including a first reticle, optical focussing means arranged to produce a Fourier transform of the first reticle, the transform being superimposed on a second reticle located at an image position of the optical sensor with respect to the field of view wherein the first and second reticles are such as to form a mutually complementary pair with respect to sensitising the optical sensor to the object, and an eyepiece or detector means positioned to receive light after transversal of the focussing means and the reticles.

2. An optical sensor according to claim I wherein the second reticle is located at a focal plane of the optical focussing means.

3. An optical sensor according to claim 2, wherein there is provided means to cause relative movement between the first and second reticles, the relative movement being arranged to be appropriate to the complementary nature of the two reticles.

4. An optical sensor according to claim 3, wherein the relative motion comprises rotation of one reticle relative to the other.

5. An optical sensor according to claim 3 or 4 wherein the second reticle is stationary with respect to the field of view or modulates the field of view at a much slower rate than the first reticle.

6. An optical sensor according to any one preceding claim wherein the first reticle comprises a one-dimensional periodic grid.

7. An optical sensor according to any one preceding claim wherein the second reticle comprises opaque spokes having translucent regions therebetween and radiating from a common centre.

8. An optical sensor according to any one of claims 1 to 6 wherein the second reticle comprises a sectored disc in which the sectors are alternately opaque and translucent.

9. An optical sensor according to any one of claims 1 to 6 wherein the second reticle comprises opaque spiral arms diverging from a common centre and defining translucent regions therebetween.

10. An optical sensor according to any one of claims 7 to 9 wherein the translucent regions and the opaque spokes, sectors or arms are replaced by optical filter materials giving each a respective wavelengthdependent optical transmissivity.

11. An optical sensor according to any

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (27)

**WARNING** start of CLMS field may overlap end of DESC **. the intense source 106, but transparent to the corresponding emission from the weak source 107. Figure 9(d) shows the resulting image at the focal plane of the optical sensor (see the focal plane x2y2 of Figure 1). The intense real image of the intense source 106 has been replaced by a space 108, but the location of the source 106 can still be inferred by extrapolation of the arms of a cross 109, as in Figure 9(b). The space 108 is occupied by a cross pattern 110 at the centre of which is the weak source 107. The cross patterns 109 and 110 are simply the Fourier transforms of the lattice reticle with respect to illumination by the intense and weak sources 106 and 107 respectively. This arrangement therefore makes it possible to locate simultaneously a weak source and a comparatively dazzling, closely adjacent source. A further optional modification is to make the annular transparent region of the second reticle an optical filter, the filter being transparent to both weak and intense source emission spectra, but at least partially opaque to broad-band light such as the solar spectrum.Such a filter enhance the contrast of each source with respect to the background light. Within the scope of the present invention many combinations of reticles may be employed each appropriate to a particular task. Reticles may be linear, rectangular or circular producing corresponding images. If a particular application dictates the production of a complicated image at the focal plane of a system, the corresponding aperture plane reticle required may be produced by photographing the image through a Fourier transforming lens; this is because the reticle is itself the Fourier transform of the required image. It will be appreciated that there are very many complementary pairs of reticles which are suitable for particular spatial filtering applications; and may accordingly be used for modulation purposes in accordance with the invention, i.e. each pair may be made either specific to a target of a particular geometry, or insensitive to the spatial characteristics of a scene. Two standard works are available. An Introduction to Fourier Optics J. W. Goodman (McGraw Hill), and An Atlas of Optical Transforms, Harburn, Taylor and Welberry (Bell), which are useful to suggest reticle pairings appropriate to a variety of spatial filtering applications. It is therefore to be understood that the scope of the present invention is by no means restricted to the periodic grid, spoked wheel, sectored disc and other reticles hereinbefore described. Furthermore, the embodiments described above envisaged the use of a telephoto lens to produce the Fourier transform, and it will be evident that many kinds of focussing means or telescope arrangement may be used for this purpose. Particular alternative examples of appropriate optical focussing means include Cassegrainian or Newtonian reflecting telescopes and catadioptric systems. WHAT I CLAIM IS:

1. An optical sensor for detecting an object of given spatial characteristics in a field of view including a first reticle, optical focussing means arranged to produce a Fourier transform of the first reticle, the transform being superimposed on a second reticle located at an image position of the optical sensor with respect to the field of view wherein the first and second reticles are such as to form a mutually complementary pair with respect to sensitising the optical sensor to the object, and an eyepiece or detector means positioned to receive light after transversal of the focussing means and the reticles.

2. An optical sensor according to claim I wherein the second reticle is located at a focal plane of the optical focussing means.

3. An optical sensor according to claim 2, wherein there is provided means to cause relative movement between the first and second reticles, the relative movement being arranged to be appropriate to the complementary nature of the two reticles.

4. An optical sensor according to claim 3, wherein the relative motion comprises rotation of one reticle relative to the other.

5. An optical sensor according to claim 3 or 4 wherein the second reticle is stationary with respect to the field of view or modulates the field of view at a much slower rate than the first reticle.

6. An optical sensor according to any one preceding claim wherein the first reticle comprises a one-dimensional periodic grid.

7. An optical sensor according to any one preceding claim wherein the second reticle comprises opaque spokes having translucent regions therebetween and radiating from a common centre.

8. An optical sensor according to any one of claims 1 to 6 wherein the second reticle comprises a sectored disc in which the sectors are alternately opaque and translucent.

9. An optical sensor according to any one of claims 1 to 6 wherein the second reticle comprises opaque spiral arms diverging from a common centre and defining translucent regions therebetween.

10. An optical sensor according to any one of claims 7 to 9 wherein the translucent regions and the opaque spokes, sectors or arms are replaced by optical filter materials giving each a respective wavelengthdependent optical transmissivity.

11. An optical sensor according to any

one of claims 7 to 10 when dependent on claim 6 and adapted to distinguish between two spatially small light sources of similar geometry but differing spectral characteristics including a third reticle positioned in the path of light from the field of view when passing through the optical sensor, the third reticle being located with respect to the focussing means such that a Fourier transform thereof is superimposed on the Fourier transform of the first reticle and comprising a one-dimensional grid arrangement offset by rotation from parallelism with the first reticle, the said grid arrangement comprising strips transparent to both light sources disposed alternately with optical filter strips having neutral non-zero transmissivity with respect to the spectrum of the first light source and wavelength dependent transmissivity with respect to the spectrum of the second light source.

12. An optical sensor according to claim 11 wherein the first and third reticles are rotated in synchronism.

13. An optical sensor according to claim 12 and adapted to distinguish the presence of a narrow wavelength band first light source in the presence of a broadband second light source wherein the detector means is connected to an electronic filter circuit adapted to discriminate between the modulation frequencies produced by rotation of the first and third reticles relative to the second reticle.

14. An optical sensor according to any one of claims 1 to 5 wherein the first reticle is a two-dimensional lattice.

15. An optical sensor according to any one of claims 1 to 5 or 14 wherein the second reticle comprises a central opaque disc within an annular translucent area.

16. An optical sensor according to claim 15 wherein the -translucent area is an optical filter.

17. An optical sensor according to any one preceding claim wherein the focussing means comprises a telephoto lens.

18. An optical sensor according to any one preceding claim wherein there is included a field lens to focus light from the second reticle onto the detector means.

19. An optical sensor as claimed in claim 2 comprising means to rotate the first reticle on its axis, a cascaded arrangement of optical filters positioned to receive light from the second reticle, each filter being arranged to transmit a respective narrow wavelength band of the light incident upon it and to reflect the non-transmitted light on to the next filter in the cascade, and a detector positioned to receive light transmitted by at least one of the filters.

20. An optical sensor as claimed in claim 19 wherein there is provided a plutality of detectors, one detector being provided to receive light transmitted through each said filter.

21. An optical sensor as claimed in claim 19 wherein the second reticle is formed as a reflective coating on a first of said filters and includes a mirror which directs light reflected from the second reticle to a second of said filters, the light transmitted through the two filters being focussed onto the detector.

22. An optical sensor as claimed in claim 21 wherein there is included in the optical path an optical wedge attenuator which is adjustable so as to balance the transmission of light through the two filters to thereby desensitise the sensor to sunlight.

23. An optical sensor substantially as described with reference to Figures 3 and 4 of the drawings accompanying the provisional specification.

24. An optical sensor substantially as described with reference to Figures 5 and 6 of the drawings accompanying the provisional specification.

25. An optical sensor substantially as described with reference to Figures 3, 4 and 7 of the drawings accompanying the provisional specification.

26. An optical sensor substantially as described with reference to Figures 3, 4 and 8 of the drawings accompanying the provisional specification.

27. An optical sensor substantially as described with reference to Figures 3, 4 and 9 of the drawings accompanying the provisional specification.