GB2250884A - Optical image sensing array with microscan - Google Patents

Abstract

In, for example, a thermal imager with an infrared detector array 1, increased resolution is obtained by optically shifting the position of the image on the array by less than a pixel in the horizontal and/or vertical directions by means of a microscan assembly 4a, 4b. The latter comprises at least one element in the form of a segment of a solid of revolution which is rotatable about the axis 5 used to generate the solid so that it periodically covers the array 1 to stepwise shift the incident image, e.g. to produce line interlace. Oppositely shaped segments may be used to shift the image in opposite directions and they may be interleaved by other wedge shaped elements (4c, 4d, figure 8) for producing shifts in an orthogonal direction. Rotation of the elements may be synchronised with a chopper (they may be mounted thereon). <IMAGE>

Description

DESCRIPTION OPTICAL IMAGE SENSING SYSTEMS This invention relates to optical image sensing systems (for example thermal imagers comprising an array of infrared detector elements) and more particularly to the provision of microscan means in such systems. The invention also relates to rotatable microscan assemblies and to rotatable chopper assemblies suitable for use in such systems.

Optical image sensing systems are known, comprising an array of photosensing elements, an optical imaging means for forming an image on the array, and a microscan means between the imaging means and the array for periodically shifting the position of the image on the plane of the array. The paper entitled "A Hand-held Imager for 2-dimensional Detector Arrays" by R.K. McEwen on pages 105 to 111 of the IEE Publication 321 (1990) of the 4th International Conference on Advanced Infrared Detectors and Systems (AIRDS) 5th to 7th June 1990 describes one such system for a thermal imager operating in the 3 to 5ym (micrometre) waveband. The whole contents of this paper are hereby incorporated herein as reference material. The imager has a 64 x 64 array of cadmium mercury telluride elements manufactured by Philips Components.The imaging means comprises a primary f/1 optical system. The microscan means was provided as an enhancement of the imager and comprises four piezo-electric bimorphs holding an optical element which deflects the image. When the element is tilted by applying appropriate voltages to the bimorphs, the image is shifted on the array by a fraction of a pixel pitch.

Microscanning is a technique which permits the system designer to increase the resolution of an array otherwise limited by the size and number of the detector elements. The technique may be used to increase resolution by shifting the image in one dimension (with either a 1-dimensional array or a 2-dimensional array) or in two dimensions (with a 2-dimensional array). In its simplest form with a two-position shift, the effect is equivalent to an interlace of alternate frames. The idea of using a rocking or oscillating plate with a two-position tilt to displace an image was considered many years ago, see for example United Kingdom patent specification GB 0 463 863 published in 1937. Using a tilting element to shift an image is quite successful, although the arrangement can be noisy and can cause vibrations.

It is an aim of the present invention to provide an alternative form of microscanning means which (depending on the system in which it is used) may be quieter and introduce less vibration and which may even be integrated with another component of the system.

According to one aspect of the present invention, there is provided an optical image sensing system comprising an array of photosensing elements, an optical imaging means for forming an image on the array, and a microns can means between the imaging means and the array for periodically shifting the position of the image on the plane of the array by means of at least one optical element which deflects the image, which system is characterised in that the optical element is in the form of at least one segment of a figure of revolution about an axis, and the segment is rotatably mounted about an axis which corresponds to the axis of revolution of the figure and so by rotation of the segment a shift in the position of the image on the array occurs for as long as the segment is rotated in front of the array.

Thus, the invention provides a rotatable microscan means and permits operation in a continuously rotating mode. Because the axis of rotation corresponds to the axis of revolution of the figure, the deflection of the image (by refraction and/or reflection) obtained by an angled or curved facet of the segment is substantially constant for the duration of the motion of the segment in front of the array.

Various applications of a figure of revolution as an optical element are described in, for example, the following publications, the whole contents of which are hereby incorporated herein as reference material.

"The Axicon: A New Type of Optical Element by J.H. McLeod in J.Opt. Soc. Am. (Journal of the Optical Society of America) Vol. 44, No. 8, pages 592 to 597, August 1954; "Axicons and their Uses" by J.H. McLeod in J.Opt. Soc. Am.

Vol. 50, No. 2, pages 166 to 169, February 1960; "Imaging Properties of a Conic Axicon" by W.A. Edmonds in Applied Optics, Vol. 13, No. 8, pages 1762 to 1765, August 1974; "Correlation Image Formation with an Axicon" by E.N. Leith et al in J.Opt. Soc. Am. Vol. 70, No. 2, pages 141 to 145, February 1980; and, for example, United States patents 4 755 027, 4 426 696 and 4 255 021.

Sometimes optical elements of this form are referred to as "axicons"; the expression "axicon" appears to have been used first by J.H. McLeod in 1954, to denote an optical focussing element whose focussing means defines a figure of revolution about an axis and which focusses (by refraction and/or reflection) a point source on its axis of revolution as a focal zone between two points along the axis. This focal zone along the axis is not particularly relevant to the present invention which uses a different characteristic, namely that, when refracted or reflected by an axicon segment, an image undergoes a radial shift towards or away from the axis and that the shift is constant for rotation of the segment.

Although most figures of revolution used as optical elements nowadays are conical and have a surface generated by revolving a straight line about the axis, other profiles may instead be used as can be seen from, for example, the publications herein incorporated as reference material. The expressions "cone" and "conical" are used hereinafter in the description of specific embodiments of the present invention, but these expressions should be understood in their broadest sense as including any generally conical figures generated by revolving a profile (regardless of whether a straight line or somewhat curved line) about an axis. Indeed, the segment employed in accordance with the present invention may have a significantly non-rectilinear profile (for example that of a torus) and such a profile may even be advantageous in reducing optical distortions in some optical systems.In most cases, the angle between the segment profile (the line) and its axis of rotation may be within 10 degrees or so of a right-angle in order to obtain the desired radial shift of the image. In the case of a refractive optical element, the other surface may be flat, but it may sometimes be advantageous to have another conical surface (or a more general aspherical surface) as the other surface. In most cases the wedge angle between front and rear surfaces of a refractive element (or the average wedge angle in the case of non-rectilinear surfaces) will not exceed 10 degrees and may be about 1 degree or less.

As the microns can means is rotated, the image may pass successively through the segment which shifts the image and then through a gap which does not shift the image. However, it is more advantageous for the image to pass through (or be reflected by) a continuous succession of segments which shift the image alternately in opposite directions. Thus, preferably the microscan means comprises at least one pair of first and second segments of oppositely-shaped figures of revolution which shift the position of the image in opposite directions on the plane of the array, the first and second segments being arranged in succession around the axis of rotation. By providing such first and second segments, the deflection angle of each segment is reduced for the same image shift between the first and second positions, and the image traverses a similar optical path in the microns can means regardless of whether it is passing through (or reflected by) the first or second segment.

Although the present invention can be used in other bands of the spectrum (such as the visible region), it is particularly useful for infrared image sensing systems for which detector arrays with very large numbers of elements are not yet available. Thus, microscanning is particularly beneficial for infrared systems since the size and number of the infrared detector elements in the array would otherwise limit the resolution.

Since pyroelectric detectors only sense changes in temperature, it is normal to include a rotatable optical chopper in a pyroelectric detector system (between the imaging means and the pyroelectric array) for periodically chopping the transmission of the image to the array. Examples of chopped pyroelectric and ferroelectric infrared image sensing systems are described in the following publications, the whole contents of which are hereby incorporated herein as reference material: "An Uncooled Solid State Thermal Imager" by R.K. Ewen et al, on pages 93 to 99 of IEE Conference publication 321 (1990); United States patent specifications US-A-4 072 863 and US-A-4 142 207.

High performance infrared image sensing systems are based on detector elements of a cadmium mercury telluride material or other small bandgap semiconductor material. Since these are photon detectors they do not require chopping of the radiation to generate temperature changes on the elements. However, even with these detector elements it can be advantageous to use a chopper so as to provide a uniform temperature reference level from the chopper blades for background subtraction and other signal processing (e.g. uniformity correction). An example of such a cadmium mercury telluride system is described in the following publication, the whole contents of which are hereby incorporated herein as reference material: "Electronically scanned CMT Detector Array for the 8 to 14Cun Band" by R.A.Ballingall et al in Electronics Letters, 1st April 1982, vol.18, No. 17, pages 285 to 287.

The provision of a continuously rotatable microns can means in accordance with the present invention is particularly advantageous in a chopped system, in-that the rotation of the microscan segment(s) can be controlled to synchronise with the rotation of the chopper. Indeed, the microscan means can be integrated with the chopper to a greater or lesser extent as desired. Thus, both the chopper and a segment(s) may rotate about the same axis, and the segment(s) may even be mounted on the chopper.

According to a further aspect of the present invention, there is provided a rotatable chopper assembly for use in an optical imaging system, and comprising a chopper having opaque parts which separate at least two windows for transmitting an optical image periodically in the system by rotation of the chopper, characterised in that the chopper assembly further comprises for at least one of the windows an optical element which optically shifts the image transmitted by that window, which optical element comprises a segment of a figure of revolution about an axis, both the chopper and the segment(s) being rotatable about an axis which corresponds to the axis of revolution of the figure.

These and other features in accordance with the invention are illustrated specifically in embodiments of the invention now to be described, by way of example, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a simplified schematic side section of the components of an infrared image sensing system in accordance with the invention; Figure 2 is an isometric view of two segments forming optical elements of a rotatable microns can assembly in accordance with the invention; Figure 3 is a plan view of a chopper configuration which may be used in combination with the microscan assembly of Figure 2; Figure 4 is a plan view showing the segments of Figure 2 aligned with the window of the chopper of Figure 3;; Figure 5 is an isometric view of four segments of another rotatable microns can assembly in accordance with the invention; Figure 6 is a plan view of a more complex chopper configuration; Figure 7 is a plan view showing conical optical segments aligned with the windows of the chopper of Figure 6, and Figure 8 is an isometric view of two conical and two tapered optical segments which form another rotatable microns can assembly in accordance with the invention.

It should be noted that all the drawings are diagrammatic and .not drawn to scale. Relative dimensions and proportions of parts of these Figures have been shown exaggerated or reduced in size for the sake of clarity and convenience in the drawings.

The thermal imaging system of Figure 1 comprises an array 1 of photosensing elements. An optical imaging means 2, for example an objective lens, forms an infrared image on the array 1. A rotatable optical chopper 3 is present between the objective 2 and the array 1 for periodically chopping the transmission of the image to the array 1. A microscan means 4 is also present between the objective 2 and the array 1 for periodically shifting the position of the image on the plane of the array 1 by means of at least one optical element 4a which deflects the image. In its simplest form when shifting the image by half a pixel between two positions, the microns can effect is equivalent to an interlace between successive frames.

In accordance with the present invention, the optical element 4a of the microns can assembly 4 comprises at least one segment of a figure of revolution about an axis 5. This figure is of a shallow frusto-conical form in the examples shown in the drawings. The segment 4a is rotatably mounted about an axis 5 which corresponds to the axis of revolution of the figure and which extends through a plane 6 containing the plane of the array 1 so that it can be rotated in front of the array 1. In the arrangement of Figure 1, both the chopper 3 and the segment(s) 4a rotate about the same axis 5. Furthermore in the arrangement of Figure 1, the microscan assembly comprises at least one pair of first and second segments 4a and 4b of oppositely-shaped figures of revolution, both of which shapes have the rotation axis 5 as their axis of revolution.These oppositely-shaped segments 4a and 4b are arranged in succession around the axis 5 and shift the position of the image in opposite directions on the plane of the array 1.

In its simplest form the optical element 4a or 4b is formed from a figure of revolution of two straight lines about an axis 5, giving a triangular cross-section with constant wedge-angle; this wedge angle normally will not exceed 100, and may be around 10 or less with high refractive index material such as germanium or silicon. The small-angle wedge with wedge angle A (measured in radians), made of optical material with a refractive index n, will cause an angular deviation of (n-1)A to an incident ray. If the wedge is spaced from the array 1 by a distance d, then the approximate shift of the image will be d(n-1)A. Thus if the required shift is known (for example 0.025mm), then suitable combinations of wedge angle A and spacing distance d can be easily selected.Because the distance d varies slightly across the image, a more complex cross-section with one or both generating lines being curved may give a small improvement in performance.

Thus, the invention provides in the system of Figure 1 a continuously rotatable microscan assembly of two or more segments 4a and 4b which may be mounted either on the rotatable chopper 3 or separately on for example a separate rotating disc. Combining the segments 4a and 4b with the chopper 3 in a single assembly is particularly advantageous in reducing the number of components in the system and in ensuring synchronised rotation of the segments 4a and 4b with the windows of the chopper 3.

In the form illustrated in Figures 1 and 2, the segments 4a and 4b comprise infrared-transmissive material of a triangular cross-section which deflect the infrared image by refraction. The segments 4a and 4b may be made of silicon or germanium, but as the wedge angle is small it is also possible to use thin plastic for the optical element. The objective 2 illustrated as a lens in Figure 1 is also of infrared-transmissive material suitable for thermal imaging. However in an alternative embodiment reflection may be used for imaging with the objective 2 and/or for deflection with the segments of the microscan 4, in which case these optical elements need no longer be of an infrared transmissive material.

The detector elements of the array 1 may be of known form, for example pyroelectric detectors or cadmium mercury telluride photon detectors, and can be connected to known signal multiplexing and processing circuitry in silicon integrated circuits.

Figure 3 illustrates a simple spiral chopper. This particular chopper has two spiral windows 3a and 3b separated by opaque portions. Each window extends almost 1800 around the axis 5 of rotation. The chopper window allows radiation to fall on any part of the detector array 2 for approximately half of the time and blocks it for the other half of the time. The chopping action progresses across the array 1 from top to bottom, bottom to top or from side to side as determined by the location of axis 5 relative to the array. Modifications with more windows are also possible.

Figure 4 illustrates an arrangement in accordance with the present invention, in which each window 3a or 3b of the chopper is aligned with a conical segment 4a or 4b and the segments 4a and 4b of successive windows alternate between equal and opposite wedge-shapes so as to shift the position of the image in opposite directions on the plane of the array 1 by synchronised rotation of the chopper 3 and segments 4a and 4b about the axis 5. As already mentioned, the segments 4a and 4b may be conveniently mounted on the chopper 3 itself. Apart from providing a mounting area, those parts of the elements 4a and 4b obscured by the opaque parts of the chopper 3 may be omitted. When the chopper 3 has a blade configuration with 4 windows, a microscan assembly of the four conical segments of Figure 5 may be used.

Since the amount of movement of the image to achieve an interlace is quite small (for example about 25 micrometres with many present-day arrays 1), the "wedge" angle of the cone is also small and decreases with increase in the spacing d between the segments 4a and 4b and the array 1. Such small-angle segments 4a and 4b mounted on the chopper 3 close to the detector array 1 need not degrade significantly the optical image. The resulting image degradation is smaller than the pixel size and the geometric distortion of the picture (which varies from point to point of the picture) is less than one pixel. A reduction in the wedge-angle (and hence in the image degradation) is obtained by using a succession of alternate equal but opposite conical elements 4a and 4b rather than simply using a succession of elements 4a and gaps.

Figure 6 illustrates a more complex chopper configuration similar to that known from the said IEE paper "An uncooled solid state thermal imager" by R.K. McEwen et al. which has a shorter closed time relative to the open time, and is thus more suitable for use with photon detectors such as cadmium mercury telluride elements. The areas of the elements 4a and 4b which are obscured by the opaque areas of the chopper 3 are cut back so as to accommodate these elements 4a and 4b over the more than 180 degree windows 3a and 3b of the chopper of Figures 6 and 7. Figure 7 also illustrates, by way of example, how the axis 5 of the choppers may be offset by a distance y from the centre-line of the array which passes through the optical axis 8. This is to provide a more linear chopping motion over the array 1 with only small phase errors.If the shallow conical optical elements 4a and 4b are centred on the axis 5 of the chopper, then the geometric distortion caused in the picture is non-symmetrical relative to the centre-line of the picture. In many cases this non-symmetry is negligible, but if improvement is desired, the shallow conical optical elements 4a and 4b can be mounted with their axes on a separate axis 9 which is on or near the centre-line of the picture and which is mechanically linked to the axis 5 of the chopper. So long as the rotations of the chopper 3 and conical segments on their respective axes 5 and 8 are synchronised, the directions of rotation do not need to be the same and by employing more sections on one than on the other (i.e. conical segments compared with chopper windows), the rotation speeds can be different.The transition between conical segments can also be tapered or staggered to coincide with the chopper blanking period which varies continuously from top to bottom of the scan.

Other modifications are possible in accordance with the invention. Thus, for example a 2-dimensional interlace (in both X and Y directions of a 2-d array 1) can be obtained by having another rotating disc carrying conical segments with their rotational axis parallel to axis 5 but positioned out of the plane of the paper in Figure 1, for example through the point 10 of Figure 6. The speed of rotation of the second disc may differ from that of the first disc; for example, one disc may rotate at twice the speed of the other.Alternatively a 2-dimensional interlace can be obtained by having a 4 segment (or multiple of 4) assembly with two of the segments incorporating a wedge shape in the radial direction (as in Figures 2 and 5) and the other two segments incorporating a wedge shape in the tangential directions (as in Figure 8), or with some or all segments incorporating a combination of radial and tangential wedges.

As already mentioned, the segments 4a and 4b may made of a plastics material which however needs to be thin to transmit infrared wavelengths adequately. A possible means of reducing the thickness of the elements 4a and 4b is to use a construction similar to that used for "Fresnel" lenses, that is by dividing each conical wedge 4a or 4b into a number of smaller sections, so long as consideration is given to the interference and diffraction effects between portions of the light passing through different facets of the element 4a (or 4b) and also to any shadowing effect from the edges of the facets if the optical element 4a or 4b is near the image plane 6.

Although a continuously-rotating microns can assembly in accordance with the present invention integrates in a particularly useful manner in an image-sensing system with a chopper 3, a rotating microns can assembly in accordance with the invention may be used in non-chopped systems, for example in a modification of the thermal imager described in the said IEE publication 321 (1990) paper by R.K. McEwen.

From reading the present disclosure, other variations and modifications will be apparent to persons skilled in the art.

Such variations and modifications may involve equivalents and other features which are already known in the design, manufacture and use of axicons and other optical elements, and image sensing systems and component parts thereof, and which may be used instead of or in addition to features already described herein. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present application also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicants hereby give notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Claims (11)

CLAIM(S)

1. An optical image sensing system comprising an array of photosensing elements, an optical imaging means for forming an image on the array, and a microns can means between the imaging means and the array for periodically shifting the position of the image on the plane of the array by means of at least one optical element which deflects the image, characterised in that the optical element is in the form of at least one segment of a figure of revolution about an axis, and the segment is rotatably mounted about an axis which corresponds to the axis of revolution of the figure and so by rotation of the segment a shift in the position of the image on the array occurs for as long as the segment is rotated in front of the array.

2. A system as claimed in Claim 1, further characterised in that the microns can means comprises at least one pair of first and second segments which are of oppositely-shaped figures of revolution so as to shift the position of the image in opposite directions on the plane of the array, the first and second segments being arranged in succession around the axis of rotation.

3. A system as claimed in Claim 1 or Claim 2, further characterised in that a rotatable optical chopper is also included between the imaging means and the array for periodically chopping the transmission of the image to the array, the rotation of the segment(s) being controlled to synchronise with the rotation of the chopper.

4. A system as claimed in Claim 3, further characterised in that both the chopper and the segment(s) rotate about the same axis.

5. A system as claimed in Claim 4, further characterised by the segment(s) being mounted on the chopper.

6. A system as claimed in Claim 4 or Claim 5, further characterised in that the chopper has a plurality of windows separated by opaque parts, each window is aligned with a segment, and the segments of alternate windows are of oppositely-shaped figures of revolution so as to shift the position of the image in opposite directions on the plane of the array.

7. A system as claimed in any one of the preceding claims, further characterised in that the photosensing elements are infrared detector elements, and the optical imaging means and segments are designed to transmit infrared for thermal imaging.

8. A system as claimed in Claim 7, further characterised in that the infrared detector elements are photon detectors of a cadmium mercury telluride material or are pyroelectric detectors.

9. A rotatable chopper assembly for use in an optical imaging system, and comprising a chopper having opaque parts which separate at least two windows for transmitting an optical image periodically in the system by rotation of the chopper, characterised in that the chopper assembly further comprises for at least one of the windows an optical element which optically shifts the image transmitted by that window, which optical element comprises a segment of a figure of revolution about an axis, both the chopper and the segment(s) being rotatable about an axis which corresponds to the axis of revolution of the figure.

10. A rotatable chopper assembly as claimed in Claim 9, further characterised by the segment(s) being mounted on the chopper.

11. An image sensing system or a rotatable microscan assembly or a rotatable chopper assembly, having any one of the novel features substantially as described and/or as illustrated in any one of the accompanying drawings.