GB2258576A

GB2258576A - A visible light detector assembly

Info

Publication number: GB2258576A
Application number: GB9116813A
Authority: GB
Inventors: David Huw Parker
Original assignee: British Aerospace Space and Communications Ltd
Current assignee: BAE Systems Hertford House Ltd
Priority date: 1991-08-03
Filing date: 1991-08-03
Publication date: 1993-02-10
Also published as: GB9116813D0

Abstract

A visible light detector assembly (2) for detecting the presence, position, and/or magnitude of a visible light radiation producing body such as a star (3) includes a first lens means (4) and an associated first substantially linear array (6) of photodiodes (5) which receives light from the star (3) via the lens means (4) and provides position and/or magnitude information about the star (3) in a first direction and a second lens means (7) and associated second substantially array (9) of photodiodes (8) located to extend substantially orthogonally to the first array (6) to receive light from the second lens means (7) and provide position and/or magnitude information about the star (3) in a second direction at an angle to the first direction. <IMAGE>

Description

A VISIBLE LIGHT DETECTOR ASSEMBLY This invention relates to a visible light detector assembly for detecting the presence, position and/or magnitude of a visible light radiation producing body such as a star. Such an assembly may be used in a land, sea or air vehicle navigational guidance system, a spacecraft attitude and/or orbit control and/or guidance system or a spacecraft navigational guidance system.

A conventional visible light detector assembly such as a star tracker for use in a satellite navigation system uses a two dimensional array of photodiodes such as the so called charge-coupled devices (C. C. D's) in which the photodiodes are arranged in a grid pattern side by side and end to end to form a matrix with their light sensitive faces located in one plane.

These photodiodes or C.C.D's are visible lights sensitive and operable to provide output signals indicative of the intensity of light falling thereon.

A typical such conventional two dimensional array or matrix of photodiodes is shown in the accompanying Figure 1 and may be made up of, for example, an array 288 diodes long in one direction and 384 long in a right angled direction, totalling 110,592 photodiodes. The image of a body such as a star is cast onto the two dimensional array or matrix using a lens system and by imaging processing the location of the body centre and its magnitude can be deduced. If there are "m" photodiodes in the Y axis direction and"n" photodiodes in the X axis direction then the position and magnitude of the body is given in the Y direction by the position and number of photodiodes illuminated in that direction and the position and magnitude is given in the X axis direction by the position and number of the photodiodes "n" illuminated.By using suitable algorithms the position of the centre 1 of the body image can be estimated to an accuracy considerably better than the inter-diode spacing, namely by a factor of 10 to 100 times better.

However such a conventional visible light detector assembly is limited by the maximum size of the two dimensional array or matrix available and this in turn restricts the accuracy and therefore usefulness of the conventional assembly. Arrays or a matrix having 1024 x 1024 photodiodes are currently available but these large arrays and larger are difficult to manufacture to light performance specifications, are expensive and generate large amounts of redundant data.In this last context not only is data generated by a photodiode receiving visible light radiation limited by a body whose presence, position and/or magnitude is to be detected but also data is emitted by the remaining photodiodes in the field of view which see background illumination at a lower intensity and may indeed also see visible light radiation at different intensities from other light radiation emitting bodies whose position, presence and/or magnitude is not to be detected.

There is thus a need for a generally improved visible light detector assembly for detecting the presence, position and/or magnitude of a visible light radiation producing body such as a star which has improved performance such as a high resolution and greater ease of manufacture.

According to one aspect of the present invention there is provided a visible light detector assembly for detecting the presence, position and/or magnitude of a visible light radiation producing body such as a star, including a first lens means for receiving and focusing light radiation from a visible light radiation producing body, a first plurality of photodiodes disposed substantially edge-to-edge in line to form a first substantially linear array operable to receive focused light radiation from the first lens means and to provide position and/or magnitude information about the body in a first direction, a second lens means for receiving and focusing light radiation from the body, and a second plurality of photodiodes disposed substantially edge-to-edge in line to form a second substantially linear array, which second array is located to extend substantially orthogonally to the first array and is operable to receive focused light radiation from the second lens means and to provide position and/or magnitude information about the body in a second direction at an angle to said first direction.

Preferably said angle is substantially a right angle.

Conveniently the photodiodes are charge-coupled devices (C.C.D's) disposed so that the substantially linear arrays formed are strip-like in form.

Advantageously each said first and second lens means includes a cylindrical lens operable to focus the image of the body in one plane onto the associated substantially linear array.

Preferably the assembly includes a beam splitter located to receive light radiation from the body and operable to split the received light radiation into two components the first of which is passed to said first lens means and the second of which is passed to said second lens means.

Conveniently the beam splitter is a cube beam splitter operable to transmit the first component directly therethrough and to reflect and pass the second component substantially at right angles to said first component.

Advantageously the first substantially linear array extends substantially vertically to the direction of transmission of said first component and the second substantially linear array extends substantially horizontally to the direction of passage of said second component.

For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a diagrammatic view of a two dimensional array of matrix of photodiodes forming part of a conventional visible light detector assembly, Figure 2 is a plan view from above in diagrammatic form of a visible light detector assembly according to the present invention, Figures 3a, 3b and 3c are diagrammatic views of a first lens means and first substantially linear array forming part of the assembly of Figure 2 shown in side view (Fig 3a), plan view from above (Fig 3b) and in end elevation (Fig 3c) illustrating the relationship of the array and a body whose presence, position and/or magnitude is to be detected, and Figures 4a, 4b and 4c are views similar to those of Figure 3 but showing a second lens means and a second substantially linear array in side view (Fig 4a) plan view from above (Fig 4b) and end elevation (Fig 4c).

Particularly as shown in Figure 2 of the accompanying drawings a visible light detector assembly generally referenced 2 of the invention for detecting the presence, position and/or magnitude of a visible light radiation producing body such as a star 3 includes a first lens means 4, a first plurality of photodiodes 5 disposed substantially edge-to-edge in line to form a first substantially linear array 6, a second lens means 7 and a second plurality of photodiodes 8 disposed substantially edge-to-edge in line to form a second substantially linear array 9. The first lens means 4 is operable to receive and focus light radiation received from the star 3 and the first linear array 6 is operable to receive focus light radiation from the first lens means 6 to provide position and/or magnitude information about the body in a first direction y. The second lens means 7 is operable to receive and focus light radiation from the star 3 and the second linear array 9 is located to extend substantially orthogonally to the first array 6 and is operable to receive focus light radiation from the second lens means 7 and to provide position and/or magnitude information about the star 3 in a second direction x at an angle to the first direction y.

Preferably this angle is substantially a right angle.

An essential feature of the present invention is the use of substantially linearly arranged arrays of photodiodes. Arrays with any convenient number of photodiodes may be used. For example currently available are linear arrays having 4096 photodiodes in line and 6000 photodiodes in line. These arrays 6 and 9 enable a detector assembly of the present invention to cover a wider field of view more cheaply than conventional assemblies, with shorter read out time and with shorter electronic data processing time.

The photodiodes 5 and 8 are charge-coupled devices (C.C.D's) which are semi conductor elements, preferably silicon wafters, each operable, when excited, to permit the storage of charge therein proportional to the intensity of light falling thereon. Means are also provided, not illustrated, for supplying voltage across each of the semi-conductive elements forming the photodiodes 5 and 8 to drive them and means for receiving output signals therefrom indicative of the intensity of light falling therefrom.

Each lens means 4 and 7 preferably is a cylindrical lens which focuses the image of the star 3 in one plane onto the associated substantially linear array 6 or 9. Hence a respective lens means 4 or 7 has no effect in its orthogonal plane.

As shown in Figure 2 the detector assembly of the invention includes a beam splitter, which is preferably a cube beam splitter 10 located to receive light radiation 11 from the star 3 and operable to split the radiation 11 from the star 3 and operable to split the radiation 11 into two components, the first 12 of which is passed to the first lens means 4 and the second 13 of which is passed to the second lens means 7. As can be seen in Figure 2 the beam splitter 10 transmits the first component 12 directly therethrough and reflects and passes the second component 13 substantially at right angles to the first component 12.

The linear arrays are substantially strip-like inform and the first array 6 extends substantially vertically to the direction of transmission of the first component 12. The second array 9 extends substantially horizontally to the direction of passage of the second component 13. This may be seen more clearly from Figures 3 and 4.

As shown in Figures 3a, 3b and 3c the lens means 4 and array 6 generate information in the y direction. As shown in in Figures 4a, 4b and 4c the lens means 7 and the array 9 generate information in the x direction. Processed together x, y information indicating the presence, position andtor magnitude of the star 3 can be obtained.

The two lens means/array combinations basically must have A common field of view as shown in Figure 2. Imagine processing to achieve sub-diode accuracy can be used.

Ambiguities arising from the presence of several light radiation producing bodies in the field of view will not occur unless there are two such bodies of identical magnitude arranged parallel to one or other of the two arrays. This is most unlikely to occur in practice.

Even employing linear arrays of conventionally available 4,096 elements the assembly of the present application will provide a high resolution of the order of 10 times better than the resolution of current conventional visible light detector assemblies. The linear arrays are easier to cool using Peltier devices. If desired a third substantially linear array of photodiodes and associated lens means could be included, mounted at a further angle to the first two array/lens means combinations to provide for redundancy. Such a third array of photodiodes and associated lens means may be placed at any convenient angle between the first and second array/lens means combinations, preferably at 450 thereto, to provide sensing data, at reduced accuracy and resolutions on either of the X and Y axes if one or other of the first and second array/lens means combinations should fail.

Claims

1. A visible light detector assembly for detecting the presence, position and/or magnitude of a visible light radiation producing body, such as a star, including a first lens means for receiving and focusing light radiation from a visible light radiation producing body, a first plurality of photodiodes disposed substantially edge-to-edge in line to form a first substantially linear array operable to receive focused light radiation from the first lens means and to provide position and/or magnitude information about the body in the first direction, a second lens means for receiving and focusing light radiation from the body, and a second plurality of photodiodes disposed substantially edge-to-edge in line to form a second substantially linear array, which second array is located to extend substantially orthogonally to the first array and is operable to receive focused light radiation from the second lens means and to provide position and/or magnitude information about the body in a second direction at an angle to said the first direction.

2. An assembly according to claim 1, where said angle is substantially a right angle.

3. An assembly according to claim 1 or 2, wherein the photodiodes are charge-coupled-devices (C.C.D's) disposed so that substantially linear arrays formed are strip-like in form.

4. An assembly according to any one of claims 1 to 3, wherein each said first and second lens means includes a cylindrical lens operable to focus the image of the body in one body onto the associated substantially linear array.

5. An assembly according to any one of claims 1 to 4, including a beam splitter located to receive light radiation from the body and operable to split the received light radiation into two components the first of which is passed to said first lens means and the second of which is passed to said second lens means.

6. An assembly according to claim 5, wherein the beam splitter is a cube beam splitter operable to transmit the first component directly therethrough and to reflect and pass the second component substantially at right angles to said first component.

7. An assembly according to claim 6, wherein the first substantially linear array extends substantially vertically to the direction of transmission of said first component and wherein the second substantially linear array extends substantially horizontally to the direction of passage of said second component.

8. A visible light detector assembly for detecting the presence, position, and/or magnitude of a visible light radiation producing body such as a star, substantially as hereinbefore described and as illustrated in Figures 2, 3 and 4 of the accompanying drawings.