GB2208184A

GB2208184A - Optical imaging system

Info

Publication number: GB2208184A
Application number: GB8521955A
Authority: GB
Inventors: Michael Owen Lidwell
Original assignee: Thales Optronics Ltd
Current assignee: Thales Optronics Ltd
Priority date: 1984-09-11
Filing date: 1985-09-04
Publication date: 1989-03-08
Also published as: GB2208184B

Abstract

A millimetre waveband optical imaging system (2) comprises an image-forming triplet lens assembly (4) and an air-spaced divergent meniscus lens element (6). Assembly (4) is formed by a corrector lens element C in areal contact with each of the hyper-hemispherical lens element B and an objective lens element D. A radiation detecor array monolithically constructed on a semiconductor substrate A is located at the planar refractive surface of element B on which a planar image is formed by selection of the refractive indices of the various lens elements of the system (2). Meniscus lens element (6) enables fields of view of up to about 100 DEG to be achieved. <IMAGE>

Description

OPTICAL IMAGING SYSTEM This invention relates to an optical imaging system for radiation in the millimetre waveband.

At the present time most systems handling radiation in the millimetre waveband (and in the far infrared waveband) use a single detector and a form of optomechanical scanning of the image over that detector in order to obtain an output. However, recent developments have given :ise to arrays of detectors capable of handling such radiation so that it has become possible to obtain a complete image picture at one time instant. In order to use such detector arrays it is necessary to provide an optical imaging system intermediate the field of view and the array and various proposals in this respect have been made. The proposed systems however have suffered from disadvantages of having a relatively restrictive field of view in object space, having a curved image surface, and being relatively bulky in axial extent.

It is an object of the present invention to provide a new and improved optical imaging system for radiation in the millimetre waveband.

According to the present invention there is provided an optical imaging system for radiation in the millimetre waveband, comprising an image-forming triplet lens assembly formed by a corrector lens element lying intermediate and having concave refractive surfaces respectively in areal contact with a hyper-hemispherical lens element and an objective lens element, the exposed surface of the objective lens element being rendered anti-reflective, and a meniscus lens element axially separated from the objective lens element of said triplet assembly, wherein the hyper-hemispherical lens element and the objective lens element each has a refractive index of at least 2.5, the corrector lens element has a refractive index intermediate that of the hyper-hemispherical lens element and of air, and the meniscus lens element has a refractive index of less than 2.0 whereby radiation in the millimetre waveband incident on the meniscus lens element from a wide angle field of view is imaged onto a substantially planar image surface adjacent the aplanatic point of the hyperhemispherical lens element.

By virtue of the triplet lens assembly the imaging system of the present invention is compact, i.e., is of relatively short axial length. The meniscus lens element which is of relatively low refractive index enables fields of view in object space of up to about 1000 which is extremely wide angle. The system additionally provides a planar. image surface and is substantially free of unwanted internal reflections.

Preferably each refractive surfase of all the lens elements is spherical and concentricity of refractive surfaces is absent. With this arrangement ghost images are not significant.

It is essential that the exposed refractive surface of the objective lens element be provided with an antireflection coating or be otherwise rendered antireflective such as by texturing the lens surface to achieve a "moth-eye" effect. It is preferred that all other interface surfaces be anti-reflection coated.

An embodiment of the present invention will now be described by way of example with reference to the accompanying drawing which schematically illustrates an optical imaging system for radiation in the millimetre waveband.

In the drawing, the system 2 has an optical axis 10 on which are aligned a triplet lens assembly 4 and a diverging assembly 6, the arrangement being such that radiation from object space 8 and incident on assembly 6 is transmitted via assembly 4 to form an image which in this arrangement is coincident with a radiation detector array monolithically constructed on a semi-conductor substrate A.

The triplet lens assembly 4 is formed by a hyperhemispherical lens element B, a corrector lens element C, and an objective lens element D. The corrector lens element C is arranged so that its two refractive surfaces, each of which is concave, are in respective areal contact with the refractive surfaces of elements B and D. In other words, the interface 5 between elements B and C and the interface 7 between elements C and D are substantially air free. In order to minimise radiation loss by reflection loss at interface 3 between substrate A and lens element B, substrate A is similarly in areal contact with the planar surface of hyper-hemispherical lens element B.In all cases areal contact may be achieved by accuracy of manufacture and assembly of individual components in the absence of a bonding or interfacing material but it is preferred, particularly for interface 3, that an interfacing material be provided in the form of a bonding material having a thickness less than 25 microns.

The divergent assembly 6 is in the form of a single meniscus lens element F having refractive surfaces 11 and 13. Radiation passing through the air space between elements F and D passes through an aperture stop E the axial position of which is slightly adjustable in order to provide cones of radiation incident on the detector array of substrate A (and which has a length L) at near normal for all radiation beams within the field of view.

Substrate A may be made of silicon or gallium arsenide into which the detector array is lithographically engraved,the detector array being operable at 95 GHz (3 mm) and for such a substrate A it is preferred that lens element B has a refractive inde of at least 2.5.

A suitable material for lens element B is gallium arsenide which has a refractive index of 3.60, the radius of the hyper-hemisphere being such that the image surface lies at or near the aplanatic point of element B when element B is optically matched to corrector lens element C.

For the required optical matching element C requires to be made of a material having a refractive index which is intermediate that of the lens element B and of air and when element B is made of gallium arsenide it is preferred that element C is made of fused silica (having a refractive index of 1.95).

Objective element D may be made of any of a wide range of materials (transmissive to millimetre wavelength radiation) but preferably is of relatively high refractive index, i.e. greater than 2.5, in order to minimise its contribution to spherical aberration but, more importantly, to permit reduction of the curvature of refractive surface 9 which can, if excessive, give rise to unwanted surface reflectiqns. In the present example, element D is conveniently made of gallium arsenide (refractive index 3.60). In this connection if surface 9 is provided with an anti-reflection coating in order to reduce any unwanted reflection losses a coating of refractive index of 1.9 is preferred.

Meniscus lens element F preferably has a relatively low index of refraction, i.e. less than 2.0, which substantially eliminates field curvature at the image surface. It is preferred that element F be made of a plastic material such as TPX which has a refractive index of 1.45 and that surface 11 is hemispherical.

A preferred example of the system 2 is detailed in Table I which lists the materials used, the radius of curvature (mm) at each surface and the separations (mm) between surfaces. It will be noted that surface 9 in this preferred example is slightly aspheric for the purpose of removing residual spherical aberration. In this example the numerical aperture of the system is 2.0 which is desirable to match the radiation field to the substrate.

It will be understood that at least in the case of assembly 4 where reference has been made to refractive surfaces being in areal contact this is included to encompass the arrangement where anti-reflection coatings are provided at interfaces 5 and 7 in that these coatings can either be on one lens element only at each interface and/or on both lens elements at each interface and the areal contact referred to is between a coating and a lens element or between two coatings.

TABLE I

i - Inter Surface Curvature Thickness Refractive Surface (mm) (mm) Material index Element 1 Flat 5.0 Silicon 3.40 A 3 Flat 38.83 Gallium 3.60 B 5 34.60 Arsenide 5 34.60 3.89 Fused silica 1.95 C 7 107.18 7 107.18 14.0 Gallium 3. 60 D 9 138.07* Arsenide 9 138.07* 55.72 Air 1.0 11 33.55 11 33.55 l 3.64 TPX 1.45 F 13 66.35 * Slightly aspheric

Claims

CLAIMS 1. An optical imaging system for radiation in the millimetre waveband, comprising an image-forming triplet lens assembly formed by a corrector lens element lying intermediate and having concave refractive surfaces respectively in areal contact with a hyper-hemispherical lens element and an objective lens element, the exposed surface of the objective lens element being rendered anti-reflective, and a meniscus lens element axially separated from the objective lens element of said triplet assembly, wherein the hyper-hemispherical lens element and the objective lens element each has refractive index of at least 2.5, the corrector lens element has a refractive index intermediate that of the hyper-hemispherical lens element and of air, and the meniscus lens element has a refractive index of less than 2.0 whereby radiation in the millimetre waveband incident on the meniscus lens element from a wide angle field of view is imaged onto a substantially planar image surface adjacent the aplanatic point of the hyper-hemispherical lens element.
2. A system as claimed in claim 1, wherein the exposed surface of the objective lens element is rendered antireflective by means of an anti-reflection coating.
3. A system as claimed in claim 1, wherein the exposed surface of the objective lens element is rendered antireflective by texturing of the lens surface.
4. A system as claimed in claim 3, wherein said exposed surface is weakly aspheric to remove residual spherical aberration.
5. A system as claimed in any preceding claim, wherein the refractive surface of the meniscus lens element adjacent the objective lens element is hemispherical.
6. A system as claimed in any preceding claim, wherein the refractive surface of the hyper-hemispherical lens element remote from the meniscus lens element is planar and a semi-conductor substrate incorporating a radiation detector array is located in areal contact thereat.
7. A system as claimed in claim 6, wherein said semiconductor substrate is bonded to said surface by a bonding material having a thickness of less than 25 microns.
8. A system as claimed in any preceding claim, wherein the various refractive surfaces of said lens elements are free from concentricity.
9. A system as claimed in claim 1 and substantially as hereinbefore described with reference to the accompanying drawing and having the particulars set forth in Table I hereof.

9. A system as claimed in claim 1 and substantially as hereinbefore described with reference Lo the accompanying drawing and having the particulars set forth in Table I hereof.

Amendments to the claims have been filed as follo- 4. A system as claimed in claim 3, wherein said exposed surface is weakly aspheric to remove residual spherical aberration.

5. A system as claimed in any preceding claim, wherein the refractive surface of the meniscus lens element adjacent the objective lens element is hemispherical.

6. A system as claimed in any preceding claim, wherein the refractive surface of the hyper-hemispherical lens element remote from the meniscus lens element is planar and a semi-conductor substrate incorporating a radiation detector array is located in areal contact thereat.

7. A system as claimed in claim 6, wherein said semiconductor substrate is bonded to said surface by a bonding material having a thickness of less than 25 microns.

8. A system as claimed in any preceding claim, wherein the various refractive surfaces of said lens elements are free from at least two being concentric.