WO1992015036A1

WO1992015036A1 - Lens system

Info

Publication number: WO1992015036A1
Application number: PCT/GB1992/000299
Authority: WO
Inventors: Thomas David Binnie
Original assignee: Vlsi Vision Limited
Priority date: 1991-02-23
Filing date: 1992-02-20
Publication date: 1992-09-03
Also published as: GB2267580B; GB9103846D0; GB2267580A; GB9314125D0

Abstract

The present invention relates to a lens system (2) suitable for substantially direct interfacing with the image sensing surface (3) of a solid state image capture device (4). The lens system (2) comprises a lens (8) and a spacer (a) in substantially direct contact with each other. The spacer (9) preferably has a refractive index not less than that of said lens (8). The lens (8) and spacer (9) have refractive indices and are dimensioned so as to form an image in a plane at or in direct proximity to a rear face of the spacer (9) element remote from the lens (8) element, from an object. In use of the lens system (2) with the lens system (2) mounted substantially directly on the image sensing surface (3) of the image capture device (4) an optical image may be captured thereby. The present invention also relates to an image capture system including a lens system of the invention, which image capture is desirably provided with a video image display device and/or a video image recording device.

Description

LENS SYSTEM The present invention relates to lens systems and in particular to a lens system suitable for substantially direct interfacing with solid state image capture devices.

Although various very compact solid state image capture devices are used in various types of optical apparatus such as video cameras and the like, these generally employ relatively cumbersome lens systems which require, moreover, more or less complex and cumbersome mounting and positioning means for aligning the image produced by the lens system onto the image sensing surface of the image capture device.

It is an object of the present invention to avoid or minimise one or more of the above disadvantages.

The present invention provides a lens system suitable for substantially direct interfacing with the image sensing surface of a solid state image capture device, which lens system comprises a lens and a spacer in substantially direct contact with each other, said lens and spacer having refractive indices and being dimensioned so as to form an image in a plane at or in direct proximity to a rear face of said spacer element remote from said lens element, from an object, whereby in use of the lens system with said lens system mounted substantially directly on the image sensing surface of the image capture device an optical image may be captured thereby.

Thus with a lens system of the present invention, the lens system may be mounted substantially directly on the image sensing surface of a solid state image capture device to provide a focussed image thereon without the need for any complex positioning means and associated operations during manufacture of image capture systems. Furthermore by using suitable securing means e.g. suitable optical grade adhesive between the lens system and the solid state image capture device it will be appreciated that there may be provided image capture systems which are particularly resistant to rough handling which in conventional systems can result in misalignment of the lens system relative to the image capture device in use of such systems.

Thus in a further aspect the present invention comprises an image capture system comprising a lens system of the invention mounted substantially directly on the image sensing surface of a said solid state image capture device.

As used herein the expression "substantially direct contact" is used to mean that there should not be any signif cant interspace containing low refractive index material such as air i.e. no interspace having a thickness resulting in a significant optical effect. In the case of an air gap this should normally be not more than 500 μm, preferably not more than 100 μ , thick. In the case where resin or like material is used between the components adhesively to secure them together and has a refractive index comparable to that of the lens or spacer, it may be considered as an extension of the lens or spacer and thus need not be so restricted in thickness though preferably the thickness thereof should not be excessive and should be more or less similarly restricted.

Advantageously there is used a plano-convex (or possibly plano-concave - see below) lens with a substantially plane spacer for manufacturing convenience and economy but other combinations e.g. a bi-convex lens and a plan-concave spacer, may also be used. Whilst a particular advantage of the inventin resides in the possibility of having a complete lens system directly attached to the sensing surface of a solid state image capture device, it will be appreciated that this invention is also applicable to complex lens systems with a plurality of separate lenses or lens system (e.g. where it is desired to hae one lens or lens system movable relative to another), wherein one lens system is attached to the sensing surface and the other part(s) is (are) mounted separately therefrom. In one example there could be used a field-flattening lens system comprising a plano-concave lens and spacer attached directly to the sensing surface of a solid state image capture device.

Preferably the lens system is secured to said image sensing surface by an optical grade adhesive i.e. a substantially transparent optically uniform adhesive. Desirably there is used between the lens and spacer an adhesive having the same refractive index as the lens (or if preferred, as a spacer) and between the spacer and the sensing surface, an adhesive having the same refractive index as the spacer.

Preferably the spacer has a higher, most preferably a substantially higher, refractive index than the lens. Where the same refractive index is acceptable for both then it will be appreciated that the spacer could be formed integrally with the lens.

It will be appreciated that the radius (or radii) of curvature of the lens element and its refractive index may be varied through a wide range of values depending on the required performance in terms of depth field, image size, freedom from aberrations etc. In general there will desirably be used solid state image capture devices in the form of photoelectric sensor arrays (wherein photons are used to generate electric current and/or voltage or change electrical properties such as resistance etc.) which have relatively small size image sensing surfaces e.g. in the range from 0.1 to 5 cms across. Thus the lens system should in such cases desirably be formed and arranged to provide a similarly small-sized image. Where a wide angle filed of view is also required (e.g. in surveillance applications) , then a lens of relatively short focal length should be used e.g. for a field of view angle of 80 degrees the (maximum) focal length will not normally exceed 1.19 times the image height and for 60 degrees will not normally exceed 1.73 times the (maximum) image height, the (maximum) image height corresponding to half the sensing surface diamater. The use of a high refractive index spacer and the exclusion of any low refractive index material from the optical path significantly decreases aberration due to Petzval Curvature (otherwise known as curvature of field aberration) and limits spherical aberration. The lens system is therefore particularly advantageous in wide field and/or large aperture applications required for low light conditions. In general there is desirably used, for such wide angle applications, a lens element having a refractive index n<j in the range from 1.45 to 1.65, and a spacer element with a higher, refractive index n^ in the range from 1.45 to 1.85.

Various optical grade materials having suitable refractive indices are widely available. Low-dispersion glass such as type BK7 (available from various sources e.g. Schott Glaswerke) is particularly suitable for the lens element. The spacer element may be made of LaKlO glass also readily available. Other materials that may be used for the lens and/or spacer elements comprise plastics materials, although these are generally less preferred in view of their lower resistance to scratching and other damage and the lower refractive indices available. Nevertheless they may be acceptable for certain applications requiring low cost such as consumer door-entry and security cameras.

Suitable adhesive materials for use between the spacer and lens elements and between the spacer element and the solid state image capture device include optical grade epoxy resins.

In the case of other applications such as video telephones, camcorders, electronic still cameras, and imaging systems for robots etc where narrower fields of view are acceptable then there may of course be used a lens and spacer with lower refractive index differential or even with equal refractive indices.

Preferably aperture stop means are provided between the lens and spacer in order further to limit spherical aberration and reduce coma and astigmatism, in generally known manner. Thus, for example, in the case of wide angle lens systems of the invention there is generally used a fixed.-aperture stop providing an f-number in the range from f/2 to f/22.

In a preferred image capture system of the present invention the solid state image capture device comprises an integrated circuit image array sensor such as that disclosed in our earlier International patent application No. PCT/GB90/01452 (publication No. WO 91/04633) which has on-board signal processing means formed and arranged for directly providing a video signal output. Naturally though other image capture devices such as CCD, MOS and CID sensors may also be used.

Thus in another respect the present invention provides a video camera comprising an image capture system of the present invention provided with signal processing means formed and arranged for producing a video signal output.

Further preferred features and advantages of the present invention will appear from the following detailed description by way of example of a preferred embodiment illustrated with reference to the accompanying drawings in which:

Fig. 1 is a schematic perspective view of a video camera incorporating a lens system of the invention; and Fig. 2 is a schematic view of the camera of Fig. 1 in use.

Fig. 1 shows a miniature video camera 1 comprising a lens system 2 mounted directly onto the image sensing surface 3 of a solid state image capture device in the form of an integrated circuit image array sensor 4. The sensor 4 is mounted in a suitable housing 5 containing a power supply 6 and provided with a video signal output interface 7.

In more detail the lens system 2 comprises a generally hemispherical lens 8 having a radius of curvature of the order of 0.8 mm, and a cylindrical spacer element 9 of substantially larger diameter (ca. 1.7mm) and a length of 1.59mm, with an aperture stop 10 therebetween. The aperture stop 10 is of metal e.g. steel alloy with a thickness of 0.15mm and an iris diameter of 0.8mm providing an effective lens aperture of f2.0. The aperture opening is filled with clear epoxy resin 11 having a refractive index substantially similar to that of the lens 8 and secures the lens 8 and spacer 9 to each other and to the aperture stop 10.

The lens is of low dispersion glass Bk7) having a refractive index n<j of 1.568 and the spacer is of LaKlO glass which has a higher refractive index n^ of 1.7200. This combination produces low image blur and large image size (ca. 1.4mm image height from central axis). The spacer 9 has a length of around 1.59mm.

The abovedescribed lens system has an effective depth of field of from 2.0 cms to Co with a field of view angle of 90° and has an r s blur of around 5μm which is within the unit sensor pixel dimensions thereby providing a reasonably good video signal image output from the video signal output connection 7.

It will be appreciated that various modifications may be made to the above described embodiment without departing from the scope of the present invention. Thus for example the spacer element could be a composite element made up of a plurality of plane components. The lens element could also be composite though this would normally be less preferred due to the significantly increased complexity. The various surfaces of the lens system could moreover be provided with diverse coatings for e.g. reducing undesirable reflections and selective filtration of the incident light rays in generally known manner.

As shown in Fig. 2 the image capture device 4 is provided with a signal processing means 12, which conveniently (as noted above) may be in the form of on-board signal processing means in an integrated circuit image array sensor as described in our International Patent Publication No. WO91/04633, and which provides a video signal output at a suitable socket 7. The camera 1 may be coupled by suitable connections 13 to a suitable VDU (visual display unit) 14 and/or a video image recording device 15 for recording on a video image storage medium in the form of a video tape 16, though naturally other suitable media such as discs may also be used.

Claims

CLAIMS 1. A lens system suitable for substantially direct interfacing with the image sensing surface (3) of a solid state image capture device (4), characterised in that said lens system (2) comprises a lens (8) and a spacer (9) in substantially direct contact with each other, said lens (8) and spacer (9) having refractive indices and being dimensioned so as to form an image in a plane at or in direct proximity to a rear face of said spacer (9) element remote from said lens (8) element, from an object, whereby in use of the lens system (2) with said lens system (2) mounted substantially directly on the image sensing surface of the image capture device (4), an optical image may be captured thereby.

2. A lens system as claimed in claim 1 which comprises a plano-convex lens (8) and a plane spacer (9) .

3. A lens system as claimed in claim 1 which comprises a bi-convex lens (8) and a plano-concave spacer (9) .

4. A lens system as claimed in any one of claims 1 to 3 wherein said spacer (9) has a refractive index not less than that of said lens (8) .

5. A lens system as claimed in any one of claims 1 to 4 wherein said lens (8) has a refractive index n<j in the range of from 1.45 to 1.65.

6. A lens system as claimed in any one of claims 1 to 5 wherein said spacer (9) has a refractive index n<j in the range of from 1.45 to 1.85.

7. A lens system as claimed in any one of claims 1 to 6 wherein said lens system is secured to said image sensing surface by a substantially transparent optically uniform adhesive.

8. A lens system as claimed in claim 7 wherein said adhesive has a refractive index the same as at least one of said spacer (9) and said lens (8) .

9. A lens sytem as claimed in any one of claims 1 to 8 having an aperture stop (10) with an f-number in the range of from f2 to f22.

10. An image capture system comprising a lens system (2) according to claim 1 mounted substantially directly on the image sensing surface (3) of a solid state image capture device (4) .

11. A camera comprising an image capture system according to claim 10 provided with signal processing means (12) formed and arranged for producing an output signal suitable for at least one of driving a video image display device (15) and recording of a video image in a video image storage medium (17) .

12. A camera as claimed in claim 10 wherein said signal processing means (12) is coupled (13) to at least one of a video image display device (15) and a video image recording device (16) .