GB2188446A

GB2188446A - Image detector with wave-front converter to reduce undesirable reflections

Info

Publication number: GB2188446A
Application number: GB08706426A
Authority: GB
Inventors: Gudmann Slettemoen
Original assignee: Individual
Current assignee: Individual
Priority date: 1986-03-24
Filing date: 1987-03-18
Publication date: 1987-09-30
Also published as: NO861180L; GB8706426D0

Abstract

An image detector 4 has an optical imaging member 3 in the form of a lens or a mirror placed immediately in front of it for suitably changing the wave front of a coherent or partly coherent optical wave 1 arriving at the detector in order to avoid or reduce adverse effects on the image as a result of interfering internal reflections. Index matching liquid 6 may be used. The image detector 4 may be a TV cathode ray tube, TV camera tube, charge coupled device (CCD), photographic film or thermoplastic film. The optical member 3 may have an anti-reflection coating. <IMAGE>

Description

SPECIFICATION Image detector with wave-front converter The present invention relates to coherent imaging systems, especially systems that are based on use of TV cameras, and is concerned with the avoidance of undesirable reflections in such systems.

When a coherent or partly coherent light wave is incident on the front plate and/or photosensitive surface of an image detector, such as a TV camera, internal reflections will occur. Such reflections may, at best, have a disturbing influence on the quality of the image and, at worst, may completely destroy the image. Such is the case in optical systems based on optical interference. This includes classical interferometers, e.g. Michelson and Rayleigh interferometers, but the problem has become especially troublesome with the increased use of speckle interferometers and holographic systems and interferometers.

Currently, there are two known methods for eliminating such reflections. One method is based on use of an anti-reflection coating which is deposited by evaporation on one or several of the glass surfaces of the system.

The other method is based on use of an optical glass prism which is attached to the front plate of a TV camera with the aid of a viscous liquid acting as an "index matching liquid". The viscous liquid has an optical refractive index close to that of the glass of the front plate, and in this manner the reflection from the interface can be damped. The reflection coming from the other prism face is reflected onto the photosensitive face at a different angle from that of the original wave and interferes therewith in accordance with the angle. If the angle between the waves is large enough, i.e. the prism angle is large enough, the interference striation formed cannot be resolved by the TV camera, and in this manner it is also possible effectively to eliminate the reflection.

However, these methods have some serious deficiencies. If the photosensitive surface is separated from the front plate internal reflections may occur from faces that are inaccessible from outside. If the front plate has an antireflection coating deposited by evaporation the reflections may, nevertheless, become too strong, and in the case of attachment of a prism this may, in the worst case, result in an increased reflection at the interface. Further, if the incident light wave is divergent or convergent, the above mentioned methods may cause the resulting interference pattern to cover the entire image with fine striation and, thus, to destroy the quality of the image.

With the aim of avoiding these deficiencies, according to the present invention an optical imaging member is placed directly in front of the image detector, possibly being cemented or otherwise attached to the detector, for changing the wave front of a coherent or partly coherent optical wave arriving at the image detector.

In this manner it is possible to convert any degree of divergence/convergence of the incoming wave to another more desirable degree of divergence/convergence by suitable selection of the optical imaging member.

In an important special case, a positive lens is used with its front focal plane containing the point of divergence of the incident light wave, whereby the divergent light wave will be converted into a substantially plane wave.

If the front plate, the photosensitive surface, and any other internal reflecting faces of the image detector are plane parallel, reflections from these faces will also result in plane waves. In an ideal case the interference between all of these reflections will result in interference striation covering the entire resulting image, causing uniform intensity which will not affect the image quality at ali. In practice a slow or weak variation of intensity across the image may result, but in holographic interferometry (ESPI, Electronic Speckle Pattern Interferometry) the electronic video output signal from the image detector/TV camera is high pass filtered and, thus, the variations of intensity will also disappear.

The present invention will now be described in more detail with reference to the accompanying drawing which illustrates diagrammatically a particular embodiment of the invention.

The drawing shows a light wave 1 diverging from point 2. The light wave is refracted by an optical member 3, which in the present case is a plano-convex lens, so that the refracted wave exits from the optical member 3 in the form of a plane wave. This means that the front focal plane of the lens 3 coincides with point 2. In the present example the lens 3 is attached to the front glass plate 5 of an image detector 4 in the form of a TV cathode ray tube. This attachment is effected with the aid of an "index matching liquid" 6, although this is not critical.

The plane wave emitted by the lens 3 will pass through faces 7, 8, 9 and 10 and will cause several internal reflections. The face 10 in the present embodiment is separated from the face 9 of the front plate 5 and is the photosensitive face of the TV camera tube 4.

The internal reflections may be reflected a second time from the faces provided in front and will thus cause wave components propagating in the same direction and interfering with the original (direct) wave.

The internal reflections from the faces 7, 8, 9 and 10 will be plane and parallel compo nents if the reflecting faces are plane parallel, and present methods of production of TV camera tubes and CCD camera means ("Charged Coupled Devices"), etc. ensure that the different faces are very plane and parallel.

The only face not causing a plane wave is the front face 11 of the lens 3. However, there is complete access and control of this face, and it may be provided with a high quality anti-reflection coating for reducing the reflection to a minimum. The formed interference pattern is a ring pattern where striation density increases with the distance from the centre, and will only cover a small part of the image because the TV camera does not resolve the very fine rings. This reflection may be avoided if a curved mirror is used as the optical member instead of the lens 3.

In some cases it may be desirable not to convert the incoming wave front to a plane wave, but to a wave front having a different curvature designed to compensate for any possible curvature, etc. of surfaces forming part of the image detector 4.

Claims

1. An image detector having an optical imaging member placed immediately in front of it for changing the wave front of a coherent or partly coherent optical wave arriving at the image detector.

2. An image detector as defined in claim 1, in which the optical member is a lens.

3. An image detector as defined in claim 1 or claim 2, in which the optical member is provided with an anti-reflection coating.

4. An image detector as defined in claim 1, in which the optical member is a mirror.

5. An image detector as claimed in claim 4, in which the mirror is arranged to receive an inclined incident wave front.

6. An image detector as defined in any one of claims 1 to 3, in which the optical member is attached to the image detector.

7. An image detector as defined in claim 6, in which the optical member is attached to the image detector by a cementation agent consisting of a refraction index matching liquid.

8. An image detector as defined in any one of the preceding claims, in which the image detector is a TV-tube, a CCD camera (Charge Coupled Device), a photographic film, or a thermoplastic film.

9. An image detector according to claim 1, substantially as described with reference to the accompanying drawing.