GB2119125A - Control of brightness in optical sights - Google Patents

Abstract

An optical sight enables light from a distant field of view to be combined with a locally generated display. A prism structure 3 diverts part of the incident illumination to a photosensor 11 having a mask 13 positioned in front of it. The brightness of the locally generated display is adjusted so as to keep it compatible with the brightness of the viewed scene and the display is optically combined with the main image path at a prism structure 4. The mask 13 is arranged to produce a spatially weighted average level of illumination at the photosensor 11 and is preferably interchangeable to allow for varying viewing conditions. The optical sight enables a viewed scene to be observed and the level of illumination of the locally generated display controlled so as to render it sensibly compatible with that of the viewed scene even under conditions of rapidly varying brightness. <IMAGE>

Description

SPECIFICATION Optical sights This invention relates to optical sights and is particularly applicable to those sights in which an image of the viewed scene is to be combined with a locally generated visual display. Such optical sights can be used as reticules and aiming sights in some weapon systems and helmet mounted displays, or in connection with information presented to the pilot of an aircraft or the driver of a vehicle. The external view which is observed through the optical sight may have an ambient illumination ranging from full sunlight to almost total darkness, and thus some control of the brightness of the locally generated display is required, so as to render it reasonably compatible with the brightness of the viewed scene.

The brightness level can be carried by means of a manual control or by using some form of photosensor to monitor the brightness of the scene.

The use of photosensors has led to severe problems in controlling the brightness when the viewed scene has a very high brightness contrast. For example, it may be required to view scenes having high brightness areas such as sunlit white cloud and low brightness areas such as green trees, both within the same field of view. In these cases, existing automatic controls commonly produce a display brightness which is either too bright or too dim for an operator to use effectively. This difficulty can be particularly severe where the optical sight is being used to track a target which is moving rapidly against a background of variable brightness, as the brightness of the display can vary in an abrupt and unpredictable manner.

The present invention seeks to provide an improved optical sight.

According to this invention, an optical sight includes means for viewing a scene and for producing an optical image thereof; means for superimposing on said image a locally generated optical display having a brightness which is variable in dependence on the level of a spatially weighted average illumination of said image.

Generally the level of ambient illumination of the image will be directly related to that of the viewed scene, but this may not always be the case where a very dim image is intensified in brightness prior to the superimposition of the optical display. If the optical sight incorporates an image intensifier, preferably the optical display is superimposed on to the viewed scene prior to image intensification being pe-rformed. This enables the power needed to energist the optical display to be kept to a very low level, which is not directly related to the degree of image intensification provided.

Preferably, the spatially weighted average is produced buy a mask ofspatially variable light transmissive properties positioned in front of photosensor means, the output of which is used to control the brightness of said display.

The mask can take a number of different forms depending on its position relative to the main received surface of the photosensor means. If the mask is positioned in contact with a photosensor having an extensive two dimensional light receiving surface, it can conveniently take the form of gradual and localised variations in the optical transmissive properties of a neutral density filter. If, however, the mask is spaced apart from the image receiving surface of such a photosensor, it can more conveniently take the form of an opaque plate having localised apertures or windows distributed throughout its area in accordance with a required weighting profile.

The invention is further described by way of example with reference to the accompanying drawing, in which Figure 7 shows an optical sight in accordance with the invention and Figure 2 shows a typical mask for producing a spatially weighted signal.

Referring to Figure 1, light from a distant object 1 in a viewed scene is received at the first lens 2 of an optical sight and passed through two prism structures 3 and 4 to a further lens 5, which forms part of an eyepiece which produces an image of the object 1 so that it can be viewed by an observer 6. A display device 7 generates a local optical display which is injected into the prism 4 via a suitable lens 8 so that it is superimposed on the image of the object 1. The prism structure 4 consists of two blocks of glass having a mutual interface 9 which acts as a semireflecting surface. Thus while light from the viewed scene passes through the prism structure 4 to the observer 6, light from the display 7 is reflected at the interface 9 so that it is combined with the viewed scene.Typically, a display device 7 consists of an array of light emitting diodes which are energised in accordance with an optical pattern required by the observer 6.

Typically, the optical sight forms part of a weapon aiming sight, in which case the locally generated display can comprise an aiming mark or reticule which is of a localised nature and which is usually positioned centrally in the field of view. As the optical sight is used to track a moving target, it is necessary for the observer 6 to be able to clearly see the target and the locally generated display even though the target may be moving across a field of view having large and possible rapid variations in brightness.

The prism structure 3 is similar to the prism structure 4 and also contains an optical interface 10, which diverts part of the light received from the viewed scene to a photosensor 11. A suitable lens 12 serves to focus the light at a two dimensional image receiving surface of the photosensor 11, and an optical mask 13 is positioned a short distance in front of the image receiving surface so as to enable the photosensor to respond to a spatially weighted average brightness of the viewed scene.

Other forms of optical beam splitters could be used in place of the prism structures 3 and 4. In particular the beam splitter nearest to the eyepiece can advantageously comprise a dichroic reflector having a very narrow reflective pass band centred on the wavelength of the light emitted by the display device 7. As such a dichroic reflector reflects only a narrow waveband, it passes most of the light from the distant viewed scene, thereby not significantly reducing its brightness.

One typical form of the optical mask 13 is illustrated in Figure 2. It comprises a rectangular opaque plate 15 having small rectangular apertures 16 distributed in a two dimensional pattern across its surface. The distribution of the apertures is chosen in accordance with a required spatial weighting profile, and it may vary depending on the nature of the viewing conditions. Thus the mask 13 is mounted in the optical sight so that it can be readily replaced by a different mask if the viewing conditions alter significantly. For example, if the optical sight is used on board an aircraft, it may be desirable to emphasisethe brightness level of a localised portion of ground immediately below the aircraft.

Conversely, with a weapon mounted on the ground, it may be desirable to position the apertures 16 so as to give greater weight to the sky region from which targets are expected to approach. Although the apertures 16 are widely distributed over the whole area of the mask 15, so as to integrate all of the light which is received, a larger number of apertures are located in the central region of the field of view, where it is expected that the operator's attention will be concentrated so as to give greater emphasis to the brightness of that region. In order to provide a fine adjustment to the profile of the spatial weighting, it may be desirable to modify the size and shape of the individual apertures as well as their position and quantity.

If the photosensor 11 is a large area device, as would be the case if it comprises a two dimensional array of individual photodetectors, it may be more convenient to position the mask 13 so that it is in contact with the image receiving surface of the photosensor 11. In this case, it may be desirable to use a neutral filter (i.e. not predominantly colour selective) having local variations in density value. If it is not required to provide an interchangeable mask 13, the spatial sensitivity of the photosensor 11 can be controlled during its manufacture so as to provide the required spatial response.

The output of the photosensor 11 comprises an electrical signal having an amplitude representative ofthe level weighted average brightness. This signal is applied to a control circuit 17, which operates to adjust the brightness of the display 7 so as to render it compatible with that of the viewed scene. To provide coarse adjustment, or to allow for the use of interchangeable colour filters etc. in the field of view, an external adjustment 18 can be included in the brightness control 17.

If the sight includes an image intensifier device, for use under night conditions, it is preferably positioned between the prism structure 4 and the lens 5.

This enables the output of the display 7 to be fed through the image intensifier device, and reduces the power consumption of the display 7 as it can be operated at a very low level of illumination.

Claims (6)

1. An optical sight including means for viewing a scene and for producing an optical image thereof; means for superimposing on said image a locally generated optical display having a brightness which is variable in dependence on the level of a spatially weighted average illumination of said image.

2. An optical sight as claimed in claim 1 and wherein the spatially weighted average is produced by a mask of spatially variable light transmissive properties positioned in front of photosensor means, the output of which is used to control the brightness of said display.

3. An optical sight as claimed in claim 2 and wherein the mask is an opaque plate having localised apertures or windows distributed throughout its area in accordance with a required weighting profile.

4. An optical sight as claimed in claim 2 and wherein the mask is in the form of gradual and localised variations in the optical transmissive properties of a neutral filter.

5. An optical sight as claimed in claim 1 and wherein the spatially weighted average is produced by means of a photosensor device having an image receiving surface with a spatially varying sensitivity to incident illumination.

6. An optical sight substantially as illustrated in and described with reference to the accompanying drawing.