GB2139056A - Improvements in or relating to optical systems - Google Patents

Abstract

An observer (1) views a very dimly lit scene by means of an image intensifier (2). To prevent local illumination (4) from overloading the image intensifier, all such sources are pulsed (5) in synchronism faster than persistence of vision with a duty cycle of e.g. 0 &cirf& 1-1%. During each pulse the optical shutter (3), e.g. a liquid crystal light valve, is opaque, or the image intensifier is blanked electrically. <IMAGE>

Description

SPECIFICATION Improvements in or relating to optical systems This invention relates to optical systems in which a scene is to be viewed under adverse lighting conditions with the use of an image intensifier.

Typically, an image intensifier receives a relatively faint image which is converted into a corresponding pattern of charges or electrons from which a very much brighter image is generated for presentation to an observer. Image intensifier systems are very useful for permitting observation of dimly lit scenes and objects, since image intensifiers are capable of providing a very high degree of optical amplification so as to permit the eye to see in detail what, to the unaided eye, would merely be darkness. For example, clear and detailed vision is possible under conditions illuminated only by natural starlight. A difficulty which arises in the use of an image intensifier is that the performance can be impaired if an excessive level of illumination is allowed to fail upon its input optical surface.In this context, an "excessive" level may be one which is quite acceptable to the eye and may in normal circumstances be regarded as being fairly dim, but which because of the extreme sensitivity of the image intensifier, can cause gross overload.

it has been proposed to restrict the spectral sensitivity of an image intensifier by means of colour selective filters and to attenuate corresponding radiation from localised sources of radiation by means of a further spectral filter complementary to the first. However, the use of filters reduces the overall sensitivity of an image intensifier both due to the loss of that part of the spectral response in the attenuating region of the filter and by the insertion loss which occurs in the nominally transmitting region.

According to this invention, an optical system includes an image intensifier; a source of illumination; means for operating the intensifier in a pulsed manner so that it responds to applied illumination only during selected recurring periods; means for pulsing the source of illumination so that light is emitted therefrom only during selected recurring periods; and means for arranging that the two periods are mutually exclusive so that said image intensifier does not respond to light from said source.

By arranging that the image intensifier is inoperative or insensitive during those periods when the source of illumination emits light, it can be ensured that the image intensifier does not respond to that light, and, more importantly, the level of the illumination originating from the source is no longer of relevance so far as the sensitivity of the image intensifier is concerned. Thus, the image intensifier can be used by an operator in an environment which is quite brightly lit provided that the background illumination originates from a source or sources of light which are pulsed in accordance with the above manner. In this way, the other personnel may operate in levels of normal illumination and use illuminated instruments in a conventional manner, whilst the image intensifier can continue to respond to extremely low levels of illumination without risk of overload.

The operation of the image intensifier is not restricted to the visible range, but can extend to the infra-red and ultra-violet ends of the spectrum, and the term "illumination" is to be construed accordingly. As the image intensifiers are used by human operators, it is very desirable that the repetition rates at which the system is operated are above the flicker frequency of the eye, so that an operator is not directly aware of the pulsed nature of the operation of the system.

The invention is further described by way of example with reference to the accompanying drawing in which: Figure 1 illustrates an optical system in accordance with the present invention, and Figure 2 is an explanatory diagram.

Referring to Figure 1, there is shown therein an optical image intensifier system which is suitable for use in the cockpit of an aircraft. Particularly at night it is desirable for crew members to use an image intensifier to enhance the visibility of an external scene, which may be lit only by natural starlight.

Light from the external dimly lit scene is viewed by an observer 1 via an image intensifier 2, and an optical shutter 3 which is placed in front of the photo sensitive surface of the image intensifier 2 so as to control the light falling upon it. Light from various instruments and background illumination is invariably present in a cockpit and is necessary to permit the pilot and crew members to perform their essential functions. Light of this kind is indicated as originating from the source 4.

Although the display light and background illumination may be at a relatively very low level, nevertheless, their presence can cause serious degradation of the operation of the image intensifer 2, as such devices can be extremely sensitive to light. An image intensifier is able to view a scene illuminated only by natural starlight and to so amplify it as to present it to a viewer as though it were normal daylight scene. Even relatively low level light generated within a cockpit, or adjacent to the light receiving surface of the image intensifier 2 can cause gross overloading of the optical capabilities of the image intensifier.

This difficulty is overcome by controlling the optical shutter 3 and the display and illumination source 4 by means of a common pulse generator 5.

The generator 5 is operative to energise the illumination source 4 only whilst the optical shutter 3 is closed, i.e. non-transmissive so that light is prevented from reaching the image intensifier 2. The pulse repetition periods, and the duration of the pulses produced by the generator 5 are determined by a control circuit 7. Typically, the shutter 3 is a liquid crystal device of a kind which can be rapidly switched between opaque and transparent states by the application of suitable signals.

Typical waveforms are illustrated in Figure 2, and it will be seen that the shutter is rendered open, i.e.

light transmissive, for most of the time, but it is closed for short intervals of duration t1. The illumi nation source 4 is rendered active only for very short periods each of duration t2 and it is arranged that the periods t2 fall wholly within the slightly longer period t1. This ensures that no light which is generated by the illumination source 4 is allowed to fall upon the input surface of the image intensifier 2.

In practice, any instrument lights and background illumination can be constituted by solid state light sources, such as light emitting diodes, and it is possible to operate such devices with an extremely high mark to space ratio. The box 4 represents all of such sources of light, but in practice the individual sources will be spaced apart from each other at different locations. For example, the period for which the source 4 is energised so as to emit light may be between 1% and 0.1% of the total time, yet the pulses of light emitted during these very short periods can be very intense so as to provide an average level of illumination which is quite acceptable. If the source of illumination is of the kind which cannot be turned on and off rapidly, then it could incorporate, or be used in conjunction with, an optical shutter similar to shutter 3.

The repetition rate of the switching waveforms applied by the pulse generator 5 is made sufficiently rapid so that flicker is not apparent to an observer. In practice, the repetition frequency will be of the order of 60 Hz or higher.

Although in Figure 1 the periods during which the image intensifier receives external light are determined by the optical shutter, this need not necessarily be the case. Typically, the optical shutter 3 will be constituted buy a liquid crystal lighttransmissive device, as previously mentioned. Such devices are now capable of operating at the required speed and whilst they are in an optically transmissive state their optical attenuation is very low. Alternatively, however, the control signal from the pulse generator 5 can be applied directly to the image intensifier so that its optically sensitive surface does not respond to incident light whilst it is in an "off" state, so that the presence of the optical shutter is no longer required.

Direct gating of the image intensifier can present advantages, as the liquid crystal material causes small but significant insertion loss of incident light.

Claims (5)

1. An optical system including an image intensifier; a source of illumination; means for operating the intensifier in a pulsed manner so that it responds to applied illumination only during selected recurring periods; means for pulsing the source of illumination so that light is emitted therefrom only during selected recurring periods; and means for arranging that the two periods are mutaully exclusive so that said image intensifier does not respond to light from said source.

2. A system as claimed in claim land wherein the pulse repetition rates of said periods are the same.

3. A system as claimed in claim 2 and wherein the length of each period for which the image intensifier is non-responsive to applied illuminations is longer than the length of each period for which the source of illumination emits light.

4. A system as claimed in claim 2 or 3 and wherein the means for operating the image intensifier in a pulsed manner includes an optical shutter located in front of the input surface of the image intensifier, the shutter comprising a switchable liquid crystal device.

5. An optical system substantially as illustrated in and described with reference to Figure 1 of the accompanying drawing.