GB2284485A - Improvements in or relating to surveillance apparatus - Google Patents

Abstract

A surveillance apparatus including an active filter comprising an optically transparent substrate (4) having a silver halide coating (6). A high intensity laser pulse entering the surveillance apparatus activates the filter thereby increasing its optical density to provide protection for the eye of the user of the apparatus. <IMAGE>

Description

IMPROvE,rtEJS IN OR BMULE'G TO SURVEILLANCE APP1RATUS This invention relates to surveillance apparatus and is particularly concerned with the protection of the human eye and other sensors from pulses of high intensity visible or near infrared radiation.

If high intensity infra-red, visible or ultra-violet radiation strikes the human eye it can cause temporary or permanent blindness.

A battlefield observer using surveillance apparatus such as binoculars or a tank gun-sight is therefore vulnerable to attack from lasers. There are high intensity pulsed lasers capable of emitting a debilitating amount of radiation in a single pulse lasting less than 25 nano seconds.

An observer can be readily protected from infra-red and ultraviolet radiation by permanently located filters interposed between his eyes and his field of view. For example, infra-red and ultraviolet filters can be chosen to have unrestricted transmission in the visible portion of the electro-nianetic spectrum so as to afford the observer permanent protection from infra-red and ultra-violet radiation without reducing the quality of his observations.

Attacks from lasers emitting in the visible portion of the electro-magnetic spectrum can also be countered without seriously compromising observational efficiency if the wavelength of the laser emission is known in advance. For example, to protect against a frequency-doubled Neodymium-Gla8s laser which emits light with a wavelength of 530 nm an interference filter placed in the optical path will protect the observer whilst having only a small effect on the observations. However, when the wavelength of the laser is not known broad-band protection in the visible portion of the spectrum becomes necessary and in order to minimise degredation of normal observational efficiency it is desirable that this protection is activated only when excessive radiation occurs.

A similar problem occurs in the infra-red spectral region with regard to protecting thermal imagers against laser damage. Carbon dioxide lasers can be tuned to emit a number of different wavelengths in the region 9.2 to 10.8 micrometers, and with isotope doping or Raman shifting this range can be extended still further. Provision of passive filter protection to a thermal imager operating in the 8 to 12 micrometer region so as to protect against all carbon dioxide laser lines would be likely to reduce the performance of the imager to an unacceptable level, so again radiation activited broad-band protection is necessary.

Various protection devices are known which exhibit a large increase in optical density once activated - hereirafter referred to as active filters. The simplest device is perhaps the mechanical shutter but since it cannot operate on time scales much less than a millisecond it is too slow to protect against high intensity pulsed lasers. Electro-optical devices are known vhich can be activated sufficiently quickly to provide this protection but they have high pre-activated optical densities which degrade the normal optical performance of the instrument in which one is incorporated.

It is an object of the present invention to provide a surveillance apparatus with an active filter which can activate quickly enough to provide protection against high intensity pulsed lasers with minimal impairment of surveillance performance.

In accordance with the present invention a surveillance apparatus includes an active filter comprising an optically transparent substrate having a silver halide coating, the active filter being located substantially at an intermediate focal plane of the apparatus.

Ideally, the optical density of the silver halide coating should be as low as possible and it has been found by experiment that this optical density decreases with the size of crystals present in the coating. Preferably, the silver halide coating is formed from crystals having diameters less than lym, ie microcrystals.

A preferreSmethod for forming the coating so as to have the preferred crystal structure comprises deposition in - vacuo of the silver halide upon the substrate. Optical densities of acceptably low level may be achieved at deposition rates in the order of 1 in.

The silver halide coating is preferably of silver bromide.

The silver halide coating may be provided with a protective coating of an optical transparent oxide, for example silicon monoxide, also depositedin-vaouo.

The substrate may comprise an optical flat or may conveniently be an existing optical component of the optical apparatus to which the silver halide coating is applied.

An embodiment of the invention will now be described by way of example only with reference to the accompanying drawings of which Fiare 1 is a perspective view of an active filter comprising a substrate, a silver bromide coating and a protective silver monoxide layer; and Figure 2 is a digrammatic representation of one optical path of a pair of binoculars incorporating the active filter shown in Figure 1.

Referring to Figure 1 there is shown active filter 2 comprising a disc-shaped glass optical flat 4 on which has been deposited in vacuc, a coating of silver bromide 6 and a protective coating of silica monoxide 8.

The preferred mehtod of manufacture of the active filter 2 is as follows. The substrate is first cleaned in order to reduce the rubber of impurities present on the substrate which can lead to clouding of the silver bromide coating. The substrates are soaked for about 5 hours in a detergent and subsequently in water and 2proponol. This procedure is also found to improve the longevity of the coatings during storage and use. The silver bromide coating 6 is thermally evaporated onto the substrate in 2 conventional vacuun coating plant (not shown) the glass substrate being heated to appro- C inately 60 C during evaporation of the silver bromide from a tungsten boat. This substrate temperature is high enough to give good adhesion Or the coating to the substrate (which increases with substrate ze-e-aturej without significantly degrading its trans- @@@@@@@@ compared to thole obtained at the optimum deposition temperature of about room temperature, 100 C being the @@@@@@@@@@@@@ -.

The silver bromide coating is preferably deposited as fast as possible, in the present example at about 4 jizt per minute, a total thickness oi 1.5 m being deposited.

Immediately after deposition the coatings so formed exhibit broadly neutral optical densities (where optical density is defined as log 10(Ii/It), Ii being the incident laser irradiance and It being the transmitted laser irradiance) of about 0.2 throughout the visible spectral region, independent of coating thickness. The coatings appear to be stable in artificial light but will rapidly darken in direct sunlight to a neutral optical density of about 0.5, again independent of thickness of the coating.

The activation speed and overall efficiency of the resulting active filter is dependent on the passive optical density of the coating at the incident laser wavelength. Higher passive optical densities result in lager optical density changes at lower incident energy densities of the laser on the coating. Desirably, therefore, the active filters are UV irradiated after manufacture.

An "active protection factor" can be defined as a ratio of the intensity of laser radiation which would have been transmitted by an active filter with no time dependent activity to that actually transmitted. Similarly a "passive protection factor" can be defined as the ratio of intensity of laser radiation which would have been transmitted by a filter with no passive optical density to that actually trnsmitted. The "total protection factor" is the ratio of intensities of the incident and transmitted irradiences which is equal to the product of tbe passive and active optical densities.

Typically, an active filter having a 1.25)un silver bromide coating will have active and passive protection factors of 14 and 3.2 respectively at an energy density of around 100 mJ/mm2giving a total protection factor of about 50: active filters having thicker coatings are expected to produce larger protection factors. The transmitted laser energy density is found to be practically constant over the range of incident laser energy densities from about 20 to 100 mJ/mm2, any increase in the active protection factor with energy density being almost totally compensated by the increased incident energy density.

In the present embodiment the silver bromide coating 6 is protected by the silicon monoxide layer 8, also deposited in vacuo, thickness of 0.1cm which improves its abrasion resistance and environmental stability. The resulting filter 2 is then exposed to a 4 watt mercury lamp for about 10 minutes to achieve maximum enhancement of the activation speed and efficiency. The presence of the silicon monoxide layer 8 has the effect of inhibiting the 1w darkening of the silver bromide layer 6 ; consequently, the passive optical density of the filter is raised from 0.2 to a maximum of only 0.25.

However, the enhancement of the active protection factor exhibited by the filter is found to be about the same as IJY irradiated active filters not having a silicon monoxide protective coating.

When subjected to a pulse of laser light of wavelength 530 nm of energy between 10 and 100 mJ/mm and full width at half maximum of approximately 25 nano seconds, the energy density transmitted is typically 2 mJ/mm2. Similar protection is provided at other optical wavelengths.

It is expected that thicker coatings of silver bromide will afford protection against laser pulses of higher energy.

The usefulness of any active laser protection device depends upon the protection factors achieved and the incident laser energy densities required to produce them. An active filter according to the present invention with protection factors approaching 50 at energy densities of the order of 100 mJ/mm is not useful when incorporated into simple goggles. An incident energy density of 100 mJ/ mm results in a transmitted energy density of about 2 nJ/mm which is still very much above the damage threshold of the human eye.

However, when an observer uses surveillance apparatus even relatively low, but hazardous, energy densities incident on the objective of the apparatus will result in exceedingly high energy densities in the vicinity of an intermediate focal plane of the apparatus. This will be sufficient to activate the filter effectively and provide an adequate degree of protection even when taking into account the magnification of the apparatus itself. For example, a laser operating at 530 mm having a nominal energy of 2.5J and beam divergence of 0.5 mrad will, if aimed from a distance of 5 km at a sight of X8 magnification having a 30 mm diameter entrance pupil, result in about 200,iJ of laser energy entering the apparatus. This assumes that about half the laser energy is lost during atmospheric propagation and that there are no transmission losses within the sight. The energy density at an intermediate focal plane of the apparatus will be of the order of 16 J/mm2, sufficient to activate the filter efficiently. In these circumstances the active filter reduces the energy reaching the user's eye from about 200 pJ to about 4 11J, an energy at which eye damage is unlikely to occur at any wavelength.

The greater intensity of the energy density near the focal plane enhances the activation speed of the filter, a fast speed being needed to provide a high active protection factor against short pulses of laser radiation.

The decomposition of silver bromide to silver that results when it is struck by a laser pulse is not spontaneously reversible, but an advantage of an active filter according to the present invention is that only that part struck by the laser become opaque and thus only a small area of the field of view will suffer from a degraded image quality after an attack. Placing the active filter in the vicinity of an intermediate focal plane of the surveillance apparatus ensures that the opaque area is very small and perhaps undetectable by the user.

It is necessary that the coating remains intact and attenuating after the first laser pulse so that subsequent pulses incident of the same position will not be transmitted. Therefore, if the energy density at the focal plane is expected to be too high for the mechanical survivability of the coating it is advantageous to move the shutter some small way from the focal plane.

Referring now to Figure 2 there is shown diagrammatically the active filter 2 of Figure 1 incorporated in one of the optical paths of a pair of binoculars 10. The active filter 2 is mounted between the objective lens 12 and eye-piece 14 so that the silver bromide coating 6 covers the entire field of view. The filter 2 is at an intermediate focal plane P. The box 16 of Figure 2 is representative of the shortening prism present in the optical paths of binoculars.

The filter 2 is arranged so that silver bromide coating 6 is on the side of the optical flat 4 which will be facing the surveyed scene during use of the binoculars so that any laser beam entering the binoculars 10 will strike the silver bromide coating 6 without first having passed through the optical flat 4 which will slightly attenuate the laser therefore reducing the efficiency of the filter 2 of the binoculars.

It will be appreciated that active filters according to the present invention can also be used to protect sensors such as from laser attack.

Claims (10)

CLA S

1. A surveillance apparatus including an active filter corprising an optically transparent substrate having a silver halide coating, the active filter bering located substantially at an intermediate focal plane of the apparatus.

2. A surveillance apparatns as claimed in claim 1 in which the silver halide coating has been formed upon the substrate by deposition invacuo, the substrate being maintained at a temperature of between 20 C and 1000C during deposition.

3. A surveillance apparatus as claimed in either one of claims 1 and 2 in which the silver halide coating is microcrystalline.

4. A surveillance apparatus as claimed in any one of claims 1 to 3 in which the silver halide coating has an optically transparent, abrasion resistant over-coating.

5. A ;urveillance apparatus as claimed in claim 4 in which the overcoating is an oxide.

6. A surveillance apparatus as claimed in claji: 5 in which the owe is silicon mono;ride.

7. A surveillance apparatus as claimed in any one of claims 1 to 6 in which the passive optical density of the silver halide coating h--s been increased to a maxirnum stable level by ultra-violet irradiation.

8. A surveillance apparatus as claimed in any one of claims 1 to 7 in which the coated side of the substrate is disposed so as to face a surveillance scene.

9. An active filter substantially as hereinbefore described with reference La Figure 1.

10. A surveillance apparatus substantially as hereinbefore described with reference to Figure 2.

10. A surveillance apparatus substantially as hereinbefore described with reference to Figure 2.

Amendments to the claims have been filed as follows CLAIMS 1. A surveillance apparatus including an active filter comprising an optically transparent substrate having a silver halide coating, the active filter being located substantially at an intermediate focal plane of the apparatus.

2. A surveillance apparatus as claimed in claim 1 in which the silver halide coating has been formed upon the substrate by deposition invacuo, the substrate being maintained at a temperature of between 60 C and 100 C during deposition.

3. A surveillance apparatus as claimed in either one of claims 1 and 2 in which the silver halide coating is microcrystalline.

4. A surveillance apparatus as claimed in any one of claims 1 to 3 in which the silver halide coating has an optically transparent, abrasion resistant over-coating.

5. A surveillance apparatus as claimed in claim 4 in which the overcoating is an oxide.

6. A surveillance apparatus as claimed in claim 5 in which the oxide is silicon monoxide.

7. A surveillance apparatus as claimed in any one of claims 1 to 6 in which the passIve optical density of the silver halide coating has been increased to a maximum stable level bg ultra-violet irradiation.

8. A surveillance apparatus as claimed in any one of claims 1 to 7 in which the coated side of the substrate is disposed so as to face a surveillance scene.

9. An active filter substantially as hereinbefore described with reference to Figure 1.