GB2310049A - Photochromic active optical filters - Google Patents

Abstract

A photochromic active optical filter (1) for protecting optical equipment against laser, especially pulsed laser, attack, which consists of a partly transparent container filled with a mixture (6) of an optionally - substituted transition metal carbonyl or a substituted transition metal * small Greek pi *-cyclopentadienyl dissolved in an organic solvent. When the mixture (6) is subjected to low power, high intensity uv, visible or ir light from, for example, a pulsed laser, it undergoes an extremely rapid increase in optical density.

Description

PHOTOCHROMIC ACTIVE OPTICAL FILTERS This invention relates to electro-optics and is particularly concerned with photochromic filters for filtering high intensity ultra violet, visible, or infra-red radiant energy. The invention further relates to photochromic materials, and to surveillance apparatus incorporating photochromic filters.

If high intensity infra-red, visible, or ultra-violet radiation strikes the human eye it can cause temporary or permanent blindness, even if the eye is only subjected to such radiation for extremely short periods of time. A battlefield observer is therefore vulnerable to attack from short bursts of coherent radiation emitted from lasers, particularly if the observer is using surveillance apparatus such as binoculars or a tank gun sight which would further intensify such radiation before striking the observer's eye. There are high intensity pulsed lasers capable of emitting a debilitating amount of radiation in a single pulse lasting less than 25 nano-seconds (ns).

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

Attacks from lasers emitting in the visible portion of the electromagnetic 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-Glass laser which emits light with a wavelength of 530 nm, an interference filter placed in the laser's 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 degradation of normal observational efficiency it is desirable that this protection is activiated only when excessive radiation occurs. Furthermore, a serious limitation of interference filters of certain designs is the high transmission for all wavelengths ashen the light impinges on these filters in off-axis orientations.

Various protection devices are known which exhibit a large increase in optical density once activated (Optical density is defined as log10 (Ii/It), where Ii is the intensity of radiation incident on the device and It is the intensity of radiation transmitted through the device). These devices are hereinafter referred to as active optical filters. The simplest device is perhaps the mechanical shutter but since it cannot operate on timescales much less than a millisecond it is too slow to protect against high intensity pulsed lasers. Some protection devices, such as certain types of sunglass lens, rely on certain photochromic materials for providing the large increase in optical density required for the device to act as an active optical filter.

Photochromic materials are materials whose optical density and/or colour alters on exposure to or withdrawal from certain types of electromagnetic radiation, and some photochromic materials, although remaining essentially transparent at low levels of radiations, are known to become activated on exposure to high intensity radiation and so are useful in active optical filters. These useful photochromic materials, hereinafter referred to as photochromic active optical filter materials, become increasingly opaque with increasing radiant energy input resulting in a sharp decline, above a certain input radiant energy threshold, in the percentage of radiant energy transmitted. however, one disadvantage of these known,useful photochromic materials is that their reaction time in becoming opaque is relatively slow, typically of the order of microseconds, which is again slow to protect against high intensity pulsed lasers. A further disadvantage of these known photochromic active optical filter materials is that many of them undergo a change in optical density, once subjected to high intensity radiant energy, which is irreversible.

It is one object of the present invention to provide a photochromic active optical filter which overcomes the above disadvantages and is thereby capable of providing protection -against high intensity pulsed lasers with minimal impairment of surveillance performance.

Other'objects and advantages of the present invention will become apparent from the following detailed description thereof.

According to the present invention there is provided a photochromic active optical filter for protecting against high intensity electromagnetic radiation which comprises an orgaometallic compound, selected from optionally-substituted transition metal carbonyls and subs titllted transition :tietai -cyclopentaaIenyls, dispersed is an or0-n1 compound which is substantially transparent to at least one of in5ra-red (ir), visible, and ultra-violet (uv) light and which is capable of reacting with the organometallic compound to produce a photochromic active optical filter material which, when subjected to high intensity light falling within the it, visible, and uv frequency range, increases in optical density.

The photochromic material may provide protection for one, some, or all frequencies within the frequency range.

The filter will generally be substantially planar in shape and of uniform thickness in order to provide protection against intense radiation incident over a wide area if desired but without introducing optical aberations which might be produced by using a curved filter or one of non-uniform thickness.

The organic compound may be a solid, or may be a liquid solvent.

When the organic compound is a liquid solvent then the filter will comprise the photochromic material disposed within a container at least a portion of which is substantially transparent to at least one of ir, uv and visible light.

The photochromic material of the filter of the present invention filters high intensity radiant energy by undergoing an extremely rapid increase in optical density when subjected to such energy, thereby considerably reducing the amount of incident radiant energy transmitted through the materials. Where the -2 incident energy consists of coherent light of 2-4MW mm intensity, the reduction in the amount of incident radiation power transmitted is found to be as much as 200 fold. The change in optical density of the present materials is found to occur generally within a few nanoseconds; thus they are capable of providing protection against pulsed lasers.

It is a considerable advantage of the present invention that when the source of high intensity radiation is removed, the present materials rapidly reverse to their initial low optical density state.

This means that when the present materials are used in an active optical filter for protecting the human eye or other electromagnetic radiation sensors, even if a large portion of the filter is subjected to high intensity radiation attack, full visibility is rapidly restored once the radiation ceases and the photochromic materials are returned to a state in which they are capable of resisting further such attacks.

In one embodiment of the present invention, -the photochromic material within the filter comprises the photo SJMtneS sen product of an organic compod, especIally a liquid organic solvent, and an optionally substituted Group VIA transition metal carbonyl of general formula I

in which M isa Group VIA transition metal atom in an oxidation a state of zero, -N-D-N- is a conjugated heterocyclic diimine group in which D is an organic group containing at least two carbon-carbon double bonds, y is O or 1, and m is (6-2y).

Although the photochromic material within the filter of the present invention may comprise a Group VIA transition metal hexacarbonyl dissolved in an organic solvent, it is found that unlike substituted transition metal carbonyls, the photochromic materials photosynthesised from mixtures of unsubstituted Group VIA transition metal hexacarbonyls and organic solvents generally readily dissociate into their carbonyl/solvent precursors when the source of radiant energy required for photolysis is removed. One exception to this is where the solvent consists of acetonitrile, which forms isolatable complexes with unsubstituted transition metal carbonyls which do not readily dissociate, but the resulting photochromic material is found not to undergo an increase in optical density when subjected to high intensity visible, ultra-violet or infra-red radiation and so falls outside the scope of the present invention. The presence of a useful photochromic material can therefore only be maintained by subjecting the ur.zubstIt-ed carbonyl/solvent mixture to a continuous source of radiation (usually W light).

Preferably, therefore, the transition metal carbonyl component of the present photochromic materials are substituted carbonyls which form complexes with various organic solvents at room temperature by first undergoing photolysis at low levels of electromagnetic radiation. The carbonyls are preferably substituted with a conjugated diimine group which facilitates the substitution of one further carbonyl ligand by a solvent molecule or a species supplied by the solvent. One preferred group of substituted transition metal carbonyls of general formula I is where y is 1 and where the conjugated diimine group i4--Dz comprises either 1,10 phenathroline or derivatives thereof or 2,2' bipyridyl or derivatives thereof.

Examples of such preferred metal carbonyls are chromium tetra carbonyl 1,10 phenanthroline and chromium tetracarbonyl 2,2' bipyridyl. These preferred carbonyls react with various organic solvents at low levels of visible radiation (450-550 nm, eg in normal daylight), undergoing a colour change to form stable complexes which are found to comprise useful photochromic materials in accordance with the present invention. These complexes are however easily oxidised and so must be prepared and used in the absence of air. The increase in the optical density of these complexes on exposure to high intensity electromagnetic radiation is found to be indirectly proportional to the radiation intensity.

In a further embodiment of the present invention, the photochromic material within the filter comprises the photosynthesised product of an organic compound, especially a liquid organic solvent, and a transition metal TT-cyclopentadienyl dimer of general formula II.

in which Mb is a group VIA transition metalespecially molybdenum or tungskn,in an oxidation state of 1.

An example of such a dimer is molybdenum cyclopentadienyl tricarbonyl dimer. These dimers are especially preferred because they are found to undergo a multi-stage reaction with the solvent in the presence of visible radiation (450-550nm) and air and/or moisture to form stable complexes, which, when subjected.to -2 bursts of 2-4 MW mm intensity coherent light, are found to reduce the instantaneous amount of incident radiation transmitted through them by as much as 200 fold within a matter of nanoseconds.

In one yet further embodiment of the present invention, the photochromic material within the filter comprises the optionally photosynthesised product of an organic compour.d, especially a li < wnd organic solvent, and a compound of general ò=ula III

in which M is a metal selected from iron, cobalt and nickel, A is c a group selected from stannyl, silyl, alkyl and halide, Y is triphenylphosphine, m is O or 2, and n is O or 1 provided that when mis 0, n is 1 and further provided that when mis 2, n is 0. One preferred group of compounds of general formula III is where Mc is iron and m is 2. An example of an organometallic compound within this preferred group is Tt-cyclopentadienyl triphenylsilane iron dicarbonyl where A is triphenylsilane. Solutions of these preferred compounds in various organic solvents are stable to both normal daylight and air and act as photochromic materials in accordance with the present invention. One further preferred group of compounds of general formula III is where M is nickel, c m is 0, and A is a halide. An example of an organometallic compound within this further preferred group is Tr-cyclopentadienyl triphenylphosphine nickel chloride. This further preferred group of organometallic compounds when dissolved in certain organic solvents act as photochromic materials in accordance with the present invention both before and after undergoing photosynthesis with the organic solvents.

The present invention further provides an organometallic compound suitable or use in a photochromic active optical filter , which compound comprises a substituted transition metal Tf-cyclopentadienyl of general formula III

in which M is a metal selected from iron, cobalt and nickel, A is C a group selected from stannyl, silyl, alkyl and halide, Y is triphenylphosphine, m is O or 2 and m is O or 1 provided that when m is O n is 1 and further provided that when m is 2, n is 0. One preferred group of components of general formula III is where M c is iron and m is 2. An example of an organometallic compound within this preffered group is Tr-cyclopentadienyl triphenylsilane iron dicarbonyl where A is triphenylsilane. Solutions of these preferred compounds in various organic solvents are stable to both normal daylight and air, and act as photochromic materials in accordance with the present invention. One further preferred group of compounds of general formula III is where M is nickel, c m is 0, and A is a halide. An example of an organometallic compound within this further preferred group is Tr-cyclopentadienyl triphenylphosphine nickel chloride. This further preferred group of organometallic compounds, when dissolved in certain organic solvents act as photochromic materials in accordance with the present invention both before and after undergoing photosynthesis with the organic solvents.

e ov c compounds erlod in the filters of the present invention are preferably substantially transparent to low levels of visible radiation. They must be capable of either coordinating to certain transition metals in particular states of oxidation, or they must be capable of supplying a species capable of coordination. Aprotic solvents are found to be especially suitable for the purpose of the present invention, producing photochromic materials which undergo an exceptionally large increase in optical density when subjected to high intensity radiation. Examples of solvents which may be used include tetrahydrofuran, cyclohexane, acetone, acetonitrile, benzene, diethyl ether, ether acetate, methanol, dichloromethane, and chloroform.

The thickness of the photochromic material in the filter will depend at least in part on the concentration of the metal carbonyl in the solvent. The optical density of the filter, both before and during exposure to high intensity radiation is indirectly proportional to both the path length of the radiation through the photochromic material in the filter and concentration of the organometallic compound in the photochromic material. A filter having a high optical density when subjected to high intensity radiation is clearly desirable, but if this is achieved by increasingtheoptical path length and/or incrasing organometallic compound concentration, then the optical density of the filter in low levels of radiation will also be high and so the performance of a radiation sensor or the human eye behind the filter will be impaired. A reasonable compromise between optical clarity during normal use and protection performance against high intensity is found using a filter containing photochromic material having an organomezallic co.pomd contentration of between O.C5 and 10 mmol.l .cm (where "cm" refers to the path length in centimeters of radiation through the photochromic material within the filter).

The active optical filter of the present invention may be used on its own for filtering high intensity radiant energy, or may be used in conjunction with surveillance--app r tusfor magnifying radiation before being received by a radiation sensor or the human eye. Where the filter is used in surveillance apparatus, the filter is located in the optical path of the apparatus, and is preferably located substantially at an inter mediate focal plane of the apparatus. Location of the filter at an intermediate focal plane has the effect of reducing to a minimum the area effected by high intensity radiation (particularly a laser pulse) and, since the increase in optical density of the present photochromic materials is generally found to be indirectly proportional to incident radiation intensity, it also results in the greatest reduction in the amount of radiation transmitted because the filter is located where radiation intensity within the apparatus is focussed to a maximum.

Methods of preparing photochromic active optical filter materials in accordance with the present invention will now be described by way of example only. In the following Examples, all the organic solvents used were of Analar (registered Trade Mark) grade, unless otherwise stated.

Example 1 Chromium hexacarbonyl (BDH Chemicals Ltd) was dissolved at room temperature at a concentration of 1 mmol*in cyclohexane, and was then continuously subjected to uv light (250-350 nm) to yield a pale yellow product solution. The photosynthesis reaction which formed the product was found to be reversible as the pale yellow colour slowly disappeared when the source of uv light was removed.

Example 2 A product solution was prepared by an identical method to that described in Example 1 above, except that acetone was employed as the solvent instead. Again, the product solution was pale yellow in appearance, and the reaction which produced it was found to reverse slowly when the source of uv light was removed.

Example 3 Molybedenum hexacarbonyl (Fluorochem Ltd, 27.2% carbon) was dissolved at room temperature at a concentration of 1 mmolvin cyclohexane, and was then continuously subjected to uv light (250-350 nm) to yield a pale yellow product solution by a reversible photosynthesis reaction similar to that of Example 1.

Example 4 Tungsten hexacarbonyl (Fluorochem Ltd, 20% carbon) was dissolved at room temperature at a concentration of 1 mmol*in cyclohexane, and was continuously subjected to uv light (250-350 nm) to yield a pale yellow product solution by a reversible photosynthesis reaction similar to that of Example 1.

* per litre of solvent Example 5 Chromium tetracarbonyl phenanthroline was prepared by refluxing l.lg chromium hexacarbonyl (BDH Chemicals Ltd) and 0.95g 1,10 phenanthroline in 50ml toluene for two hours under nitrogen. The deep red crystals of chromium tetracarbonyl phenanthroline which formed on cooling were filtered and washed with 60 ml hexane before drying under vacuum.

The chromium tetracarbonyl phenanthroline as prepared was dissolved at room temperature in the absence of air at a concentration of 0.1 mol*in acetonitrile. The resulting red solution was exposed to daylight in a sealed airtight transparent container, whereupon the solution underwent an irreversible photosynthesis reaction to yield a pale yellow-brown product solution.

Example 6 Deep red crystals of chromium tetracarbonyl phenanthroline were prepared by the method described above in Example 5, and were then dissolved at room temperature in the absence of air at a concentration of 0.2 mmol*in acetone. The resulting red solution was exposed to daylight in a sealed airtight transparent container, which gave rise to a pale yellow-brown product solution by an irreversible photosynthesis reaction.

Exanrple 7 Chromium tetracarbonyl phenanthroline crystals prepared by the method described in Example 5 were dissolved at room temperature in the absence of air at a concentration of 0.2 mmol*in dichloromethane.

The resulting red solution was exposed to daylight in a sealed airtight transparent container which again gave rise to a pale yellow brown product solution by an irreversible photosynthesis reaction. Example 8 Molybedenum cyclopentadienyl tricarbonyl dimer

(Fluorochem Ltd, 39.7%C 2.2%H) in deep red crystalline form was dissolved at room temperature at a concentration of 1 mmol*in acetonitrile. The resulting deep red solution was exposed to daylight and air and/or moisture, which irreversibly changed the colour of the solution first to a less intense orange and then to a pale blue product solution.

* per litre of solvent Example 9 The method of preparation of Example 8 was repeated at a tricarbonyl dimer concentration of 0.5 mmol*in acetonitrile.

Example 10 The tricarbonyl dimer starting material of Example 8 was dissolved at room temperature at a concentration of 0.1 mmol*in chloroform, and was subsequently exposed to daylight and air and/ or moisture to change the colour of the solution irreversibly from deep red first to a less intense orange and finally to a pale blue product solution.

Example 11 TT -Cyclopentadienyl triphenylsilane iron dicarbonyl (rr-C=H=)(SiPh )Fe(CO) (Ph=phenyl) was prepared by first reducing 25 mmol of iron cyclopentadienyl dicarbonyl dimer

(Fluorochem Ltd, 47.2%C 3%H) with 1% sodium amalgam in 200 ml of tetrahydrofuran (THF) solvent to give NaFe(CO)2(tT-C H ). The 55 THF solvent had previously been distilled over sodium for 3 hours before use. The NaFe(CO)2(TT-C5H5) in the THF solvent solution was mixed and reacted with an excess (78.5 mmol) of triphenyl silyl chloride (97%, Aldrich Chemicals Ltd) at room temperature to yield an orange-brown solution. The THF solvent was then removed at room temperature under vaccum. The resulting oily residue was sublimed to given an orange waxy solid consisting of Il-cyclopentadienyl triphenylsilane iron dicarbonyl. All operations involved in the preparation, except the removal of THF, were performed under an atmosphere of nitrogen.

The above orange waxy solid was dissolved at room temperature at a concentration of 1 mmol *in acetonitrile, to yield a yellow product solution which was found to be stable towards air and daylight.

Example 12 TT-Cyclopentadienyl triphanylsilane iron dicarbonyl prepared in accordance with the method of Example 11 was dissolved at room temperature at a concentration of 4 mmol in chloroform to yield a yellow product solution which was found to be stable towards air and daylight.

* per litre of solvent Example 13 lT-Cyclopendadienyl triphenylphosphine nickel chloride (rT-C5H5)Ni(PPh3)C1 was prepared by refluxing nickelocene (?luorochem Ltd, 62.8%C 5.8%H) with nickel dichloro bistriphenylphosphine Ni(PPh3)2Cl in THF.

Ni(PPh3)2C1 was prepared by mixing 2.0g of NiCi2 .6H20 (Fisanes, analytical reagent) in looms of hot ethanol with 100 ml ethanolic solution containing 5.78g of triphenylphosphine (BDH Chemicals Ltd., general purpose reagent). The mixture was refluxed for 30 minutes. The blue crystalline product which was formed on cooling the refluxed mixture was filtered and washed with cold ethanol.

The cyclopentadienyl derivative was prepared by refluxing 1.9 g nickelocene and 6.5 g NiC12(PPh3)2 in 150 ml THF under a nitrogen atmosphere for 5 hours. The reaction mixture rapidly acquired the deep red-purple colour of the product (rr-C5H5)Ni(PPh3)Cl. The THF solvent was removed at reduced pressure and the residue dissolved in hot benzene and filtered.

Hexane was added to the benzene filtrate, and the resulting mixture was cooled to OOC to give red-purple crystals of the product. These crystals were then filtered off.

The above red-purple crystals were dissolved at room temperature in chloroform, in the absence of air, at a concentration of 0.28 mmol*.

Example 14 W-cyclopentadienyl triphenylphosphine nickel chloride prepared by the method described in Example 13 was dissolved at a concentration of 0.28 mmol,in the absence of air, in acetone.

The resulting red solution was exposed to daylight which irreversibly changed the colour of the solution to lilac.

* per litre of solvent In order to measure their effectiveness in protecting against a pulsed laser of wavelength\, samples of each of the product solutions prepared in accordance with the above Examples were transferred to a standard liquid spectroscopic cell with a 1 cm path length. The initial optical density (OD)of each sample in the cell was first determined, by subjecting each sample to low intensity electromagnetic radiation also of wavelength j(sample optical density ODS is defined as log 10(its / it5) wheren It is the intensity of the incident radiation on the sample and It is the intensity of the radiation tramsmitted through the sample). This established the normal optical clarity of the solution, and gave a good indication of the extent to which the solution, when used as a photochromic active optical filter material in association with a surveillance device, would be likely to impair the normal performance of the device.

Each sample-filled cell was then subjected to a potentially debilitating pulse of laser radiation of energy in excess of 50 mJ and full width at half maximum (FWHM) of approximately 35 ns.

The source of laser radiation used on the sample of solution prepared in accordance with Example 9 above was a lseodymium- Glass laser (J.K. Systems 2000) with Pockels cell Q switching, which was capable of emitting single laser pulses of wavelength 1060 nm.

For testing samples of the solutions prepared in accordance with all other Examples, the above laser included a CDA crystal frequency doubling facility which halved the emitted laser pulse wavelength to 530 nm. The sample-filled cell was placed at the focal point of a 300mm focal length condensing lens positioned in the path of the incident laser beam to the cell. The total energy of the laser pulse incident on the sample cell (Esi) was measured using a calibrated transmission energy meter (GEC GF or TF) permanently positioned between the lens and the laser, and the intensity of the pulse transmitted through the sample cell (1t5) was measured using a fast photodiode (RS BP X 65).

The value of Ei and the variation of Is with time over the duration of the pulse were compared with corresponding results obtained from firing a laser pulse, from the same source and of the same wavelength and FWHM, at a reference cell placed at the same focal point. The reference cell was selected to have a similar initial OD to that of the sample cell, but differed from it in that the reference cell contained a passive optical filter material ie a material whose optical density remained constant when subjected to high intensity radiation. The instantaneous optical density of the sample cell over and above its initial OD, t ODS, was calculated over the duration of the pulse from the following equation I

where r refers to the reference cell and s to the sample cell.

The results of laser testing carried out on samples of product solutions prepared in accordance with the above Examples are given in Table 1 below. The samples of Examples 1 to 4 inclusive required continuous irradiation with a uv lamp placed close to the sample cell in order to retain the sample liquid in an activated state for attenuating laser pulses. The samples of all other Examples remained in an activated state without any supplmentary source of low level radiation, and so could be used in total darkness if necessary before being struck by a laser pulse. The transient value of nODS for each sample was calculated after an elapse of 25 ns from the beginning of the pulse in order to determine the performance of each sample as a photochromic active optical filter material.

25 ns was approximately the length of time taken for the laser pulse to reach its maximum and most damaging intensity.

TABLE 1

Example Ds t ODS Es (mJ) Example OD I s 1 0.15 1.7 76.5 2 '6'0.15 1.5 82.1 3 'A0.15 1.8 76.8 4 0.15 1.5 77.2 5 0.1 1.85 79.6 6 0.15 1.1 75 7 0.2 1.1 70.5 8 0.08 2.4 1 110.5 9 0.5 0.85 132 10 0.1 1.3 135.5 11 9 O.1 1.2 110 12 J\ 0.15 2.0 88 13 0.3 1.0 126 14 0.3 1.9 126 All the samples tested were found to attain their maximum A ODS at or before 25 ns from the beginning of the laser pulse, with most achieving their maximum in about 15 ns. Furthermore the optical densities of all the samples were found to reverse to their previous, initial levels in a very short period of time from the end of the laser pulse, indicating that the sample solutions tested do not suffer permanent alteration as a result of a pulsed laser attack, nor do they suffer loss of optical clarity for any length of time after such an attack.

Embodiments of an active optical filter and a surveillance apparatus in accordance with the present invention will now be described by way of example only with reference to the accompanying drawings of which Figure 1 is a perspective view of an active optical filter containing a photochromic active optical filter materialXaccording to the present invention.

Figure 2 is a side elevation of the active optical filter illustrated in Figure 1, Figure 3 is a sectional elevation of the active optical filter illustrated in Figure 2 along the line X-X, and Figure 4 is a diagrammatic representation of one optical path of a pair of binoculars, incorporating the active optical filter of Figure 1 shown in section.

Referring first to Figures 1, 2 and 3 there is shown an active optical filter 1 consisting of a pair of disc-shaped optical flats 2,2 mounted in parallel arrangements at either end of a tubular collar 3. The optical flats 2,2 are designed to pass visible radiation within the frequency range 400 to 750 nm only, and are conveniently as thin as possible to reduce to a minimum laser attenuation which would otherwise reduce the efficiency of the filter 1. The optical flats 2,2 are shown fitted into shouldered portions 4,4 respectively at either end of the collar 3, such that the space bounded by the optical flats and the collar provides a fluid-tight compartment 5. The optical flats may either be a tight push fit into the shouldered portions 4,4 , or may be retained therein by adhesive or some other form of retaining means. The depth of the shouldered portions 4,4 are such that the optical flat 2, 2 , when in position, are flush with the ends of the collar 3. The compartment 5 is filled with a liquid photochromic active optical filter material 6. Filling is effected through an entry port 7 in the sidewalls of the collar 3. The port 7 is sealed closed to prevent loss of liquid 6 by a removable plug 8.

Referring now to Figure 4 there is shown the active optical filter 1 of Figures 1, 2 and 3 incorporated in one of the optical paths of a pair of binoculars 10. The active optical filter is mounted between the objective lens 12 and eye-piece 14 so that the active optical filter liquid 6 covers the entire field of view. The filter 1 is shown situated with an intermediate focal plane F of the binoculars 10 passing through the medium 6. The box 16 ofFigure 4 is representative of the shortened prism present in the optical paths of binoculars.

Where the active optical filter 1 contains photochromic optical filter material prepared in accordance with the methods of Examples 1 to 4 inclusive, there is also provided a source of uv light within each optical path of the pair of binoculars adjacent the filter 1 eg a small uv lamp and an associated power supply (not shown). The uv light source is required to keep the medium in an activated state for protecting against laser attack when the binoculars are used.

It will be appreciated that active optical filters according to the present invention can also be used to protect sensors such as vidicons from laser attack. They may also be used as laser safety devices for protecting instrumentation, equipment and personnel in the vicinity of high power tunable lasers.

Claims (1)

1. Claims

   1. A photochromic active optical filter for protecting against high intensity electromagnetic radiation which comprises an organometallic compound, selected from optionally-substituted trw sition metal carbonyls and substItuted transition metal TT- cyclopentadienyls, dispersed in an organic compound ';hlch is substent PlljF transparent to at least one of infra red (ir), visible, and ultra-violet (uv) light and which is capable of reacting with the organometallic compound to produce a photochromic active optical filter material which, when subjected to high intensity light falling within the ir, visible, and uv frequency range, increases in optical density.

   2. A filter according to claim 1 wherein the organic compound is a liquid organic solvent and the filter comprises the photochromic material disposed within a container at least a portion of which is transparent to at least one of ir, visible, and uv radiation.

   3 A filter according to claim lor cla 2 wherein the photochronIc active optical filter material comprises the photosyn thesised product of the organic optionally substituted Group IVA transition metal carbonyl of general formula I

   wherein M is a Group VIA transition metal in an oxidation a state of zero-N--D--N- is a conjungated heterocyclic diimine group in which D is an organic group containing at least two carbon-carbon double bonds, y is 1 or 0, and m is (6-2y).

   4. A photochromic filter according to claim 3 wherein the con jugated diimine group -N-D--N- is either 1,10 phenanthroline or derivatives thereof or 2,2 bipyridyl or derivatives thereof.

   A A photochromic filter according to either claim 3 or claim 4 wherein the organic compound comprises tetrahydrofuran, benzene, diethyl ether, ethyl acetate, methanol, cyclohexane, acetone, acetonitrile, dichloromethane or chloroform, provided that when y is 0, the organic solvent does not comprise acetonitrile.

   6. A photochromic filter according to claim 1 cr claim 2 where the photochromic material comprises the photosynthesised product of the organiccomrourd and a substituted transition metal W-cyclopentadienyl dimer of general formula II

   wherein m is a Group VIA transition metal in an oxidation state of 1.

   7. A filter according to claim ó wherein iLb is molybdenum or tungsten.

   0. A photochromic filter according to claim 1 or claim2 wherein the photochromic material comprises the optionally photo synthesised product of the organic solvent and a substituted transition metal fr-cyclopentadienyl of general forumula III

   wherein M is a metal selected from iron, cobalt and nickel, c A is a group selected from stannyl, silyl, alkyl, and halide, Y is triphenylphosphine, m is O or 2, and n is O or 1 provided that when m is O,n is 1 and further provided that when m is 2, n is 0.

   9. A photochromic filter according to claim 8 wherein M is Fe and m is 2.

   10. A photochromic filter according to claim 9 wherein A is triphenysilane.

   11. A photochromic filter according to claim 8 wherein M c is Ni, m is O ,and A is a halide.

   12. A photochromic filter according to any one of the pre ceding claims 8 to 1; wherein the organic co=round comprises tetrahydrofuran, benzene, diethyl ether, ethyl acetate, methanol, cyclohexane, acetone, acetonitrile, dichlorom ethane, or chloroform.

   ,,*. A photochromic filter according to any one of the pre ceding claims wherein the product of concentration of the organometallic compound in the photochromic material and the thickness of the photochromic material within the filter exposed to radiation passing through the filter is between 0.05 and 10 mmol.l .cm .

   14. A photochromic filter according to any one of the preceding claims wherein the filter is substantially planar.

   15. A photochromic filter according to any one of the preceding claims wherein the filter is of uniform thickness.

   16. A photochromic active optical filter according to claim 1 substantially as hereinbefore described with particular reference to the drawings and any one of the examples.

   A A surveillance apparatus incorporating the photochromic active optical filter according to any one of the pre ceding claims, the filter being located in the optical path of the apparatus.

   13. A surveillance apparatus according to claim 17 wherein the photochromic filter is located substantially at an inter mediate focal plane of the apparatus.

   19 Surveillance apparatus according to claim 17 substantially as hereinbefore described with particular reference to Figure 4.

   2. An organometallic compound suitable 2or use in a photochromic active optical filter which comprises a substituted transition metal 7 -cyclopentadienyl of general formula III

   wherein M is a metal selected from iron, cobalt and nickel, C A is a group selected from stannyl, silyl, alkyl, and halide, Y is triphenylphosphine, m is O or 2, and n is O or 1 provided that when m is 0p is 1 and further provided that when m is 2, n is 0.

   21. A compound according to claim 20 wherein M is Fe c and m is 2.

   22. A compound according to claim 21 wherein A is triphenylsilane.

   23. A compound according to claim 2G wherein M is c Ni, m is 0, and A is a halide.

   24. An organometallic compound according to claim 20 substantially as hereinbefore described with particular reference to either Example 11 or Example 13.

   Amendments to the claims have been filed as follows 1. Surveillance apparatus incorporating a filter located in the optical path of the apparatus for providing protection against high intensity electromagnetic radiation, wherein the filter is a photochromic active optical filter comprising an organometallic compound dispersed in an organic comspound which is substantially transparent to at least one of infra red (ir), visible, and ultraviolet (uv) light and which reacts with the organometallic compound to produce a photochromic active optical filter material selected from (i) the photosynthesised product of the organic compound and a substituted Group VIA transition metal carbonyl of general formula I

   wherein M is a group VIA transition metal in an oxidation a state of zero, and -N--D--N- is a conjugated heterocyclic diimine group in which D is an organic group containing at least two carbon-carbon double bonds; (ii) the photosynthesised product of the organic compound and a substituted transition metal TX -cyclopentadienyl dimer of general formula II

   wherein M is a Group VIA transition metal in a oxidation b state of 1; and (iii) the optionally photosynthesised product of the organic

   Pcoln cuna selvent and a substitutcd transition metal Th -cyclopentadienyl of general formula III

   wherein M is a metal selected from iron, cobalt, and c nickel, A is a group selected from stannyl, silyl, alkyl and halide, y is triphenylphosphine, m is O or 2, and n is O or 1 provided that when m is 0, n is 1 and further provided that when m is 2, n is 0.

   2. Apparatus according to claim I wherein the conjugated diimine group -N--D--N- in formula I is either 1, 10 phenanthroline or derivatives thereof or 2,2 bipyridyl or derivatives thereof.

   3. Apparatus according to claim 1 wherein M in Formula II is b molybdenum or tungsten.

   4. Apparatus according to claim 1 wherein in Formula III M is C Fe and m is 2.

   5. Apparatus according teclai; 4 wherein A is triphenylsilyl.

   6. Apparatus according to claim 1 wherein in Formula III M is c Ni, m is 0, and A is a halide.

   7. Apparatus according to any of the preceding claims wherein the organic compound is a liquid organic solvent and the filter comprises the photochromflic material disposed within a container at least a portion of which is transparent to at least one of ir, visible, and uv radiation.

   8. Apparatus according to claim 7 wherein the organic compound comprises tetrahydrofuran, benzene, diethyl ether, ethyl acetate, methanol, cyclohexane, acetone, acetonitrile, dichloromethane, or chloroform.

   9. Apparatus according to any one of the preceding claims wherein the product of concentration of the organometallic compound in the photochromic material and the thickness of the photochromic material within the filter exposed to radiation passing through the -1 filter is between 0.05 and 10 mmol.l .cm.

   10. Apparatus according to any one of the preceding claims wherein the filter is substantially planar.

   11. Apparatus according to any one of the preceding claims wherein the filter is of uniform thickness.

   12. Apparatus according to any one of the preceding claims wherein the apparatus is a radiation magnifying apparatus and includes an objective lens located between the field of view under surveillance and the filter.

   13. Apparatus according to any one of the preceding claims wherein the filter is located substantially at an inLermediaie focal plane of the apparatus.

   14. Surveillance apparatus according to claim 1 substantially as hereinbefore described with particular reference to the drawings.

   15. An organometallic compound suitable for use in a photochromic active optical filter for surveillance apparatus, which comprises a substituted transition metal Tt -cyclopentadienyl of general formula III

   wherein M is a metal selected from iron, cobalt and nickel, c A is a group selected from stannyl, silyl and halide Y is triphenylphosphine, m is O or 2, and n is O or 1 provided that when m is 0, n is 1 and further provided that when m is 2, n is 0 and A is stanyl or silyl.

   16. A compound according to claim 15 wherein M is Fe and m is C 2.

   17. A compound according to claim 16 wherein A is triphenylsilyl.

   18. A compound according to claim 15 wherein M is Ni, m is 0, c and A is a halide.

   19. An organometallic compound according to claim 15 substantially as hereinbefore described with particular reference to either Example 11 or Example 13.