GB2287099A

GB2287099A - Photochromic filter assemblies

Info

Publication number: GB2287099A
Application number: GB9509104A
Authority: GB
Inventors: John Patrick Foley; Howard Sutton
Original assignee: Pilkington PE Ltd
Current assignee: Qioptiq Ltd
Priority date: 1992-03-27
Filing date: 1993-03-24
Publication date: 1995-09-06
Anticipated expiration: 2013-03-24
Also published as: GB2287099B; GB9509104D0

Abstract

An optical device (10) (e.g. the eye) is protected from exposure to very high levels of illumination (12) by the provision of a photochromic filter (11) which attenuates the transmitted visible light. The filter is of limited thickness for maximum efficiency being equal to the penetration depth for the UV activation wavelengths and is situated between lenses which expand and then diminish the incident beam 12 such that the filter is of sufficient area to be below saturation. <IMAGE>

Description

PHOTOCHROMIC FILTER ASSEMBLIES This invention relates to optics and to photochromic filter assemblies in particular.

Photochromic filters are already known and when exposed to wavelengths within an activation waveband they become less transmissive than previously to wavelengths within a transmission waveband. Typically the activation waveband is blue and ultra-violet light whilst the transmissive waveband is the visible spectrum which, when the filter is activated, can have an attenuation level of more than 99%. One known form of photochromic filter comprises a dispersion of photochromic materials or dyes in a carrier medium such as polyurethane.

Photochromic filters have already been proposed for use in protecting sensitive optical devices such as the human eye from saturation illumination levels such as occur with nuclear flash exposure. However, the known filters fail to get sufficiently dark when exposed to very high illumination levels and the resulting protection is inadequate.

We have now determined that the principal limiting factor in the performance of photochromic filters under high illumination levels is that the activation wavelengths are absorbed by those photochromic materials of the filter which are adjacent the surface of incidence of the illumination. This continues even after this layer of material has fully darkened. This creates a limited penetration depth (D) for activation wavelengths and the performance of the filter cannot be improved either by making the filter thicker or by increasing the concentration of the photochromic materials in the filter.

It is an object of the present invention to provide a new and improved method of protecting an optical device from high illumination levels. A further object is the provision of photochromic filter assemblies for use in the method.

According to one aspect of the present invention there is provided a method of protecting an optical device from high illumination levels comprising mounting in front of the device a plurality of photochromic filters and directing actuation wavelengths from the incident illumination to be incident on a face of each filter sufficient to activate each filter to the saturation level.

Preferably each filter has a thickness substantially equal to the penetration depth D.

Preferably two of said filters are disposed back-to-back and conjoined to form a single element, the actuation wavelengths being directed to be incident on each of the exposed faces of the element.

The present invention also provides a photochromic filter assembly comprising a plurality of photochromic filters and means for directing actuation wavelengths from incident illumination to be incident on a face of each filter sufficient to activate each filter to the saturation level.

Conveniently said directing means comprises an ultra-violet beamsplitter.

Preferably each filter has a thickness substantially equal to the penetration depth D. Preferably two of said filters are disposed back-to-back and conjoined to form a single element, the directing means being arranged to direct the actuation wavelengths onto each of the exposed faces of the element.

The assembly may be incorporated in a pair of goggles for human wear to protect the eyes of the wearer from nuclear flash exposure.

According to another aspect the present invention provides a method of protecting from high illumination levels an optical device having a predetermined aperture, comprising expanding the incident beam,intercepting the expanded beam with a photochromic filter, and thereafter diminishing the beam size for reception by the aperture of the device, the filter having an area which is sufficiently large to provide protection by functioning at or below its saturation level and being activated by wavelengths contained in the expanded beam.

Embodiments of the present invention will now be described with reference to the accompanying drawings in which:- Fig. 1 schematically illustrates a first embodiment; Fig. 2 shows a modification of the Fig. 1 embodiment; Fig. 3 illustrates a second embodiment; Fig. 4 shows a modification of the Fig. 3 embodiment and; Fig. 5 schematically illustrates a further embodiment.

The embodiment which is shown in Fig. 1 for protecting an optical device 10 from exposure to very high levels of illumination such as arise from nuclear flash comprises providing in front of device 10 a plurality of photochromic filters llA,llB,llC,llD such that the visible light content of the incident illumination 12 passes through all of the filters and is progressively attenuated by each. This is achieved in this embodiment by locating a first beamsplitter 13A before the first filter llA.

Beamsplitter 13A is fully transmissive to visible light wavelengths but only partly, say 50%, transmissive to actuation wavelengths (W/blue) so that 50% of the actuation wavelengths contained in the incident illumination 12 is incident on the front face of filter 11A causing it to reach saturation level. The diverted actuation wavelengths are directed by a reflector 14 to successive 50% beamsplitters 13B,13C,13D which are respectively angled to direct actuation wavelengths onto the rear faces of filters llB,llC,llD causing each to reach saturation level. The cumulative effect of each filter 11 provides maximum attenuation of the visible wavelengths prior to reaching device 10. Although each filter 11 can have any thickness, for maximum efficiency each should be substantially equal to the penetration depth D since this is both optically and cost efficient.

A W/blue edge filter 15 is positioned immediately in front of the device 10 to prevent transmission of these wavelengths to the device 10 and this has the additional effect of redirecting these wavelengths to the near face of filter l1D to enhance the actuation thereof.

In the Fig. 2 embodiment filters llA and llB are arranged back-to-back and physically conjoined to form a single element 16 which for maximum efficiency should have a thickness only twice that of the penetration depth D.

Otherwise the Fig. 2 embodiment works in the same way as Fig. 1 but with fewer components.

Figs. 3 and 4 illustrate arrangements which avoid the use of a primary beamsplitter 13A directly in front of the first filter llA. This is achieved by utilising the illumination falling in the area peripherally surrounding the first filter llA and directing that illumination or at least its UV/blue content onto the second filter llB.

In Fig. 3 the second filter llB is conjoined with the first filter llA and the single filter element 16 is peripherally surrounded by an annular element 17 which is non transmitting to visible light but transmitting to UV wavelengths. A frusto-conical UV reflector 18 directs the UV wavelengths onto the rear face of the filter element 16.

In Fig. 4 the first and second filters 11A,11B are conjoined to form a single element 16 and a third filter 11C is provided. The annular element 19 surrounding element 16 is both non-transmitting to visible light and has a 50% UV beamsplitter 19A providing illumination to the rear face of element 16. A frusto-conical UV reflector 20 directs the remaining W wavelengths onto the rear face of filter 11C.

In Fig. 5 the filter 11 is disposed in a region where the size of the illuminating beam 21 is expanded with respect to that of the initial illumination 12, the area of the filter 11 being sufficiently large to provide protection for the optical device 10 by functioning at or below its saturation level. The degree of expansion is determined from the ratio of maximum anticipated illumination to saturation level illumination. Expansion is provided by a beam expander 22. The output from the filter 11 is diminished by a beam contractor 23 for reception by the aperture 10A of the device. The filter 11 is activated by wavelengths contained in the expanded beam 21 and incident directly on the face of the filter.

Claims

1. A method of protecting an optical device from high illumination levels an optical device having a predetermined aperture, comprising expanding the incident beam, intercepting the expanded beam with a photochromic filter, and thereafter diminishing the beam size for reception by the aperture of the device, the filter having an area which is sufficiently large to provide protection by functioning at or below its saturation level and being activated by wavelengths contained in the expanded beam.

2. A photochromic filter assembly comprising a beam expander lens, beam contractor lens and a photochromic filter located between said lenses, the area of the filter being sufficiently large to provide protection for an optical device by functioning at or below its saturation level.

3. An assembly as claimed in claim 2, and substantially as hereinbefore described with reference to Fig. 5 of the accompanying drawing.