GB2203858A - Filter system countering laser dazzle - Google Patents

Abstract

Laser dazzle countering apparatus comprises two optical channels 11, 12 containing respective rejection filters A,B. Filters A and B are complementary, one transmitting where the other does not. Filter A may pass radiation within a first sequence of spaced passbands whereas filter B passes radiation within a second sequence of spaced passbands, the first and second sequences of passbands alternating in wavelength ascendency order. It is preferred that the filters A,B have a common rejection band at or about 500 nm. The channels 11, 12 may also incorporate fast light switches S1, S2 respectively blocking passage of dazzle radiation through the channel if such radiation is incident on the switch. <IMAGE>

Description

OPTICAL APPARATUS This invention relates to optical apparatus.

Typical high power lasers operate with narrow bandwidth centred on predetermined wavelengths, for example 532 nm or 694 nm, and if such high power radiation is transmitted through an optical apparatus to the eye of a human observer or to an electro-optic detector irreversible damage can occur. To avoid this problem narrow band filters for blocking radiation at the predetermined laser wavelengths are used in the optical system. However with the advent of more modern forms of laser, such as Qve lasers, laser radiation wavelengths are no longer predetermined and so the known arrangement of narrow band blocking filters is insufficient.Fortunately these modern forms of laser produce radiation of relatively moderate power levels and rather than causing irreversible damage to the detector produce short term saturation, which is reversible, and which is referred to as 'dazzle'.

It is an object of the present invention to provide optical apparatus incorporating a means of countering laser dazzle emanating from a laser operating at an unknown wavelength.

According to the present invention there is provided optical apparatus comprising means definIng a plurality of optical channels each receiving radiation from a common field of view and arranged to deliver radiation to detector means, the apparatus comprising rejection filter means whereby radiation wavelengths transmitted through one of said channels are rejected in the other or others of said channels.

The rejection filter means may comprise respective rejection filters in each optical channel.

In a two-channel example one filter may transmit only all wavelengths up to a predetermined wavelength and the other filter may transmit only all wavelengths after the predetermined wavelength, in wavelength ascendency order.

In another two-channel example one filter may transmit only within a first sequence of spaced passbands and the other filter may transmit only within a second sequence of spaced passbands, the first and second sequences of passbands alternating in wavelength ascendency order.

In practise the filters need not be precisely complementary provided that they do not have any common passband. It is preferred that the filters have one or more common rejection bands centred on the predetermined high power laser wavelengths so that in addition to countering laser dazzle high power laser radiation is blocked or severely attenuated.

The dual channel examples which have been described may be used in conjunction with a pair of human eyes functioning as a detector means in which case unwanted laser dazzle radiation will impinge upon one eye but not the other and for many circumstances this is acceptable.

If however such laser dazzle in one eye is troublesome it can be blocked by use of a fast light switch in each channel and subsequent to the filter, the switch being activated into its blocking mode by incident laser dazzle radiation. Such fast light switches may for example be of the PLZT type. It will also be appreciated that in this case the means which define the two channels are the human eyes themselves.

In another example radiation from a field of view is incident upon a beamsplitter which creates two optical channels. These two channels may subsequently be recombined. In this case the filter means may be a coating on the beamsplitter whereby the transmitted and reflected radiation beams are complementary in wavelength content.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 illustrates a first embodiment; Fig. 2 illustrates a second embodiment; Fig. 3 schematically illustrates rejection filter characteristics for the filters used in the Figs. 1 and 2 embodiments; Fig. 4 schematically illustrates an alternative form of rejection filter characteristic; Fig. 5 illustrates a practical example of the Fig. 4 characteristic; Fig. 6 illustrates a third emobidment utilising the schematic rejection filter characteristics of Figs. 3 or Fig. 4; Fig. 7 illustrates a fourth embodiment; and Fig. 8 schematically illustrates a rejection filter characteristic for the Fig. 7 embodiment.

As is shown in Fig. 1 optical apparatus 10 in its simplest form comprises a first channel 11 containing filter A and a second channel 12 containing filter B. Radiation detection in the channel 11 is effected by a human eye llA and radiation detection in channel 12 is effected by a human eye 12A. It is preferred that eyes llA, 12A, constitute the left and right eyes of a single human observer. Filters A and B of Fig. 1 are respective rejection filters having the transmittance characteristic schematically illustrated in Fig. 3 from which it will be seen that filter A transmits up to wavelength W and rejects wavelengths greater than W whilst filter B rejects wavelengths less than W and transmits wavelengths greater than W, taken in wavelength ascendency order.Accordingly filters A and B are complementary and laser dazzle radiation is blocked by one of these filters so that one eye llA, 12A, remains undazzled. In normal operation filters A,B, with the Fig. 3 characteristics present a coloured image to each eye llA, 12A, although the binocular image is coloured neutral. The colour of each monocular image can be neutralised by use of filters A,B, having the characteristics illustrated in Fig. 4 wherein each filter transmits only one sequence of spaced passbands and the passbands of filter A alternate in wavelength ascendency order with the passbands of filter B. In this arrangement it is preferred that filters A and B have a common rejection band at or about a wavelength of 500 nm.

In the second embodiment which is illustrated in Fig. 2 the two channels 11,12, in addition to respective filters A and B incorporate respective fast light switches S1, S2, which preferably are of the PLZT type. In this arrangement switch S1 is closed if and only if laser dazzle light is passed by filter A and switch S2 is closed if and only if laser dazzle light is passed by filter B.

Table I provides design details for a practical form of the filters A,B, having characteristics of the Fig. 4 type and these practical characteristics are illustrated in Fig. 5. It will be noted from Fig. 5 that this design provides filters A and B with a number of common rejection bands of which one incorporates the known high power laser wavelength of 532 nm whilst another incorporates the known high power laser wavelength of 694 nm. Accordingly with this design the apparatus 10 counters laser dazzle and blocks or severely attenuates high power laser radiation of the known irreversible damage kind. Table II sets forth the optical responses of the Table I filters.

Fig. 6 illustrates a quasi-binocular system 20 having a single incident channel 21 which is split into two channels 22,23, by means of a beamsplitter BS1, these two channels 22,23, being re-combined by beamsplitter BS2 and appropriate fold mirrors 24A, 24B, so that a single output channel 25 is provided. Beamsplitter BS1 incorporates a rejection filter having characteristics of either filter A or filter B so that beamsplitter BS1 functions to split the incident radiation beam into its wavelength complementary components. Laser dazzle light entering the apparatus 20 is controlled by the filter characteristics of beamsplitter BS1 and is selectively blocked in channel 22 or channel 23 by the fast optical switch S2, S1, provided therein.

Beamsplitter BS2 incorporates a filter which is the complement of the BS1 filter.

Fig. 7 illustrates apparatus 30 which is generally similar to apparatus 20 of Fig. 6 but apparatus 30 is arranged to split the incident radiation into three separate channels each with a fast light switch and to thereafter re-combine the radiation to provide a single channel output. In this arrangement the beamsplitting filters BS1, BS'3, BS'4 and BS2 have complementary reflectances and transmittances arranged to provide approximately one-third of the incident radiation in each channel. Fig. 8 schematically illustrates appropriate filter characteristics to achieve this function.

In each of the embodiments illustrated in Figs. 6 and 7 radiation emergent from apparatus 20, 30, may be directed to a detector means which can be either electrooptic or a human eye. In a modification of the Fig. 6 embodiment in which components 24A and BS2 are omitted the detector may be a pair of human eyes respectively viewing channels 22, 23.

It will be appreciated that the filters which have been described may be made from colour glasses, dye filters or thin film interference filters or even by a combination of these techniques. Thin film filters are preferred and the specific design set forth in Table I is a thin film filter design.

TABLE I

Component Design A C1(#D990nm), C2 (#D820nm), C2 (#D683nm) B C1(#D1070nm), C1(#D900nm), C2 (#D747nm) , C2 (#D626nm) C1 0.5L, (2H1,2L)2, (2H2,2L)8, 2H1, 2L, 2H1, 0.5L C2 0.5L, (2H1,2L), (2H2,2L)9, 2H1, 2L, 2H1, 0.5L H1 #/4 at #D of n = 1.88 H2 #/4 at # D of n = 2.34 TABLE II

% Integrated visible C.I.E. colour response coordinates Component Scotopic Photopic X Y A Component Scotopic Photopic X Y34.2 30.4 Component Scotopic Photopic X Y0.398 0.356 B Component Scotopic Photopic X Y31.9 35.8 Component Scotopic Photopic X Y0.254 0.321

Claims (6)

1. CLAIMS : 1. Optical apparatus comprising means defining a plurality of optical channels each receiving radiation from a common field of view and arranged to deliver radiation to detector means, the apparatus comprising rejection filter means whereby radiation wavelengths transmitted through one of said channels are rejected in the other or others of said channels.
2. 2. Apparatus as claimed in claim 1, wherein the rejection filter means comprise respective rejection filters in each optical channel.
3. 3. Apparatus as claimed in claim 1, wherein the plurality of channels is two in number, said filter means being arranged so that one channel transmits radiation only within a first sequence of spaced wavebands and the other channel transmits radiation only within a second sequence of spaced passbands, the first and second sequences of passbands alternating in wavelength ascendency order.
4. 4. Apparatus as claimed in claim 3, wherein said filter means provide one or more common rejection bands centred on predetermined high power laser wavelengths.
5. 5. Apparatus as claimed in any preceding claim, wherein each channel comprises a fast light switch subsequent to the filter means, the switch being activiated into its blocking mode by incident laser dazzle radiation.
6. 6. Optical apparatus as claimed in claim 1 and substantially as hereinbefore described with reference to any one of the embodiments and with reference to the accompanying drawings.