GB2328289A

GB2328289A - Controllable Light Attenuator

Info

Publication number: GB2328289A
Application number: GB9815307A
Authority: GB
Inventors: Dirk Krogmann
Original assignee: Bodenseewerk Geratetechnik GmbH
Current assignee: Bodenseewerk Geratetechnik GmbH
Priority date: 1997-08-12
Filing date: 1998-07-14
Publication date: 1999-02-17
Anticipated expiration: 2018-07-14
Also published as: FR2767391A1; DE19734810B4; FR2767391B1; GB9815307D0; DE19734810A1; GB2328289B

Abstract

A controllable light attenuator is provided to protect a detector (10) responding to electro-magnetic radiation against excessive intensities of this radiation, especially to protect the detector (10) of a target tracking missile from laser radiation which is directed at the approaching missile from the target as a counter measure. Radiation with an intensity which is dependent on the incident radiation on the detector (10) is applied to an auxiliary detector (20) which generates an electrical radiation signal. In front of the detector (10), a filter is arranged as a light attenuator (24) consisting of a material, the absorbance of which is altered when exposed to electro-magnetic radiation. If a predetermined threshold value is exceeded by the radiation signal an auxiliary light source (22) is activated from the radiation signal which exposes the light attenuator (24) to electro-magnetic radiation such that, the absorbance of the light attenuator (24) is increased and the incident radiation on the detector (10) is attenuated to a harmless level.

Description

1 2328289 Controllable light attenuator The invention relates to a

controllable light attenuator for the protection of a detector responding to clectro-magnetic radiation against excessive intensities of this radiation, especially for the protection of the detector of a target tracking missile from laser radiation which is directed at the approaching missile ftom a target as a counter measure.

Modern target tracking missiles have a seeker with an image resolving detector, for example, a CCD matrix detector. From the image of the field of view containing the target, as detected by the detector, signals are obtained by image processing. by means of which the seeker is directed towards the target and/or the missile is guided to the target. If the target detects an approaching missile, counter measures must be taken to deflect the missile. Such counter measures could include directing a high intensity laser beam at the missile. Such a high intensity laser beam can, dazzle" or if necessary, destroy the detector.

It is an object of the invention to protect a detector, especially in a missile, against excessive intensities of radiation incident on the detector.

According to the invention this object is achieved in that radiation with an intensity which is dependent on the incident intensity on the detector is applied to an auxiliary detector which generates an electrical radiation signal, (b) a filter is arranged in front of the detector as a light attenuator consisting of a material, the absorbance of which is altered on exposure to electro-magnetic radiation.

2 (c) ail auxiliary light source is arranged to be activated by the radiation signal, this auxiliary light source exposing the light attenuator to clectro-magnetic radiation, such that, the absorbance of the light attenuator is increased and the radiation incident on the detector is attenuated to a harmless level.

A filter is thus arranged in front of the detector. Conventionally, this filter is transparent to the radiation, to which the detector responds. In target tracking missiles, as a rule, this radiation is infrared radiation. An auxiliary detector detects the intensity of this radiation. To this end, the auxiliary detector is exposed to a radiation intensity, which is dependent on the incident radiation on the actual detector. If the radiation signal of the auxiliary detector exceeds a predetermined threshold value, for instance, on the occurrence of a high intensity laser beam, then the auxiliary light source is activated. The filter, in turn, is exposed to the auxiliary light source. Thereby, the absorbance of the filter is increased such that, the incident radiation on the detector is attenuated to a harmless level. That occurs optically and electronically without mechanically moving parts. Thereby, the attenuation of the radiation can occur almost free from inertia such that, the detector is not endangered. With the radiation attenuated by the filter, the detector can continue to observe the source of the radiation and guide the missile to this source and, therefore, to the target.

Preferable the light attenuator is a crystal which exhibits the,13LI1RA" effect i.e. Blue Light Induced Infrared Absorption. An example of such a crystal is potassium mobate KNbO, The,13LI1RA- effect is described, among others, in an article,Blue Light Induced Infrared Absorption in K NB 0," J Opt. Soc. Am B vol. 11 No. 10 (1994). In general such an induced infrared absorption is obtained with a suitably doped crystal, if the wavelength of the second wavelength range is approximately half as large as the wavelength of the first wavelength range.

Modifications of the invention are the subject matter of dependent claims.

An embodiment of the invention is described in detail hereinafter with reference to the accompanying drawings.

3 is a schematic illustration of a controllable light attenuator for the protection of a detector responding to electro-magnetic radiation against excessive intensities of this radiation.

Fig.2 Fig.3 is a measuring diagram and shows the absorbance a for a crystal consisting of potassium niobate for infrared radiation as a function of the blue light intensity' bl.u.

shows schematically, the cooling of the detector and light attenuator.

Referring to Fig. 1, numeral 10 designates a detector, which responds to infrared radiation and is exposed to infrared radiation as electromagnetic radiation from a first wavelength range in an imaging path of rays 12 indicated schematically only. This radiation is indicated by an arrow 14. In the imaging path of rays 12, a mirror partially transparent to infrared radiation inclined at 45', is arranged as a beam splitter 16. The beam splitter 16 deflects a portion of the infrared radiation directed at the detector 10 in a path of rays 18 onto an auxiliary detector 20. The auxiliary detector 20 is a photodetector responding to the infrared radiation.

The intensity of the infrared radiation incident on the auxiliary detector is a certain fraction of the intensity of the Incident radiation on the detector 10. The fraction results from the reflectance of the beam splitter 16. The auxiliary detector generates an electrical radiation signal that is proportional to the intensity of the radiation on the auxiliary detector 20 and, therefore, proportional to the intensity of the radiation on the detector 10. If this electrical radiation signal exceeds a predetermined threshold value which corresponds to an excessive intensity of the radiation directed onto the detector 10, an auxiliary light source 22 in the form of a laser diode is then activated by means of a signal evaluation circuit (not illustrated). The auxiliary light source emits blue light as electro-magnetic radiation in a second wavelength range. This second wavelength range is half the wavelength of the -Infrared- first wavelength range.

4 A light attenuator 24 in the form of a doped crystal consisting of potassium mobate K Nb 0,, for example, is arranged in the imaging path of rays. The blue light emitted from the auxiliary light source 22 is directed by a lens 26 onto the light attenuator 24. Here, the light attenuator 24 is exposed to a light beam 26 from the auxiliary light source 22 normal to the direction of the path of rays 12. The absorbance of the light attenuator 24 for infrared radiation is increased due to the light attenuator 24 being exposed to radiation from the blue light of the auxiliary light source 22. Thereby, the dazzling of the detector 10 and adverse effect of its functions caused by the excessive infrared radiation is prevented. Without the irradiation by light from the auxiliary light source 22, the light attenuator 24 is well transparent to the infrared radiation.

In the illustrated preferred embodiment, several auxiliary light sources 22 are arranged around the light attenuator 24 normal to the direction of the path of rays 12 to increase the intensity of the radiation of the second wavelength range, as indicated in Fig. 1 with two auxiliary light sources.

According to the paper by Mabuchi, Polzlk and Kimble mentioned above, the absorption of infrared radiation operating with blue light is the stronger, the colder the crystal imaging light attenuator 24 is. The crystal is heated due to strong absorption of infrared radiation with the increase in the absorbance. Therefore, it is advantageous to cool the light attenuator 24. Also the operation of the detector 10 is improved by cooling. For this reason, the image resolving detector 10 in target tracking missiles is conventionally arranged in the cavity of a Dewar vessel and cooled by a Jouule-Thomson cooler. The Dewar vessel consists of a pot-shaped outer portion and an inner portion arranged within the outer portion and at a space therefrom, the outer and inner portions being interconnected along the rims of the,pots" and form the evacuated cavity therebetween. In the arrangement illustrated in Fig. 3, the detector 10 is arranged on the end surface of the inner portion of a Dewer vessel 30. The Dewar vessel contains a Joule-Thomson cooler by which the detector 10 is cooled. The light attenuator 24 is connected thermally to the cooled detector. The Jouule-Thomson cooler 32 cools not only the detector 10 but also, simultaneously, the light attenuator 24. The outer portion of the Dewer vessel 30 is provided with regions 36, 38 on its outer peripheral surface in the region of the light attenuator 24, which are transparent to the radiation of the auxiliary light source 22

Claims

Claims

A controllable light attenuator for the protection of a detector responding to electro -magnetic radiation against excessive intensities of this radiation, especially for the protection of the detector of a target tracking missile from laser radiation which is directed at the approaching missile from a target as a counter measure, characterised in that (a) radiation with an intensity which is dependent on the intensity incident on the detector is applied to an auxiliary detector which generates an electrical radiation signal, (b) a filter is arranged in front of the detector as a light attenuator consisting of a material, the absorbance of which is altered on exposure to electromagnetic radiation, (c) an auxiliary light source is controllable from the radiation siggnal, which exposes the light attenuator to electro-magnetic radiation, such that, the absorbance of the light attenuator is increased and the radiation incident on the detector is attenuated to a harmless level.
2. A controllable light attenuator according to claims 1, characterised in that, (a) the detector is exposed to radiation from a first wavelength range.

(b) the auxiliary light source emits radiation from a second vavelength range and 6 (c) the absorbance of the light attenuator of radiation from the first wavelength range is increased on exposure to radiation from the second wavelength range.
3.

A controllable light attenuator according to claims 1 or 2, characterised in that, the detector responds to infrared radiation.
4.

A controllable light attenuator according to one of the claims 1 to 3, characterised in that, the wavelength of the radiation in the second wavelength range is approximately half the wavelength of the radiation in the first wavelength range.
5.

A controllable light attenuator according to claims 4, characterised in that, the first wavelength range comprise infrared radiation and the second wavelength range comprise blue radiation.
6. A controllable light attenuator according to claim 5, characterised in that, the light attenuator exhibits a blue light induced, increased infrared light absorption effect (BLIIRA-Effect).
A controllable light attenuator according to claim 6, characterised in that, the light attenuator consists of potassium niobate (K Nb 0,).
8. A controllable light attenuator according to anyone of the claims 1 to 7, characterised in that, the light attenuator is cooled by a cooling device.
9. A controllable light attenuator according to claim 8, characterised in that, the detector (10) is cooled by a cooling device and the light attenuator is arranged in front of the detector in thermal contact with the cooling device of the detector.

7
10. A controllable light attenuator according to one of the claims 1 to 9, characterised in that, (a) the detector path of rays (b) a beam splitter the light attenuator is exposed to electro-magnetic radiation in an imaging and is arranged in the imaging path of rays in front of through which a portion of the radiation directed on the detector is deflected to the auxiliary detector.
11. A controllable light attenuator according to claim 10, characterised in that, the light attenuator is exposed to a light beam from the auxiliary light source normal to the direction of the path of rays.
12. A controllable light attenuator according to claim 11, characterised in that, a plurality of simultaneously activated auxiliary light sources are arranged around the light attenuator.
13. A controllable light attenuator according to claim 11 or 12, characterised in that, (a) the detector is arranged on the end side of a pot-shaped inner portion in the cavity of a Dewar vessel, the cooling device being located in the Dewar vessel, (b) the light attenuator of the Dewar vessel thermally therewith.

is located in front of the detector in the cavity with the detector screened and connected (C) in the side wall of an outer portion surrounding the inner portion of the Dewar vessel forming the cavity, windows region of the light attenuator the auxiliary light source.

are formed in the which are transparent to the radiation of 8
14.

A controllable light attenuator for the protection of a detector responding to electromagnetic radiation against excessive intensities of this radiation substantially as hereinbefore described with reference to and as shown in the accompanying drawings.