GB2526506A

GB2526506A - Covert marking apparatus

Info

Publication number: GB2526506A
Application number: GB9507796.2A
Authority: GB
Inventors: Keith Loder Lewis; Jonathan Ray Milward
Original assignee: Qinetiq Ltd
Current assignee: Qinetiq Ltd
Priority date: 1994-05-03
Filing date: 1995-04-13
Publication date: 2015-12-02
Anticipated expiration: 2015-04-13
Also published as: GB9408938D0; GB2526506B; GB9507796D0

Abstract

Covert marking apparatus suitable for marking, for example, a rendezvous point, comprising a source 1 of monochromatic unmodulated radiation with which an object is illuminated at an intensity which is too low to be noticed in light of ambient radiation. Viewing of the scene through a suitable optical filter 3 which blocks radiation of different wavelength from that selected causes the illuminated object to stand out from the rest of the scene.

Description

COVERT MARKING APPARATUS

This invention is concerned with the covert marking of a rendezvous position.

A common problem encountered by troops during parachute drops, especially at night, concerns the marking of rendezvous points.

The troops are scattered over a wide area and have to rendezvous at a point which is typically marked by non-covert means such as a torch. Such means are equally visible to enemy forces. There is a long telt need for a means of indicating a rendezvous point which can only be detected by friendly forces.

The current invention offers a solution to these problems by exploiting the principle of spectral contrast enhancement. By this principle, an object is subjected to illumination with radiation of a selected wavelength, the power of the illumination being such that is not noticeable in the presence of ambient illumination such as moonlight. However, with the aid of suitable means for blocking ambient illumination of different wavelengths from that selected, the illuminated object stands out from the rest of the scene.

The invention has other applications in, for example, beach marking where a point on shore could be marked such that it is visible only to forces out at sea, or drop zone marking, where a natural feature visible from the air could be marked to designate a drop zone for troops or supplies.

According to this invention, apparatus for covertly marking an object comprises:-means for illuminating an object with radiation of a selected wavelength; means for detecting the illuminating radiation and means for blocking radiation of different wavelengths from that selected.

In a preferred embodiment the means for blocking radiation of different wavelengths from that selected is a narrow bandpass filter which has a high transmission at the selected wavelength.

In a further preferred embodiment the means for illuminating an object with radiation of a selected wavelength is a laser.

In a further preferred embodiment means for illuminating an object with radiation of a selected wavelength is a laser diode.

A further preferred embodiment includes means for controlling the temperature of the laser diode.

A further preferred embodiment includes:-a laser diode producing radiation in the near infrared region of the electromagnetic spectrum; infrared imaging equipment sensitive to the radiatipn so produced and a narrow bandpass filter which has a high transmission at the wavelength of the radiation produced.

A further, preferred embodiment includes means for modulating the output of the illuminator.

The invention will now be described by way of example only with reference to the following diagrams in which:-figures la -id illustrate the principle of spectral contrast enhancement, figure 2 shows a schematic representation of the current invention, figure 3 represents the illuminator used in a particular embodiment of the invention and figure 4 illustrates the shape of the collimated beam produced by the illuminator of the current invention.

Referring to figure la, the night vision equipment used in this particular embodiment is equipped with a generation III image intensifier tube whose sensitivity extends from about 560nm to about S9Onm with a maximum at about 8lOnm.

Referring to figure ib, the object to be marked is illuminated with radiation of a wavelength which falls within the sensitivity range of the night vision equipment. In this embodiment the illuminating radiation was chosen to coincide with the maximum sensitivity of the night vision equipment.

Referring to figure ic the scene is viewed through a narrow bandpass filter which blocks substantially all radiation except that of the selected wavelength. Therefore, the system response (figure id) is limited to the passband of the filter and any object illuminated with radiation within this passband stands out from the rest of the scene.

Referring to figure 2 this particular embodiment comprises an illuminator 1, a night vision sight 2 and a narrow bandpass optical filter 3.

The object to be covertly marked 4 (eg a bush or a tree) is illuminated by the illuminator 1 with radiation of a wavelength which is selected from the operating range of the night vision sight and, in this case, is in the near infrared region of the electromagnetic spectrum.

The night vision equipment 2 used in this particular embodiment comprises a standard "KITE" (TM) sight (Mk IV) available from Pilkington PE Ltd., St Asaph, UK.

The power of the illuminating radiation is such that it is swamped by ambient illumination such as moonlight and is invisible when using standard night vision equipment alone.

However, when the illuminated object is viewed using the night vision sight 2 in conjunction with the filter 3 it stands out from the rest of the scene. -The filter 3 was supplied by OCLI Optical coatings Ltd, Ridge Way, Hillend Industrial Park, Nr. Dunfermline, Fife, KYL1 SFR, UK and comprises a 3mm thick 7.62cm diameter RG780 glass substrate coated on either side. One of the coatings is a long wavelength cut off, the other is a Fabry-Perot etalon type band pass filter. The short wavelengths are cut of f by the RG780 glass substrate. This filter has a. transmission in excess of 80% at SlOnin and of less than 1% average from 400nm to l000nm outside passband.

Referring to figure 3, the illuminator 1 comprises at least one laser diode 5 which operates at the desired wavelength. The laser diode 5 is accompanied by associated diode driver circuitry 6, a battery 7 and collimating optics 8.

Incorporation of more than one laser diode, each of which operates at a different wavelength, would allow illumination of different objects at different wavelengths. The different objects could then each be viewed with the aid of a suitable narrow bandpass filter which is transmissive at the appropriate wavelength.

The laser diode used in this embodiment was a LTO1O available from Access Pacific Ltd., Kyxnbrook School House, Kintholton Road, Keysoe, Bedford MK44 2HH.

Typically, the output wavelength of a laser diode varies with temperature and in theory, large fluctuations in temperature could cause the output wavelength of the device to drift outside the passband of the filter used. In practice however, the passband of the filter used in the current invention is wide enough to contain the range of wavelengths normally produced by the diode. Should use of a filter with a narrower passband be required (for example in a system where illumination at more than one nominal wavelength is used) then means for maintaining the temperature of the diode or diodes is commercially available in the f on of, f or example, Peltier coolers.

Direct exposure of the night vision equipment to sunlight would cause damage but for training and boresighting purposes during the day, a neutral density filter is supplied which greatly reduces the amount of light which may enter the equipment. The narrow bandpass filter used in this embodiment of the invention offers similar protection and by using this instead of the neutral density filter, this embodiment of the invention may be used during daylight hours. This offers an additional degree of covertness in that enemy forces would not be scanning with night vision equipment during daylight hours.

Under these or other circumstances it may be desirable to make the illuminating radiation easier to detect by friendly forces.

This can be achieved by modulating the illuminating radiation by, for example, periodically switching the diode on and off and the illuminator may include suitable means for doing so.

Referring to figure 4, the collimated beam produced by the illuminator 2 used in this particular embodiment takes the form of a cone with a substantially rectangular cross section. If the illuminator is placed, for example, 1Mm from the object to be illuminated then the illuminated area would be 3.lm by 1.Sxn.

Although the embodiment described above operates in the infrared region of the electromagnetic spectrum, this is not seen as limiting. For example the principle of spectral contrast enhancement could also be exploited in the visible region and in this case the means for detecting the illuminating radiation may be the human eye.

Claims

CLAIMS.1. Apparatus f or covertly marking an object comprising:-means for illuminating an object with radiation of a selected wavelength; means for detecting the illuminating radiation and means for blocking radiation of different wavelengths from that selected.
2. The apparatus of claim 1 where the means for blocking radiation of different wavelengths from that selected is a narrow bandpass filter which has a high transmission at the selected wavelength.
3. The apparatus of claim 1 where the means for illuminating an object with radiation of a selected wavelength is a laser.
4. The apparatus of claim 3 where the means for illuminating an object with radiation of a selected wavelength is a laser diode.
5. The apparatus of claim 4 and further including means for controlling the temperature of the laser diode.
6. The apparatus of claim 1 and further including a laser diode producing radiation in the near infrared region of the electromagnetic spectrum; infrared imaging equipment sensitive to the radiation so produced and a narrow bandpass filter which has a high transmission at the wavelength of the radiation produced.
7. The apparatus of any of the preceding claims and further including means for modulating the output of the illuminator.CAmendments to the claims have been filed as followsCLAIMS1. Apparatus for covertly marking and detecting an unmarked object comprising:-means for illuminating an unmarked object with unmodulated radiation of selected wavelength; means for detecting the radiation reflected by the unmarked object; and means for blocking substantially all the reflected radiation except that of the selected wavelength, said means for blocking being located between the unmarked object and the means for detecting the illuminating radiation, said means for blocking being a narrow bandpass filter which has a high transmission at the selected wavelength. - 2. The apparatus of claim 1 where the means for illuminating an unmarked object with radiation of a selected wavelength is a laser.3. The apparatus of claim 2 where the means for illuminating an unmarked object with radiation of a selected wavelength is a laser diode.4. The apparatus of claim 3 and further including means for controlling the temperature of the laser diode.5. The apparatus of claim 1 and ftirther including a laser diode producing radiation in the near infrared region of the electromagnetic spectrum; infrared imaging equipment sensitive to the radiation so produced and a narrow bandpass filter which has a high transmission at the wavelength of the radiation produced. x6. The apparatus of any preceding claims wherein the apparatus comprises a means for illuntating the unmarked object with illumination having a.wavelength that falls within the sensitivity range of the means for detecting the reflected radiation. C:7. The apparatus of any preceding claims wherein the apparatus comprises a means for illuminating the object with illumination having a wavelength that falls within the sensitivity range of the means for detecting the reflected radiation.