EP1598632A1

EP1598632A1 - A target system

Info

Publication number: EP1598632A1
Application number: EP04011850A
Authority: EP
Inventors: Anna-Karin Holmer; Arnold Fredriksson
Original assignee: Saab AB
Current assignee: Saab AB
Priority date: 2004-05-19
Filing date: 2004-05-19
Publication date: 2005-11-23
Also published as: US20080026346A1; WO2005111528A1

Abstract

The present invention relates to a target system for a weapon effect simulation system, said target system comprising a retro-reflecting prism having at least three back surfaces arranged to an incident simulation beam and wherein at least one of the back surfaces comprises a first, selectively reflecting layer arranged to reflect a simulation beam of a predetermined optical wavelength range and being substantially transparent to optical wavelengths outside said range. The system is characterized in that a second layer is mounted on the first layer, said second layer being arranged to receive beams transmitted through said first layer and said second layer being of a material having an index of refraction sufficiently high to avoid total internal reflection between said first and second layer within a predetermined range of incident angles.

Description

FIELD OF THE INVENTION

The present invention relates to a target system according to the preamble of claim 1.

BACKGROUND OF THE INVENTION

There exist today weapon effect simulation systems for use in combat training and/or shooting practice and which are arranged to simulate the effects of a specific weapon type. The weapons in one such weapon simulation system are provided with a simulation unit comprising a laser, a receiver for laser radiation and a hit evaluation unit, and the targets are provided with retro-reflecting prisms. The laser is arranged to emit a simulation beam simulating firing of the weapon. The retro-reflecting prism(s) of the targets is arranged to retro-reflect the simulation beam. The receiver of the simulation unit is arranged to receive the retro-reflected simulation beam and the hit evaluation unit is arranged to perform hit evaluation based on the received simulation beam.
However, there are some problems associated to use of the above described weapon effect simulation system. The retro-reflecting prisms tend to give rise to exposing reflections of incident ambient light and laser radiation of wavelengths other than the simulation wavelength. This is especially not desired in combat training, as the training is intended to be as realistic as possible. Further, if a laser range measurement unit is used by any real or simulated weapon system, there is a risk that the laser range measurement unit might be damaged if the emitted laser range finder beam intercepts a target retro-reflector unit, since the laser beam of the range finder is reflected with very little damping.
US 6 139 323 relates to an optical weapon effect simulation method for training of soldiers at least at two weapons, wherein each weapon is equipped as an attacking system as well as a target system. The attacking system includes a laser pulse transmitter arranged to transmit laser signals of at least two different wavelengths and a measurement unit arranged to detect signal reflections. The target system is provided with at least one retro-reflector with an integrated selective filter, an optical receiver with a selective filter and evaluation electronics. The selective filter is intended to ensure that only laser pulses of a defined wavelength are reflected/accepted by the target system. The attacking system then can identify target type based upon the wavelength of the laser signal received from the target system. However, the retro-reflectors as described in US 6 139 323 have the drawback of unwanted specular reflections for wavelengths other than the used simulation wavelength. Thus, the above described problems with exposing and damaging reflections is not solved.

SUMMARY OF THE INVENTION

One object of the present invention is to avoid total internal reflection for a useful range of incidence angles and inhibit unwanted for wavelengths other than the simulation wavelength, thus solving the problems described above.
This has been achieved by means of a target system in accordance with claim 1 comprising a retro-reflecting prism having at least three back surfaces arranged to an incident simulation beam and wherein at least one of the back surfaces comprises a first, selectively reflecting layer arranged to reflect a simulation beam of a predetermined optical wavelength range and being substantially transparent to optical wavelengths outside said range, wherein a second layer is mounted on the first layer, said second layer being arranged to receive beams transmitted through said first layer and said second layer being of a material having an index of refraction sufficiently high to avoid total internal reflection between said first and second layer within a predetermined range of incident angles.
One advantage of the retro-reflecting prism arrangement is that it does no give rise to internal reflections within the predetermined range of incidence angles and thus allow for specular reflections only for the simulation wavelength.
The basic function of the retro-reflecting prism is known in the art. In brief, a retro-reflecting prism has three reflecting back surfaces, meeting at right angles to each other, thus making up a right angle corner. Parallel incident light entering the prism will be reflected three times against these surfaces (once at each surface) and return in the opposite direction and parallel to the incidence angle, for a useful range of incidence angles.
The first layer is preferably coated on the back surface(s). The first layer ensures that back surface(s) reflects only light beams having a predetermined wavelength or a wavelength lying within a predetermined wavelength range. The first layer is therefore arranged to reflect beams having said predetermined wavelength or lying within said wavelength range. Other wavelengths are transmitted through the first layer.
The second layer is tightly mounted to the first layer so that no air gap is present between the layers. Thereby it is secured that the light beams exiting the first layer enters the second layer and thus are prevented from being reflected by the air back into the first layer. If the index of refraction of the second layer is sufficiently close to the index of refraction of the first layer, preferably substantially the same as the index of refraction of the first layer, a retro-reflecting prism arrangement is provided which reflects almost no light beams outside said predetermined wavelength/wavelength range.
In accordance with one embodiment of the invention, an intermediate layer is interposed between the first layer and the second layer. The intermediate layer is for example an adhesive layer. The three layers are, as described above, tightly mounted to each other so that no air gap is present between the layers. Further, the index of refraction of the intermediate layer is preferably substantially equal to the index of refraction of the first layer.
If the first and second layers are arranged on all back surfaces of the prism arrangement, it is ensured that the prism arrangement will not reflect beams other than those of the predetermined wavelength or wavelength range even when the light beams are incident from angles outside the predetermined range of incidence angles and only hit one or some of the back surfaces and thus are not properly retro-reflected. Therefore, the retro-reflecting prism arrangement causes almost no reflections from ambient light.
In accordance with one embodiment of the invention, the second layer is arranged to absorb the transmitted wavelengths.
In accordance with another embodiment of the invention, the second layer is diffuse reflecting and therefore has a diffuse reflecting back surface. One advantage of the diffuse reflecting back surface is that it absorbs little heat, whereby the temperature gradient of the back surface mirror is small, which results in preserved high accuracy of the retro-reflecting prism.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig 1 shows a known retro-reflecting prism
Fig 2 shows a beam incident on the retro-reflecting prism of fig 1 and specularily reflected at the back surfaces of the prism so that it is retro-reflected back from the prism.
Fig 3 shows a schematic side view of a retro-reflected beam
Fig 4 shows a schematic view of a selectively retro-reflected beam according to one embodiment of the invention
Fig 5 shows a schematic view of a selectively retro-reflected beam according to a second embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In fig 1, a solid retro-reflecting prism 1 is viewed from the front, and consisting of a corner cube. It has a front surface 9 and three back surfaces 2 and makes up a solid volume. As shown in figure 2, a beam 3 incident on the retro-reflecting prism will be specularly reflected in each of the three back surfaces 2 so that the beam is retro-reflected back 4 from the prism. The direction of the retro-reflected beam 4 is shown schematically in figure 3.
In the technique according to the prior art, there need not be any coating on the back surfaces 2 of the prism since the refraction index difference between the material of the prism and air is sufficient enough to give total internal reflection for a useful range of incidence angles. The back surfaces could be metal coated to obtain a wavelength dependent reflection in which case the not reflected part of the beam would be absorbed in the metal layer. However, all practical metal layers usable at the simulation wavelength, which is typically about 900 nm, will give a rather high reflectance also within the visible spectrum, thus giving rise to unwanted reflections from ambient light. One suggestion could be to apply a wavelength selective reflecting coating, for example a multi-layer dielectric coating, on at least one and preferably on all three of the back surfaces 2 of the prism 1. However, this would not work on its own since, in the same manner as for a non-coated prism, there will be total internal reflection between the last surface of the reflecting coating and air. The total internal reflection is not wavelength dependent, but all wavelengths within a very wide spectrum would be reflected in spite of the wavelength selective coating.
Figure 4 shows a first embodiment of this invention, where the back surfaces of the retro-reflecting prism are coated with a wavelength selective reflecting coating 6 and a material 7 of sufficiently high refractive index is glued to the back surfaces on top of the coating. Thus, total internal reflection at the last surface of the reflection coating is avoided for a range of incidence angles and the wavelengths not reflected will be transmitted into the glued on material. This is shown in figure 4 where a part of the incident light within a selective wavelength range is retro-reflected 4 and the other part, outside the selected wavelenght range, is transmitted 5. In figure 4 the function is only shown schematically for just one back surface. At least one and preferably all three back surfaces of the retro-reflecting prism should be configured in this way. The glued on material is made absorbing for the wavelengths transmitted into said material. The absorbing material could for example be a colour filter, an IR high-pass filter or an IR band-pass filter. In this embodiment the retro-reflecting prism would look black.
In fig 5, the glued on material is made to diffusely reflect the light transmitted into said material. (As light we include wavelengths from UV to IR.) Figure 5 shows a schematic view of the function of this embodiment, where a part of the incident light within a selected wavelength range is retro-reflected 4 and the other part, outside the selected wavelenght range, is diffusely reflected 8. Said diffusely reflecting material could for example be an opal glass or plastic, or a glass or plastic plate having its back surface frosted (either by etching or grounding or some other process), the material having an appropriate index of refraction according to this invention. In this embodiment the retro-reflecting prism would look matt white.
In an extended example, the glued on material is made to diffusely reflect the light transmitted into said material, as is described in relation to fig 5. Further, a wavelength selective reflecting layer is put on top of the diffusing material. This could for example be some kind of paint. In this embodiment the retro-reflecting prism would look matt and have the same colour as the reflecting layer. This embodiment could be used to camouflage the retro-reflecting prism.
It is assumed that an appropriate anti-reflection coating is used for the front surface 9 of the prism.

Claims

A target system for a weapon effect simulation system, said target system comprising a retro-reflecting prism having at least three back surfaces arranged to retro-reflect an incident simulation beam and wherein at least one of the back surfaces comprises a first, selectively reflecting layer arranged to reflect a simulation beam of a predetermined optical wavelength range and being substantially transparent to optical wavelengths outside said range,
characterized in that a second layer is mounted on the first layer, said second layer being arranged to receive beams transmitted through said first layer and said second layer being of a material having an index of refraction sufficiently high to avoid total internal reflection between said first and second layer within a predetermined range of incident angles.
A target system according to claim 1,characterized in that the second layer is arranged to absorb at least parts of the transmitted beam.
A target system according to claim 1, characterized in that the second layer is arranged to diffuse reflect at least parts of the transmitted beam.
A target system according to claim 1,characterized in that the second layer comprises an IR high-pass filter.
A target system according to claim 1, characterized in that the second layer comprises an IR band-pass filter.
A target system according to claim 1, characterized in that the index of refraction of the second layer is substantially equal to the index of refraction of the first layer.
A target system according to claim 1, characterized in that the first layer comprises a multi-layer dielectric coating.
A target system according to claim 1, characterized in that an adhesive layer is interposed between the first and second layer.
A target system according to claim 8, characterized in that the index of refraction of the adhesive layer is substantially equal to the index of refraction of the first layer.
A target system according to claim 1, characterized in that a third, wavelength selective reflective layer is mounted on top of the second layer.
A retro-reflecting prism having at least three back surfaces arranged so as to retro-reflect an incident optical beam and wherein at least one of the back surfaces comprises a first, selectively reflecting layer arranged to reflect an optical beam of a predetermined optical wavelength range and being substantially transparent to optical wavelengths outside said range,
characterized in that a second layer is mounted on the first layer, said second layer being arranged to receive optical beams transmitted through said first layer and said second layer being of a material having an index of refraction sufficiently high to avoid total internal reflection between said first and second layer within a predetermined range of incident angles.