PROTECTION OF A MONOSTATIC OR QUASI-MONOSTATIC LASER TELEMETER

The present invention relates to a method for protecting a monostatic or quasi-monostatic laser rangefinder, by adding an optical module to the rangefinder in front of its optical output. It also relates to the telemetry assembly thus constituted.

Laser rangefinders are distance measuring devices which emit a laser beam of radiation, called primary radiation, and which collect a part of this radiation, called return radiation, which has been backscattered or retro-reflected by a target distant from the rangefinder. Determining the radiation propagation time between the range finder and the target, accumulated for the outward and return propagation paths, provides a measure of the distance to the target from the range finder. In general, such a range finder therefore has an optical output for emitting the laser beam of primary radiation, and an optical input for collecting part of the return radiation.

There are commonly three types of range finders, depending on the separation distance between an optical center of a transmitting pupil which is associated with the optical output, and an optical center of a receiving pupil which is associated with the optical input. This separation distance is called the center distance of the transmitting and receiving pupils, and the three types of range finders are:

- monostatic rangefinders, for which the distance between the axes of the transmitting and receiving pupils is zero;

- quasi-monostatic rangefinders, for which the distance between the axes of the transmitting and receiving pupils is less than or equal to 100 mm (millimeter); and

- bistatic range finders, for which the distance between the axes of the transmitting and receiving pupils is greater than 100 mm.

Figures 1a to 1c illustrate these three types of range finders: Figure 1a for a monostatic rangefinder, Figure 1b for a quasi-monostatic rangefinder, and Figure 1c for a bistatic rangefinder. In these figures, the reference PU21 designates the exit pupil of the range finder, also called the emission pupil, and the reference PU22 designates the entry pupil of the range finder, also called the reception pupil.

Among these laser rangefinders, monostatic or quasi-monostatic ones are used for many applications, because of their ease of installation and transport. Indeed, for a monostatic or quasi-monostatic rangefinder, the optical output and input are superimposed or juxtaposed, and the rangefinder is designed so that a direction of propagation of the primary radiation which is emitted by the optical output is identical to a direction of propagation of the part of the return radiation which is collected during a measuring operation of the range finder, the directions of propagation being opposite. Thus, a monostatic or quasi-monostatic range finder can be made up of a single block, which integrates all the radiation emission and reception components. As opposed to monostatic or quasi-monostatic rangefinders, bistatic rangefinders are composed of a block for transmitting the primary radiation and a block for receiving part of the return radiation, which are separate. The use of a bistatic range finder then requires to precisely characterize the relative positions of the two blocks, and to orient the direction of emission of the primary radiation and the direction of collection of the return radiation so that these two directions are aligned. intersect significantly at the target.

The present invention is therefore limited to the field of monostatic or quasi-monostatic laser rangefinders, that is to say laser rangefinders for which the optical output and the optical input are juxtaposed or superimposed with an interaxis of pupils of emission and reception which is zero or less than or equal to 100 mm.

In general, the power of the part of the return radiation which is collected through the optical input of the range finder is much less than the intensity of the laser beam of primary radiation which is emitted by its optical output. Indeed, in most situations of use of a monostatic or quasi-monostatic laser rangefinder, the target causes a significant absorption of the primary radiation, and / or reflects this primary radiation with an angular widening of the beam which is important. The optical sensor which is dedicated, inside the range finder, to the detection of the return radiation then has a high sensitivity, which is suitable for these most common conditions of use where the radiation to be detected is much less intense than the radiation that is emitted.

However, there are particular situations of use in which the target retro-reflects a significant part of the primary radiation, with a direction of propagation of the reflected radiation which is identical to that of the primary radiation but with a direction of propagation of the return radiation. which is opposite to that of primary radiation. This is the case, in particular, when the target is a metallic reflector with three faces which are arranged in the corner of a cube. The power of the part of the return radiation which is collected by the optical input of the monostatic or quasi-monostatic laser rangefinder is then greater than the tolerance limit of the rangefinder optical sensor, so that this sensor is then damaged or destroyed.

However, in practice, the high sensitivity of the range finder sensor which is necessary for the most common conditions of use, is the opposite with a tolerance limit which is also high for this optical sensor.

From there, a first object of the invention consists in having a protection which is capable of preserving a monostatic or quasi-monostatic laser rangefinder against damage caused by the particular situations of use which have just been described.

An additional aim is to have such protection which is compact, inexpensive, and which does not make the use of a monostatic or quasi-monostatic laser range finder more complex.

To achieve at least one of these goals, a first aspect of the invention proposes a method of protecting a monostatic or quasi-monostatic laser range finder, when the range finder has an optical output for

emit a laser beam of primary radiation, and an optical input to collect part of a return radiation, and when the optical output and input are juxtaposed or superimposed, with an interaxial of transmitting and receiving pupils which is zero or less than or equal to 100 mm. The method of the invention applies when the range finder is designed so that a direction of propagation of the primary radiation which is emitted by the optical output is identical to a direction of propagation of the part of the return radiation which is collected during 'a range finder measurement operation. The method comprises fixing an optical module in front of the optical output of the laser rangefinder of the monostatic or quasi-monostatic type, to transversely shift the laser beam of primary radiation which is emitted by this optical output. The optical module used is such that the direction of propagation of the primary radiation downstream of the optical module is identical to the direction of propagation of the primary radiation between the optical output of the range finder and the optical module. Further, the optical module is adapted to produce a transverse offset length which is between 10cm (centimeter) and 35cm, preferably between 15cm and 20cm, when measured perpendicular to the direction of propagation.

For this, the optical module of the invention comprises two reflector assemblies which are arranged on a path of the laser beam of primary radiation which is emitted by the range finder equipped with the optical module, downstream of the optical output of the range finder, so that the primary radiation is reflected by one then the other of the two reflector assemblies. Further, each reflector assembly is adapted to apply a deflection to the laser beam of primary radiation, and the respective deflections which are applied by one and then the other of the two reflector assemblies are opposed.

Thus, an optical module which is used to produce the protection function according to the invention has a simple optical structure, this structure being able to be limited to the two reflector assemblies maintained in suitable positions.

In addition, since the deviations that are applied to the

primary radiation beam by one then the other of the reflector assemblies are opposed, the optical module can be adapted so that the parallelism between the propagation directions of the primary radiation beam upstream and downstream of the optical module is not altered by an inclination of this module around at least one axis.

The optical module, when it is attached to a monostatic or quasi-monostatic laser range finder, therefore has the effect of transversely shifting the pupil emitting the primary radiation, away from the pupil receiving the return radiation. Then, when the target retro-reflects the primary radiation producing only a small angular dispersion, such as for example a reflecting target which has the shape of a re-entering cube wedge, the power of the return radiation is mainly contained in a section of beam which is little larger than the size of the emission pupil of the range finder, and this return radiation is returned to the optical output of the module. However, thanks to the invention, it is offset transversely with respect to the optical input of the range finder, so that this optical input is outside the main part of the return beam. The power of the part of the return radiation which is collected, and therefore which receives the optical sensor of the range finder, is then reduced to an extent which is sufficient to prevent damage to this optical sensor.

In addition, given the cross-sectional dimensions of the laser beam of the primary radiation, the value of the transverse offset, between 10 cm and 35 cm, is sufficient to guarantee the protection of the optical sensor of the rangefinder, whether it is monostatic or quasi-monostatic. According to the invention, this transverse offset value is produced by an optical module which may itself have small dimensions and cause limited bulk. In other words, the optical module which is used in the invention transforms the monostatic or quasi-monostatic range finder into a bistatic range finder within the meaning of the types of range finders which have been listed above, but without the general drawbacks of bistatic range finders.

Generally, the two reflector assemblies can be selected so that it is not necessary to precisely adjust one or two orientation angle (s) of the entire optical module relative to the range finder, while retaining the identity between the directions of propagation of the primary radiation upstream and downstream of the optical module. The optical module can then be attached to the rangefinder quickly and easily.

Advantageously, each reflector assembly can be adapted to apply to the laser beam of primary radiation a deviation which is substantially independent of at least one orientation angle of this reflector assembly. By “substantially independent deviation” is meant that a first order derivative of the deviation which is applied to the laser beam of primary radiation by the reflector assembly considered, with respect to the angle of orientation of this reflector assembly, is nothing. In other words, the reflector assemblies can be selected so that it is not necessary to precisely adjust one or two orientation angle (s) of each reflector assembly, which reduces the cost of designing and assembling the optical module. .

Generally for the invention, the optical module which is used can advantageously also be arranged so as not to transversely shift the part of the return radiation which is collected by the range finder during a measurement operation thereof.

In first embodiments of the invention, each reflector assembly of the optical module used can comprise a portion of a material which is transparent to the primary radiation, this portion having three reflective flat faces which are arranged in superposition on zones of three respective faces of a cube, distributed around a vertex of this cube. Thus, when the optical module is fixed in front of the optical output of the range finder, each reflector assembly reverses the direction of propagation of the primary radiation while maintaining the same direction of propagation, but producing an elementary transverse shift of the laser beam of primary radiation. The transverse shift which is effective for the laser beam of primary radiation between the downstream side of the optical module and an interval which is intermediate between the optical output of the range finder and the optical module, is then a vector addition of the elementary transverse shifts which are produced by the two reflector assemblies. Advantageously, each portion of the transparent material may have the shape of a bar, with a first longitudinal end of the bar which comprises a reflective flat face, and with a second longitudinal end of the bar, opposite the first longitudinal end, which comprises a dihedral reflecting at right angle, with an edge of this dihedron which is perpendicular to the reflecting plane face of the first longitudinal end of the bar. In other words, each reflector assembly may have the shape of a bar which is inscribed in a re-entrant cube corner. Each reflector assembly of these first embodiments of the invention can then be called bar-corner-of-cube.

In second embodiments of the invention, each reflector assembly of the optical module used may comprise an optical bracket based on a pentaprism which has two reflective flat faces, so that when the optical module is fixed in front of the outlet rangefinder optics, each reflector assembly deflects at right angles the direction of propagation of the primary radiation. For such second embodiments of the invention, a mechanical connection between the two reflector assemblies is preferably rigid or undeformable.

In third embodiments of the invention, each reflector assembly can be a plane mirror, and the optical module is arranged so that the two plane mirrors are parallel. Then, the identity between the directions of propagation of the primary radiation upstream of the two plane mirrors and downstream thereof is not altered if the two mirrors are modified in orientation while remaining parallel to one another . In particular, the optical module can comprise a rhombohedron of transparent material, two opposite faces of which are reflective to produce reflections of the primary radiation which are internal to the rhombohedron of transparent material. Thus, each of the two reflecting faces of the rhombohedron forms one of the two reflecting assemblies. Preferably

(degree) and 50 °, in particular equal to 45 °.

In addition, one of the following characteristics can advantageously be applied in improvements of the invention, alone or in combination of several of them:

each flat reflecting face of the reflector assemblies of the optical module used can have a surface which is less than 9 cm 2 , preferably less than 4 cm 2 . The optical module can thus be of reduced dimensions and be compact;

- The optical module used can further include at least one optical diasporameter, which is adapted to compensate for a lack of identity between the direction of propagation of the primary radiation downstream of the optical module and the direction of propagation of the primary radiation in the interval between the optical output of the range finder and the optical module, when the optical module is fixed in front of the optical output of the range finder; and

- The optical module used may further comprise removable fixing means, which are suitable for fixing this optical module in a removable manner on the monostatic or quasi-monostatic laser rangefinder, so that the fixed optical module is effective for the laser beam of primary radiation that is emitted by the optical output of the rangefinder.

The resulting rangefinder assembly, comprising the laser rangefinder and the optical module attached to its optical output, can be integrated into an optical sight assembly, an optical aiming assembly, an optical target designation assembly, or a multiple sensor head. which is intended to characterize an environment.

Other features and advantages of the present invention will appear in the following description of examples of non-limiting implementations, with reference to the accompanying drawings, in which:

- Figures 1a to 1c, already described, illustrate a distinction between three types of range finders: monostatic, quasi-monostatic and bistatic; - Figure 2 is a perspective view of an optical module which can be used in a first embodiment of the invention;

- Figure 3 illustrates the geometric principle of design of a reflector assembly as used in the embodiment of Figure 2;

- Figure 4 corresponds to Figure 2 for a second embodiment of the invention;

- Figure 5 corresponds to Figure 2 for a third embodiment of the invention; and

- Figures 6a and 6b show two examples of use of the invention.

For the sake of clarity, the dimensions of the elements which are shown in these figures do not correspond to actual dimensions or to actual dimensional ratios. In addition, identical references which are indicated in different figures designate identical elements or which have identical functions.

In accordance with FIG. 2, a quasi-monostatic laser range finder 20 comprises an optical output of laser radiation 21, as well as an optical input 22 which is juxtaposed with the output 21. Such a quasi-monostatic range finder conforms to FIG. 1b . A monostatic range finder 20 can be used equivalently, in which the optical output 21 and the optical input 22 are superimposed, as shown in Figure 1a. The range finder 20 is designed to produce a laser beam Fi, called the primary radiation laser beam, through the optical output 21. The beam Fi has a parallel beam structure, and can have a wavelength which is between 0.360 µm ( micrometer) and 3 miti, e.g. equal to 1.5 miti, with a beam diameter that is less than 12 mm (millimeter), for example. In that case,

The optical input 22 is intended to collect a part F R of the radiation from the laser beam Fi, after propagation of the beam Fi to a target (not shown), backscattering by the latter and propagation back from the target to 'to optical input 22. F Rhas been called return radiation in the general part of this description. A separation distance between the respective optical axes of pupils of the optical output 21 and of the optical input 22 is therefore less than or equal to 100 mm. Such an operation of a monostatic or quasi-monostatic laser rangefinder, to provide a measurement of the distance away from the target with respect to the rangefinder, is known to those skilled in the art. In general, due to the fact that the backscattering of the beam Fi by the target is distributed through a large solid angle, and / or because the backscattering by the target is accompanied by absorption, the power of the part of the radiation of return F Rwhich is collected by the optical input 22 of the range finder 20 is much smaller than that of the laser beam Fi. For this reason, the optical input 22 generally has a cross-sectional area which is greater than that of the optical output 21. Reference number 23 denotes an optical sensor which is used inside the range finder 20, downstream of the optical sensor. optical input 22, for detecting the collected portion of the radiation back F R .

However, in certain situations, the reflection of the laser beam Fi on the target can be intense, and directed precisely towards the optical output 21 of the range finder 20. This is the case, in particular, when the target is a re-entering reflecting cube corner. Then, the radiation power which is received by the sensor 23 is very high, so that this sensor 23 risks being damaged. But in such situations, the return radiation F Rforms a quasi-parallel or parallel beam, which has a limited cross section. Then, shifting the optical output 21 from the optical input 22 is sufficient to eliminate most of the risks of degradation of the detector 23. An shift length which is between 10 cm and 35 cm, and possibly between 15 cm and 20 cm, is sufficient. Those skilled in the art will understand that shifting the optical output 21 from the optical input 22 means moving the effective transmitting pupil and the receiving pupil of the range finder 20 away from each other.

For this, two reflector assemblies, designated respectively by the references 1 and 2, are combined in an optical module 10 which is intended to be mounted in front of the optical output 21, to move transversely the effective emission pupil of the laser beam Fi, in order to away from the optical input 22. Preferably, the optical module 10 does not affect the part of the

return radiation F R which is collected by the optical input 22.

In accordance with a first embodiment of the invention which is illustrated by FIG. 2, each reflector assembly 1, 2 can be formed by a re-entrant and reflecting corner of a cube. In this case, the primary radiation laser beam Fi is reflected by each reflector assembly 1, 2 parallel to itself, by reversing its direction of propagation. The two reflector assemblies 1 and 2, combined to reflect the laser beam Fi one after the other, return this laser beam Fi still parallel to the direction of propagation that it had between the optical output 21 of the range finder 20 and the module 10, and with a direction of propagation which is also identical. However, each reflector assembly 1, 2 shifts the emission pupil of the beam Fi, and the total offset of this emission pupil results from the vector addition of the two elementary offsets which are produced by the reflector assembly 1 and by the reflector assembly 2, respectively. In the figure, L denotes the length of the total offset.

According to a preferred embodiment of the invention, each reflector assembly 1, 2 may consist of a rectilinear bar of transparent material, for example glass, the end faces of which are superimposed on three faces of a cube that are adjacent to the same vertex of the cube. Figure 3 shows the principle of such a superposition for the reflector assembly 1. Pi, P 2 , ..., P 7 denote seven visible vertices of the cube, and Ai, A 2 and A 3denote the three edges of the cube which converge at the vertex Pi. The bar of the reflector assembly 1 can have any cross section, but preferably with a planar side face which constitutes both the entry face and the exit face of the laser beam F1 in the bar. A first longitudinal end Lu of the bar can consist of a flat face So which is superimposed on a part of the face P 1 P 3 P 7 P 4 of the cube, and the other longitudinal end L12 of the bar can be straddling the edge Ai, between the vertices Pi and P 2 . The longitudinal end L12 of the bar is thus constituted by a flat face Si which is superimposed on a part of the face P 1P 2 P 5 P 3 of the cube, and also by another plane face S 2 which is superimposed on a part of the face P1P2P6P4 of the cube. In other words, the faces Si and S 2 form a dihedral at right angles, with an edge of this dihedron which is

perpendicular to the face So. The end faces So, Si and S 2 are advantageously metallized so as to be reflective for the primary radiation. Then, if the laser beam F 1 enters the glass bar of the reflector assembly 1 through one of its side faces, then being reflected on one then on the other of the two faces Si and S 2 of the end L 12 , internally to the bar, it can propagate longitudinally in the glass bar until it is reflected by the face So of the end Lu before coming out through the same lateral face as that of its entrance . In addition, refractive deviations that affect the laser beam F 1at its entry into the glass bar, and at its exit from this same bar, can be opposed so that the direction of propagation of the laser beam F 1 is not modified between before its entry and after its exit from the bar, but only the direction of propagation of the laser beam F 1 is reversed. The length of the glass bar determines that of the offset between the exit pupil which is effective for the laser beam F 1 after the reflector assembly 1, and the emission pupil of the optical exit 21. Those skilled in the art will understand that the reflector assembly 1 which has just been described can be turned over, so that the laser beam F 1enters the glass bar to be first reflected by the end face So, then propagates longitudinally in the glass bar, and is then reflected by the end faces Si and S 2 before exiting the bar of glass.

It is known that the reversal of the direction of propagation of the laser beam F 1 along the same direction of propagation, as has just been described for use of the glass bar from the end L 12 towards the end Lu, or from the end Lu towards the end L 12 , is obtained whatever the angular orientation of the bar as long as the laser beam F 1 remains inside the bar between its two ends. In other words, the reversal of the direction of propagation of the laser beam F 1 along its initial direction of propagation is independent of two angles of polar coordinates, which define the longitudinal orientation of the bar, when each of the two angles vary in an interval of non-zero length.

The reflector assembly 2 may have a similar constitution: it may consist of another cube corner glass bar, one of the

l_ 2i ends can be formed by a face portion of the cube, and the other end L 2 2 can straddle an edge of the cube. In FIG. 2, Fi designates the laser beam of primary radiation in the interval between the optical output 21 of the range finder 20 and the reflector assembly 1, F I2 designates the same laser beam but between the two reflector assemblies 1 and 2, and F 2still designates the same laser beam but after having been reflected by the reflector assembly 2. The respective longitudinal directions of the two glass bars of the reflector assemblies 1 and 2 can form any angle between them, in projection in a plane which is perpendicular to the laser beam Fi. In addition, the two reflector assemblies 1 and 2 can be spaced from each other by any distance along the beam F I2 .

It is repeated that an advantage of the embodiment of the invention which has just been described in relation to FIGS. 2 and 3, lies in the fact that no angular alignment is necessary for the two reflector assemblies. 1 and 2, whether it is relative to each other, or whether it is relative to the direction of emission of the laser beam Fi at the level of the optical output 21 of the range finder 20. Indeed, in general terms for this type of implementation of the invention which is based on a geometry of re-entrant and reflecting cube corners, the deviations of the laser beam which are produced by the two reflector assemblies 1 and 2, are each equal to 180 ° in value absolute, and therefore automatically opposite in meaning.

FIG. 4 corresponds to FIG. 2 when each of the reflecting elements 1 and 2 consists of a pentaprism of transparent material, for example glass. The pentaprism is an optical component designed to deflect a light beam at right angles, well known to those skilled in the art and belonging to the category of components called optical brackets. Each pentaprism has a flat entry face, two flat reflecting faces denoted S'i and S ' 2 , and a flat exit face, in the order of the faces along the path of the laser beam Fi. Preferably, the reflective flat faces S'i and S ' 2are metallized to increase the intensity of each internal reflection of the laser beam Fi. Each of the two pentaprisms which form the reflector elements 1 and 2 deflects the laser beam of primary radiation Fi by 90 ° (degree), one after the other. In FIG. 4, Di and D 2 denote the deviations of the laser beam Fi which are produced respectively by the two pentaprisms, each equal to 90 ° in absolute value but opposite in direction. Also in a known manner, each deviation Di or D 2of the laser beam Fi which is produced by one of the pentaprisms is independent, in the first order, of an orientation angle of this pentaprism around a direction denoted D which is parallel simultaneously to the two flat reflecting faces of the pentaprism. “First order” means that the deviation of the laser beam Fi varies little around the value 90 ° when the orientation of the pentaprism around the direction D is varied to a limited extent, on either side of the orientation for which the laser beam Fi passes perpendicularly through each of the entry and exit faces of this pentaprism. More precisely, the derivative of the deviation of the laser beam Fi which is produced by the pentaprism, with respect to the orientation of this pentaprism around the direction D, is zero when the deviation is equal to 90 °.

However, the parallelism between the laser beam Fi on the one hand, such as exists in the gap between the optical output 21 of the range finder 20 and the reflector assembly 1, and the laser beam F 2on the other hand, as the latter exists downstream of the reflector assembly 2, may be altered by a defect in orientation of at least one of the pentaprisms, especially such a defect in orientation around another direction direction D. To compensate for such a defect, it is possible to add, inside the optical module 10, an optical diasporameter 3, preferably between the reflector assemblies 1 and 2, but also possibly upstream of the reflector assembly 1 or downstream of the reflector assembly 2. In a way which is also known to those skilled in the art, the optical diasporameter 3 comprises two transparent plates 3a and 3b juxtaposed, each of them having two opposite faces which form between them an angle which is small, but not zero, for example between 0.1 ° and 1.5 °, the two blades having the same angle value. So,

Figure 5 corresponds to Figure 2 for implementation modes

of the invention where the optical module 10 comprises two parallel plane mirrors 1 and 2. For example, to ensure that the two plane mirrors 1 and 2 remain parallel, they can be formed by two opposite faces of a rhombohedron 4 made of transparent material solid. These two opposite faces can be made reflective by metallization or deposition of stacks of multiple dielectric layers. A wedge angle α of some of the side faces of the rhombodedron 4, which are located between the opposing reflective faces, may be equal to 45 °. The laser beam Fi of the primary radiation then penetrates into the rhombohedron 4 via one of the lateral faces of the latter, called the entry face, then is reflected by the mirror face 1 internally to the rhombohedron 4. It propagates then towards the mirror face 2 in the transparent material of the rhombohedron 4,I2 , then is reflected by the mirror face 2 again internally to the rhombohedron 4, and emerges from the rhombohedron 4 via another side face thereof, which is opposite the side entry face, forming the laser beam F 2 which is parallel to the beam Fi. The mirror faces 1 and 2 have sufficient areas, preferably greater than 1.0 cm 2 , to reflect a main part of the energy of the laser beam Fi as a function of the cross section of this beam. In addition, the direction of propagation of the laser beam F 2 remains identical to that of the laser beam Fi when the rhombohedron 4 is inclined about any axis.

For the three embodiments which have just been described, with reference to Figures 2, 4 and 5 respectively, the reflector elements 1 and 2 have sufficient cross sections to transmit a main part of the energy of the laser beam Fi . For this, reflective flat faces which are areas greater than 1.0 cm 2 may be sufficient, depending on the cross section of the laser beam Fi.

The two reflector assemblies 1 and 2, and possibly also the optical diasporameter 3, are maintained in relative positions which are fixed inside the module 10. Advantageously, the module 10 and / or the output 21 of the range finder 20 can (can ) be equipped with at least one assembly ring 21 'upstream of the reflector assembly 1, which is adapted to removably assemble the module 10 on the range finder 20, in front of the optical output 21. The range finder monostatic or quasi-monostatic laser 20 which is thus equipped with the optical module 10 has been called a rangefinder assembly in the general part of the present description. Possibly also, the module 10 can be fixed on the monostatic or quasi-monostatic laser rangefinder 20 within the rangefinder device, that is to say without the possibility of disassembly. In that case,

Optionally, a telemetry assembly as obtained by a method in accordance with the invention can be incorporated into a multiple sensor head which is intended to characterize an environment. The multi-sensor head assembly can then be mounted on a swiveling support, with the optical inputs, optical outputs, and sensor detection windows that are grouped together in a limited effective head area. In this case, it may be advantageous to use an implementation of the invention of the type of FIG. 2, and in which a non-zero angle between the longitudinal directions of the two glass bars allows the module 10 to be placed in the effective area of ​​the multi-sensor head without masking an optical input, optical output or detection window of any sensor.2 is also indicated in these two figures.

It is understood that the invention can be reproduced by adapting or modifying secondary aspects thereof with respect to the embodiments which have been described in detail above, while retaining some of the advantages cited. It is recalled that the main advantage is to provide protection of the monostatic or quasi-monostatic laser rangefinder against a specular retro-reflection of the laser beam of primary radiation by a target, during which the return beam would be superimposed on the laser beam of radiation. primary.
CLAIMS

A method of protecting a monostatic or quasi-monostatic laser range finder (20), said range finder having an optical output (21) for emitting a laser beam of primary radiation (Fi), and an optical input (22) for collecting a part of a return radiation (F R ), the optical output and input being juxtaposed or superimposed, with a transmission and reception pupil center distance which is zero or less than or equal to 100 mm, and the range finder being designed so that a direction of propagation of the primary radiation which is emitted from the optical output is the same as a direction of propagation of the part of return radiation which is collected during measuring operation of the range finder,

the method comprising fixing in front of the optical output (21) of the range finder (20), an optical module so as to transversely shift the laser beam of primary radiation (Fi),

the optical module (10) being such that a direction of propagation of the primary radiation downstream of the optical module is identical to a direction of propagation of said primary radiation between the optical output of the range finder and said optical module, with a length (L) of transverse offset which is between 10 cm and 35 cm, preferably between 15 cm and 20 cm, measured perpendicular to the direction of propagation, said optical module comprising two reflector assemblies (1, 2) which are arranged on a path of the laser beam of primary radiation which is emitted by the range finder equipped with the optical module, downstream of the optical output of the range finder, so that the primary radiation is reflected by one then by the other of the two reflector assemblies,each reflector assembly being adapted to apply a deflection to the laser beam of primary radiation, and the respective deflections which are applied by one then the other of the two reflector assemblies being opposed.

2. The method of claim 1, wherein the optical module (10) is further arranged not to shift transversely the part of the

return radiation (F R ) which is collected by the range finder (20) during the measurement operation of said range finder, said return radiation resulting from backscatter or back reflection of the primary radiation by a target distant from the range finder.

3. The method of claim 1 or 2, wherein each reflector assembly (1, 2) of the optical module (10) comprises a portion of a material which is transparent to primary radiation, said portion having three reflective flat faces (So , If, S 2) which are arranged in superposition on zones of three respective faces of a cube, distributed around a vertex of the cube (Pi), so that when the optical module is fixed in front of the optical output (21) of the range finder (20 ), each reflector assembly reverses a direction of propagation of the primary radiation (F1) while maintaining the same direction of propagation, but producing an elementary transverse shift of the laser beam of primary radiation, the effective transverse shift for said laser beam of primary radiation between downstream of the optical module and an interval which is intermediate between the optical output of the range finder and said optical module, being a vector addition of the elementary transverse shifts which are produced by the two reflector assemblies.

4. The method of claim 3, wherein each portion of the transparent material has a bar shape, with a first longitudinal end (Lu , L21) of the bar which comprises a reflective planar face (So), and with a second longitudinal end ( L12, L 22 ) of the bar, opposite to said first longitudinal end, which comprises a reflecting dihedron at right angles, with an edge of said dihedron which is perpendicular to the reflective planar face of the first longitudinal end of the bar.

5. The method of claim 1 or 2, wherein each reflector assembly (1, 2) of the optical module (10) comprises an optical bracket based on a pentaprism which has two reflective flat faces (S'1, S ' 2 ), so that when the optical module is fixed in front of the optical output (21) of the range finder (20), each reflector assembly deflects at right angles the direction of propagation of the primary radiation (Fi).

The method of claim 1 or 2, wherein each reflector assembly (1, 2) of the optical module (10) is a plane mirror, and the optical module is arranged so that the two plane mirrors are parallel.

7. The method of claim 6, wherein the optical module (10) comprises a rhombohedron (4) of transparent material, two opposite faces of which are reflective to produce reflections of the primary radiation which are internal to the rhombohedron of transparent material, each of which. two reflecting faces of the rhombohedron forming one of the two reflecting assemblies (1, 2).

8. Method according to any one of the preceding claims, in which each reflective flat face of the reflector assemblies (1, 2) of the optical module (10) has an area of ​​less than 9 cm 2 , preferably less than 4 cm 2 .

9. Method according to any one of the preceding claims, wherein the optical module (10) further comprises at least one optical diasporameter (3) adapted to compensate for a lack of identity between the direction of propagation of the primary radiation (Fi) downstream of the optical module and the direction of propagation of said primary radiation in the interval between the optical output (21) of the range finder (20) and said optical module, when the optical module is fixed in front of the optical output of the range finder.

10. Method according to any one of the preceding claims, in which the optical module (10) further comprises removable fixing means, which are adapted to fix the optical module removably on the monostatic or quasi-monostatic laser rangefinder ( 20), so that the attached optical module is effective for the primary radiation laser beam (Fi) which is emitted from the optical output (21) of the range finder.

11. Method according to any one of the preceding claims, in which the laser range finder (20) and the optical module (10) fixed in front of the optical output (21) of said laser range finder, are included in a range finder assembly, said range finder assembly being integrated with an optical sight assembly, an optical aiming assembly, an optical target designation assembly, or a multi-sensor head that is intended to characterize an environment.