The field of the invention is that of a passive infrared imaging system such as cameras and thermal imagers operating in the MWIR band (3-5 μιη) and / or in the LWIR band (8-12 μιτι) using a cooled detector and an opto-mechanical device of variable focal imaging also designated optical zoom.

To optimize the scope, cost and usability of such a system, one wishes to have a continuous zoom Grand Champ (GC) - Small Field (PC) with a configuration Extra Small Field (TPC) Optical with a PC / TPC ratio of class 1 .5, without the TPC setup would burden, not hindrance nor renchérisse the system.

There is currently ongoing IR zooms mono-opening structure and without flow.

Such continuous zoom average ratio of GC-PC (typically of the order of 6) currently equip IR systems market, such as the twin Sophie ZS and Sophie XF Thales Optronics.

Conventionally, such a zoom shown in Figures 1 A and 1 B, comprises, in order on the optical axis z of the system:

- A fixed GF1 leading group of F1 focal length, telephoto-type (a priori).

- A mobile group Gm1 diverge acting controller, who works with a variable magnification gm1 negative.

- A mobile Gm2 convergent group acting as a compensator, which works with a variable magnification gm2, negative also.

- A constant group GF2 relaying to the detector the intermediate real image provided by the three groups Gf 1, Gm1 and Gm2. The GF2 group works with a constant magnification and negative GF2 which is generally between -2 and -1.

A folding mirror 2 is disposed in the GF2 group to optimize the footprint.

The exit pupil of the system coincides with the cold diaphragm 3 of the detector 1, which serves as the aperture stop of the zoom range on continuous GC-PC. Using a field lens 4 generally

GF2 present in the group, it is possible to constrain the position and the aberrations of the entrance pupil of the PC, to minimize the diameters of the components of the leading group GF1, which weigh significantly on the cost of the optics. GC, usually the entrance pupil is virtual and it is ensured that the beams remain inside the envelope useful beams PC.

The focal zoom F is expressed as F = F1. gm1. gm2. GF2. Zooming, ie, the zoom effect is due to changes in product gm1. gm2, which is minimum and maximum GC PC.

The assembly is optimized so that PC, the absolute value of gm2 (also denoted Igm2l) remains less than about 0.85. It is known that this condition makes it possible to use the compensator (Gm2) as a focusing group for the entire continuous area GC-PC. Indeed, the axial sensitivity of the compensator Gm2, which connects the defocus image at the axial movement thereof is expressed as (1 - (gm2) 2 ) (GF2) 2 .

Therefore, if Igm2l is sufficiently far from 1 .0, then a small axial movement of the tandem induced defocusing of the image; ie Gm2 can act as a close focusing group or can be used to compensate for thermal drifts of the combination.

Similarly, if the product gm1. gm2 remains less than about 0.85 PC configuration, then it is possible to use the tandem Gm1 -GM2 as a focal group on the entire GC-range PC.

Obviously, with this type of optical architecture it is possible to obtain a continuous zoom larger report to cover a GC-TPC need. Roughly speaking, the diameter of the head components is then multiplied by a factor equal to the PC / TPC report, which has a significant impact on the size and cost of the equipment.

Another known solution is to use an infrared zoom GC-TPC dual opening, structureless flow. We have seen that the elongation of the focal length of a zoom to a TPC course is accompanied by an increase of the effective diameter of the head components, and therefore an additional cost. Nevertheless, it is possible to limit the diameter of the front lens (front lens of the front group) while ensuring a perfectly healthy photometric performance. For this, we reduced

useful TPC opening before inserting the cryostat a diaphragm bordered by a structural flow reduction mirror. Such a device is described more in §5 of the article by J. Vizgaitis: Dual F / number resource for camping 3rd generation FLIR Systems, Proc. Of SPIE Vol. 5783. It is found for example in ATTICA M-ER camera company Airbus Defense & Space, which is described in the article "New Thermal Imager for long range surveillance" by J. & H. Fritze Schlemmer AMA Conference 2013 . such a solution only partially solves the problem posed to the extent that it uses a mechanism dedicated to the switching of the TPC aperture diaphragm, which is clearly not very economical.

You can also add an afocal TPC before a GC-PC back in. Indeed, for a TPC from a GC-PC zoom, simply mount magnification afocal unit PC / TPC before the zoom lead group. In doing so, the GC is also reduced to a PC / TPC factor. Such afocal, Galilean type has at least two IR components of large diameter, a priori expensive. This solution is absolutely not respond to the problem, or in terms of weight and size, either in terms of cost or in terms of ergonomics.

Accordingly, to this day remains a need for an IR imaging system simultaneously giving satisfaction to all of the above requirements, in terms of mass, size, cost, and ergonomics.

The system of the invention uses an infrared zoom with two mobile groups, continuous range GC-PC and with additional configuration TPC. This lens has the following characteristics:

- The continuous range operates in a conventional way, its aperture stop coinciding with the cold shield of the cryostat.

- The TPC pattern, original, works with a smaller effective numerical aperture than the GC-PC range with a relatively large proportion of flow structure, because it is not reduced. The TPC of the aperture stop is formed by the support of one of the

lenses of the zoom head group, and is imaged in the vicinity of the cold screen. There is no mechanism dedicated to opening change.

More specifically the invention relates to a passive IR imaging system having its optical axis:

- an array detector placed in a cryostat comprising a cold stop,

- an optical imaging device on the detector, to focus continuously varying a focal length of GC configuration F G c at a focal length F PC configuration PC with in said zoom range and a constant numerical aperture of a diaphragm opening coinciding with the cold diaphragm of the cryostat, said optical device comprising:

o a head group convergent fixed position and constant focal comprising at least one integrated lens in a mechanical maintaining, diameter determined by the PC configuration,

o a first mobile unit and a second leaving group capable of being positioned to ensure zooming between F G c and F PC , as well as the focusing of the image on the detector,

o a transmission group of fixed position and constant image magnification, adapted to image the aperture stop in order to limit the diameter of the envelope of the useful rays PC on the lenses of the front group.

It is mainly characterized in that the optical device comprises a TPC configuration of focal length F T predetermined PC, with the first and second mobile units positionable to obtain the focal length F T pc, and aperture stop materialized in the mechanical holding the leading group.

The imaging system according to the invention is thus equipped with a continuous zoom GC-PC Cold pupil, with an additional configuration TPC operating at reduced opening with a hot pupil materialized in the head group and imaged in the vicinity of cold pupil of continuous zoom.

The back comprises two movable groups ensuring both the changing control and the focusing of the image on the detector.

The zoom is essentially dimensioned for the continuous function GC-PC, and there is no mechanism or opto-mechanical module exclusively dedicated to TPC.

Optical components are oversized for the needs of the TPC function.

TPC does not further forced drive mechanisms and compensating the GC-PC zoom; in particular, it is not necessary to improve their resolutions for the TPC.

According to a first embodiment, the two movable groups are adjacent, the first is divergent with a negative variable magnification, and the second convergent with a negative magnification variable also. When the GC configuration is 20 °, the PC configuration at 3 °, and the TPC configuration at 2 °, the product of the magnifications of the first and second mobile units is typically greater than 1 .2 TPC configuration, between 0.8 and 0.85 PC configuration, and greater than 0.12 GC configuration.

According to a second embodiment, the imaging system further comprises a fixed group convergent variable magnification and negative, between the two competing mobile units which have a negative variable magnification. When the GC configuration is 20 °, the PC configuration at 3 °, and the TPC configuration at 2 °, the image carrying group works with a constant magnification of small absolute value, typically less than 0.1.

The invention also relates to an IR camera cooled or cooled IR binoculars which comprise a passive IR imaging system as described.

Other features and advantages of the invention will become apparent from reading the following detailed description, given by way of example and with reference to the accompanying drawings in which:

the already described Figures 1 show schematically in cross section an IR imaging system according to the prior art, GC configuration (fig 1a) and PC (Figure 1 b),

Figures 2 show schematically in cross section a first example of IR imaging system according to the invention GC configuration (fig 2a), PC (fig 2b) and TPC (fig 2c),

Figures 3 show schematically in cross section a second exemplary IR imaging system according to the invention GC (fig 3a) configuration, PC (Fig 3b) and TPC (fig 3c)

4 shows an example of mechanical holding of the leading group.

From one figure to another, the same elements are identified by the same references.

As shown in the two examples of Figures 2 and 3, the IR imaging system according to the invention has on its optical axis z:

- A 1 matrix detector cold shield,

- an optical imaging device on the detector, with continuously variable of the GC configuration zoom PC configuration and operating without vignetting with a constant number of N opening. F Note the current focal length of the zoom, F G c its focal GC configuration and F PC its focal PC configuration.

- on the continuous range GC-PC, an aperture stop located at the cold diaphragm 3 of the cryostat in order to optimize the thermal measurement resolution (NETD or acronym of the English expression "Noise Equivalent Temperature Difference") of the system. More rarely, the aperture diaphragm is placed just upstream of the detector cryostat encapsulating 1, and bordered by a stream of reducing mirror to find a pseudo cold pupil, guaranteeing a healthy photometry. In these two cases, zoom operates with a very low level flow structure, which maximizes the imaging system performance while facilitating the correction of non-uniformities image. If it is possible to control variations in the stream of residual structure (Narcissus effect,

The optical device comprises, in order on the optical axis z:

- a GF1 headgroup convergent fixed position and fixed focal length F1 of which the lens or lenses are mounted in a mechanical holding 5 an example is shown in Figure 4 for two lenses,

- two mobile groups Gm1 and Gm2 positioned to ensure continuous zooming between F G c and F PC and focusing the image on the detector 1 (focus) function,

- an image carrying GF2 group of fixed position and fixed magnification GF2, capable of transporting the pupil in the input space

According to the invention, the leading group GF1, which has a strong impact on the cost of optics, is sized at most just way to pass without vignetting useful bundles of continuous zoom GC-PC in all thermal conditions taking into account any imperfections. For this, the positions of PC and GC entrance pupils and the pupil aberrations are forced to maintain the envelope useful beams within a reasonable diameter comparable to the diameter of the pupil PC input . To this end, the GF2 group generally comprises a field lens. Typically, the diameter of the front lens is oversized by about 10% to 20% compared to the nominal diameter of the entrance pupil PC.

According to the invention, the optical device further comprises a configuration of TPC focal length F T predetermined PC. Starting from the PC configuration, the Gm1 group must be moved away from one group Gf for the focal length F T pc-Position Gm2 group is also modified so as to maintain focus on the detector. The continuous zoom GC-PC and configuration optimize TPC is so coupled with a conventional optical design software.

TPC, the support 51 of one of the lenses of the front group Gf 1 (the lens L2 in the example of Figure 4) acts as the aperture stop, and the effective numerical aperture is reduced compared to that of the continuous zoom. Thus, assuming that the head GF1 lens is oversized by about 20% compared to the diameter of the entrance pupil of the PC (to ensure freedom from vignetting on the entire GC-range PC), we obtain a number of useful N opening T pc which is:

Since the diameter of the casing of the PC bundles is close to the diameter of the pupil of PC input, the TPC of the aperture stop, located in the head block, is naturally imaged in the vicinity of the cold stop, to + or - few millimeters thereof, less than 5 mm, for example.

Ultimately, TPC configuration, the system works with a significant proportion of flow structure, effective to degrade the NETD to, which goes against the usual design rules of IR imaging systems cooled. However, as the diaphragm alone may vignetting useful beam is close to the image of the pupil, the flow structure presents only low-frequency spatial variations, so that the non-uniformities induced picture can be easily compensated using a table correction gains of the pixels, and a limited number of correction tables pixel offsets.

Consequently, profit is largely positive: no significant extra cost compared with a continuous zoom GC-PC, it has a built TPC configuration and photometry control.

two examples of the imaging system will now be described in detail according to the invention, but the scope of the invention is not limited to the detailed description that follows. Based on a continuous zoom 20 ° (GC) -3 ° (PC) with TPC 2 °, suitable for a MWIR cooled detector 1 having 640 x 480 elements in 15 μηι. It is assumed that the cold diaphragm 3 of the cryostat open at F / 3.9, is located about 1 mm from one focal plane where the detector 1.

In the first example, the imaging system according to the invention described in connection with Figures 2A, 2B and 2C, using a zoom with two adjacent movable groups. It comprises, in order along the optical axis z: - a fixed group GF1 F1 focal length telephoto-type (a priori) with for example two lenses, one convergent L1, the other divergent L2 mounted on a mechanical holding 5 shown in figure 4.

- A mobile group Gm1 diverge acting controller, who works with a variable magnification gm1 negative.

- A mobile Gm2 convergent group acting as a compensator, which works with a variable magnification gm2, negative also.

- A constant group GF2 relaying to the detector 1 the intermediate real image provided by the three groups GF1, Gm1 and Gm2. The GF2 group works with fixed magnification and negative GF2; in our example it is close to -1 .5, more typically it is between -2 and -1.

The exit pupil coincides with the cold diaphragm 3 of the detector 1, which serves as the aperture stop of the zoom range on continuous GC-PC. Using a field lens 4 arranged near the intermediate focal plane PFi GF2 present in the group, it is possible to constrain the position of the entrance pupil of the PC in the vicinity of GF1 group in order to minimize the diameters of the components of this group, which significantly weigh on the overall cost of the zoom. GC, the entrance pupil is mostly virtual.

We know that F = F1. gm1. gm2. GF2. The magnification GF2 is fixed, the focal variation is obtained by variation of the product gm1 gm2 which is minimum and maximum GC TPC.

The set is optimized so that PCs, gm1. gm2 remains less than about 0.85. This condition makes it possible to use the drive-compensating tandem as a focusing group for the entire continuous area GC-PC. Indeed, the axial sensitivity of the tandem Gm1 -GM2, which connects the defocus image at the axial movement thereof is expressed as [1 - (gm1 gm2.) 2 ] (GF2) 2 .

It follows that if the product gm1. gm2 is sufficiently far from 1 .0, then a small axial movement of the tandem induced defocusing of the image; that is, the tandem can act as a close focusing group, and may also serve to compensate for thermal drifts of the combination. Hence the interest.

However, if the product gm1. gm2 is very close to 1 .0, while axial movement of the tandem-Gm1 Gm2 hardly induces

defocus of the image; Under these conditions we can not use mobile groups Gm1 and Gm2 dedicated to zooming the zoom to focus on an early stage, or athermalization.

Advantageously, the TPC configuration is obtained by exceeding the singular point in which gm1. gm2 = 1 .0. Since F T PC PC is about 1 .5, we then gm1. gm2> 1 .2, which allows to use the tandem as a focusing group, as the continuous range GC-PC. In this case we have gm1. gm2 = 0125 GC, 0833 in PC and 1 .25 TPC.

Is described in connection with Figures 3A, 3B and 3C, a second example of an imaging system according to the invention using a zoom lens with a fixed group between the two moving groups. Zooming comprises, in order on the optical axis z:

- A fixed focal length F1 group GF1 with such a single lens.

- A mobile group Gm1 diverge playing the role of controller, which works with variable magnification gm1 and negative.

- A fixed group GF3 converge, who works with a magnification gf3 variable and negative.

- a mobile group Gm2 divergent acting as a compensator, which works with a variable magnification gm2, negative and large in absolute value (lower than -15, typically).

- A constant group GF2 relaying to the detector 1 the intermediate image supplied from the four groups GF1, Gm1, Gm2 and GF3; GF2 works with a constant magnification GF2, low absolute value generally less than 0.1, which means that in the intermediate space between the Gm2 groups and GF2, the beams are nearly collimated.

As in the previous example, the entire continuous area GC-PC, the exit pupil is coincident with the cold diaphragm of the detector, which therefore plays the role of the aperture stop GC-PC zooming. To minimize the diameter of the front lens (head group), it is possible to constrain the position of the entrance pupil of the PC in the vicinity thereof, possibly with the aid of a field lens 4

placed in the vicinity of the intermediate focal plane PFi in the re-imager GF2. GC, the entrance pupil is mostly virtual.

We have F = F1. gm1 .gf3. gm2. GF2. The magnification GF2 being constant, the focal length variation is obtained by varying the product gm1. gf3.gm2, whose absolute value is minimum and maximum GC TPC. Approximately in this case | gm1. gf3 .gm2 | = 4.0 GC-40 TPC.

Also on the GC-TPC beach, gm2 varies between -17.5 and -19.4, and GF2 = 0.07, so that the compensating group Gm2 can act as a focusing group. Indeed, the axial sensitivity Gm2 is expressed as [1 - (gm2) 2 ] (GF2) 2 . Given the gm2 values and GF2 are at stake, this sensitivity varies from approximately 1 .5 to 1 .8 in absolute value, which is largely consistent with an elementary step of moving the class 50 microns.

In these two examples, the TPC of the aperture stop is located in the GF1 head group which is dimensioned by the need of the PC. It follows a reduction of the useful opening of TPC compared to continuous zoom GC-PC, a lower factor F ratio TPC / FPC as the leading group should pass without vignetting useful bundles of continuous zoom in all conditions (thermal tolerances and construction), which imposes a slight oversizing of the components relative to the diameter of the pupil PC input . In our two examples, the diameter of the front lens is 60 mm, while the theoretical diameter of the PC entrance pupil is 46.9 mm. This is leveraged to the TPC, which may have an entrance pupil of about 57 mm, considering a reasonable margin of 1 .5 mm radius from the edge of the lens and the optical useful region. This margin 52 is shown in figure 4 between the edge of the lens L1 and the useful zone.

Finally, the image in the sensor space of the aperture stop is located in the vicinity of the cold diaphragm of the cryostat, so that TPC, only the diaphragm is likely to slightly vignetting useful beams. This condition ensures slow spatial variations in the structure of flow TPC.

CLAIMS

IR passive imaging system having its optical axis (z):

- a matrix sensor (1) placed in a cryostat comprising a cold diaphragm (3),

- an optical device imaging on the detector (1), continuously variable focal length from a focal GC configuration F G c, a focal length F PC configuration PC with in the zoom range a constant numerical aperture and an aperture stop located at the cold stop (3) of the cryostat comprising:

o a head group (Gf 1) fixed and constant focal position that includes at least one lens mounted in a mechanical holding (5) of diameter determined by the PC configuration,

o a first (Gm1) and second (Gm2) leaving groups capable of being positioned to ensure zooming between F G c and F PC as well as the focusing of the image on the detector (1),

o an image transmission unit (GF2) of fixed position and constant magnification, adapted to image the aperture stop in order to limit the diameter of the envelope of the useful rays PC on the lenses of the front group GF1, characterized in that the optical device comprises a configuration of TPC focal length F T pc predetermined, with the first and second mobile units (Gm1, Gm2) positioned to obtain the focal length F T pc, and aperture stop materialized in the mechanical retaining (5) head group.

IR passive imaging system according to the preceding claim, characterized in that the two mobile units (Gm1, Gm2) are adjacent and in that the first leaving group (Gm1) is divergent with a negative variable magnification, and the second leaving group (Gm2) is consistent with a negative variable magnification.

Passive IR imaging system according to the preceding claim, characterized in that the GC configuration is 20 °, the PC configuration at 3 °, and the configuration TPC 2 °, and in that the product of the magnifications of the first and second mobile groups is greater than 1 .2 TPC configuration, between 0.8 and 0.85 PC configuration and greater than 0.12 GC configuration.

Passive IR imaging system according to claim 1, characterized in that it comprises a fixed convergent group (GF3) with a variable magnification and negative, between the two mobile units (Gm1, Gm2) which are diverging and have a magnification negative variable.

Passive IR imaging system according to the preceding claim, characterized in that the GC configuration is 20 °, the PC configuration at 3 °, and the TPC configuration at 2 °, and in that the absolute value of the magnification of the group image transport (GF2) is less than 0.1.

cooled IR camera which includes a passive IR imaging system according to one of the preceding claims.

cooled IR binoculars which includes a passive IR imaging system according to one of claims 1 to 5.