IR imaging system with continuous GC-PC zoom provided with a TPC configuration

Abstract

A passive IR imaging system with a matrix-array detector in a cryostat includes a cold diaphragm, an image-forming device, of focal length that is continuously variable between F.sub.GC and F.sub.PC with, in this range of focal lengths, a constant numerical aperture and an aperture diaphragm level with the cold diaphragm, comprising a head group of fixed position and constant focal length with at least one lens in a mechanical holding means by the PC configuration, a first group and second group that are movable and positioned in order to ensure the change of focal length between and the focus of the image, an image-transport group of fixed position and of constant magnification, able to image the aperture diaphragm in order to limit the diameter of the PC useful beams on the lenses of the head group. The device comprises a TPC configuration with the first and second movable groups positioned to obtain the focal length F.sub.TPC, and its aperture diaphragm embodied in the mechanical holding means.

Claims

1. A passive IR imaging system that comprises, on its optical axis: a matrix-array detector placed in a cryostat comprising a cold diaphragm, an optical device for forming images on the detector, having a focal length that is continuously variable between a large-field (GC) configuration of focal length F.sub.GC and a small-field (PC) configuration of focal length F.sub.PC, with, in this range of focal lengths, a constant numerical aperture and an aperture diaphragm located level with the cold diaphragm of the cryostat, comprising: a head group of fixed position and constant focal length that comprises at least one lens mounted in a mechanical holding means, of diameter determined by the small-field (PC) configuration, a first group and second group that are movable and able to be positioned in order to ensure the change of focal length between F.sub.GC and F.sub.PC and the focus of the image on the detector, an image-transport group of fixed position and of constant magnification, able to image the aperture diaphragm in order to limit the diameter of the envelope of the small-field useful beams on the lenses of the head group, wherein the optical device comprises a very-small-field (TPC) configuration of preset focal length F.sub.TPC, with the first and second movable groups positioned to obtain the focal length F.sub.TPC, and an aperture diagram for the very-small-field (TPC) configuration embodied in the mechanical holding means of the head group such that the image in the detector space of this aperture diaphragm is located in the vicinity of the cold diaphragm of the cryostat.

2. The passive IR imaging system as claimed in claim 1, wherein the two movable groups are adjacent and in that the first movable group is divergent with a negative variable magnification, and the second movable group is convergent with a negative variable magnification.

3. The passive IR imaging system as claimed in claim 2, wherein the large-field (GC) configuration corresponds to 20, the small-field (PC) configuration to 3, and the very-small-field (TPC) configuration to 2, and in that the product of the magnifications of the first and second movable groups is higher than 1.2 in very-small-field (TPC) configuration, comprised between 0.8 and 0.85 in small-field (PC) configuration and higher than 0.12 in large-field (GC) configuration.

4. The passive IR imaging system as claimed in claim 1, wherein it comprises a convergent fixed group with a negative variable magnification, located between the two movable groups which are divergent and have a negative variable magnification.

5. The passive IR imaging system as claimed in claim 4, wherein the large-field (GC) configuration corresponds to 20, the small-field (PC) configuration to 3, and the very-small-field (TPC) configuration to 2, and in that the absolute value of the magnification of the image-transport group is lower than 0.1.

6. A cooled IR camera that comprises a passive IR imaging system as claimed in claim 1.

7. A pair of cooled IR binoculars that comprises a passive IR imaging system as claimed in claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other features and advantages of the invention will become apparent on reading the following detailed description, which is given by way of nonlimiting example with reference to the appended drawings, in which:

(2) FIGS. 1a and 1b, which have already been described, schematically show seen in cross section an IR imaging system according to the prior art, in GC configuration (FIG. 1a) and PC configuration (FIG. 1b),

(3) FIGS. 2a to 2c schematically show seen in cross section a first example of an IR imaging system according to the invention, in GC configuration (FIG. 2a), PC configuration (FIG. 2b) and TPC configuration (FIG. 2c),

(4) FIGS. 3a to 3c schematically show seen in cross section a second example of an IR imaging system according to the invention, in GC configuration (FIG. 3a), PC configuration (FIG. 3b) and TPC configuration (FIG. 3c),

(5) FIG. 4 shows an example of a mechanical holding means of the head group.

(6) In all the figures, elements that are the same have been referenced with the same references.

DETAILED DESCRIPTION

(7) As shown in the two examples of FIGS. 2a to 2c and 3a to 3c, the IR imaging system according to the invention comprises, on its optical axis z: a matrix-array detector 1 comprising a cold screen, an optical device for forming images on the detector, having a focal length that is continuously variable between the GC configuration and the PC configuration and operating without vignetting with a constant aperture number N. The current focal length of the zoom is denoted F, its focal length in GC configuration is denoted F.sub.GC, and its focal length in PC configuration is denoted F.sub.PC. in the continuous GC-PC range, an aperture diaphragm located level with the cold diaphragm 3 of the cryostat in order to optimize the noise equivalent temperature difference (NETD) of the system. More rarely, the aperture diaphragm is placed just upstream of the cryostat encapsulating the detector 1, and is bordered by a flux-decreasing mirror so as to achieve a pseudo cold pupil, and guarantee a healthy photometry. In the two aforementioned cases, the zoom operates with a very low level of structural flux, this allowing the performance of the imaging system to be maximized while facilitating corrections of image nonuniformities. If it is managed to control the variations in the residual structural flux (essentially the Narcissus effect) during zooming, it is possible to correct induced image nonuniformities with a single gain table valid for all the focal lengths of the GC-PC range, and a limited number of offset tables.

(8) The optical device comprises in order on the optical axis z:

(9) a convergent head group Gf1 of fixed position and of fixed focal length F1, the one or more lenses of which are mounted in a mechanical holding means 5, one example of which is shown in FIG. 4 for 2 lenses,

(10) 2 movable groups Gm1 and Gm2 positioned to ensure the continuous change of focal length between F.sub.GC and F.sub.PC and the focus of the image on the detector 1 (focus function),

(11) an image-transport group Gf2 of fixed position and of fixed magnification gf2, able to transport the pupil in the entrance space.

(12) According to the invention, the head group Gf1, which has a large impact on the cost of the optic, is exactly dimensioned, taking into account manufacturing imperfections, so as to let pass without vignetting beams useful to the continuous GC-PC zoom under all thermal conditions. To do this, the positions of the PC and GC entrance pupils and the pupillary aberrations are constrained so as to maintain the envelope of the useful beams inside a reasonable diameter, comparable to the diameter of the PC entrance pupil. To this end, the group Gf2 generally comprises a field lens. Typically, the diameter of the head lens is over-dimensioned by about 10% to 20% with respect to the nominal diameter of the PC entrance pupil.

(13) According to the invention, the optical device furthermore comprises a TPC configuration of preset focal length F.sub.TPC. Starting in the PC configuration, the group Gm1 must be moved away from the group Gf1 in order to obtain the focal length F.sub.TPC. The position of the group Gm2 is also modified, so as to maintain the focus on the detector. The continuous GC-PC zoom and the TPC configuration are optimized in a coupled way using a conventional optical-design software package.

(14) In TPC, the abutment 51 of one of the lenses of the head group Gf1 (the lens L2 in the example of FIG. 4), plays the role of aperture diaphragm, and the useful numerical aperture is decreased with respect to that of the continuous zoom. Thus, assuming that the head lens of Gf1 is over-dimensioned by about 20% with respect to the diameter of the entrance pupil of the PC (in order to guarantee the absence of vignetting in the entirety of the GC-PC range), a useful aperture number N.sub.TPC is obtained that is equal to:
N(F.sub.TPC/F.sub.PC)/1.2.

(15) As the diameter of the envelope of the PC beams is similar to the diameter of the PC entrance pupil, the aperture diaphragm of the TPC, which is located in the head block, is naturally imaged in the vicinity of the cold diaphragm, i.e. to within + or a few millimeters from the cold diaphragm; for example, to within less than 5 mm from the cold diaphragm.

(16) At the end of the day, contrary to the conventional design rules of cooled IR imaging systems, in TPC configuration the system operates with a non-negligible proportion of structural flux, this having the effect of degrading the NETD. Nevertheless, as the only diaphragm liable to vignette the useful beams is close to the image of the pupil, the structural flux only exhibits low-frequency spatial variations, and hence the induced image non-uniformities may easily be compensated for using a table of pixel-gain corrections, and a limited number of tables of pixel-offset corrections.

(17) Therefore, the net gain is very largely positive: without significant extra cost with respect to a continuous GC-PC zoom, an integrated and photometrically well-characterized TPC configuration is provided.

(18) Two examples of imaging systems according to the invention will now be described in detail, but the following descriptional details are not intended to limit the scope of the invention. These imaging systems are based on a continuous 20 (GC)3 (PC) zoom with a 2 TPC, suitable for an MWIR cooled detector 1 comprising 640480 elements of a pitch of 15 m. The cold diaphragm 3 of the open cryostat of F/3.9, is assumed to be located at about 11 mm from the focal plane in which the detector 1 is located.

(19) In the first example, the imaging system according to the invention described with reference to FIGS. 2A, 2B and 2C, uses a zoom with two adjacent movable groups. It comprises in order along the optical axis z:

(20) A fixed group Gf1 of focal length F1 and of telephoto type (a priori) with, for example, two lenses, one convergent lens L1 and one divergent L2 lens, mounted on a mechanical holding means 5, which means is shown in FIG. 4.

(21) A divergent movable group Gm1 that plays the role of variator, and which works with a negative variable magnification gm1.

(22) A convergent movable group Gm2 that plays the role of compensator, and which works with a variable magnification gm2 that is also negative.

(23) A fixed group Gf2 that relays to the detector 1 the real intermediate image delivered by the three groups Gf1, Gm1 and Gm2. The group Gf2 works with a fixed and negative magnification gf2; in our example, it is in the vicinity of 1.5, and more generally it is comprised between 2 and 1.

(24) The output pupil coincides with the cold diaphragm 3 of the detector 1, which plays the role of aperture diaphragm of the zoom in the continuous GC-PC range. Using a field lens 4 placed in proximity to the intermediate focal plane Pfi present in the group Gf2, it is possible to constrain the position of the entrance pupil of the PC to the vicinity of the group Gf1, in order to minimize the diameters of the components of this group, which add significantly to the overall cost of the zoom. In GC, the entrance pupil is most often virtual.

(25) It is known that F=F1.Math.gm1.Math.gm2.Math.gf2. Since the magnification gf2 is fixed, the variation in focal length is obtained by varying the product gm1 gm2, which is minimum in GC and maximum in TPC.

(26) The assembly is optimized so that in PC, gm1.Math.gm2 remains below about 0.85. This condition makes it possible to use the variator-compensator tandem as focal group for the entirety of the continuous GC-PC range. Specifically, the axial sensitivity of the Gm1Gm2 tandem, which relates the defocus of the image and the axial movement of the tandem, is expressed by [1(gm1.Math.gm2).sup.2] (gf2).sup.2.

(27) It may be deduced therefrom that if the product gm.Math.gm2 is sufficiently far from 1.0, then a small axial movement of the tandem induces a defocus of the image; in other words, the tandem may play the role of near focal group, and may also serve to compensate for thermal drifts in the combination. Hence its interest.

(28) In contrast, if the product gm1.Math.gm2 is very close to 1.0, then an axial movement of the tandem Gm1Gm2 induces practically no defocus of the image; under these conditions it is not possible to use the movable groups Gm1 and Gm2 dedicated to the zooming to focus the zoom on a nearby scene, or for athermalization.

(29) Advantageously, the TPC configuration is obtained by going beyond the singularity at which gm1.Math.gm2=1.0. As F.sub.TPC/F.sub.PC is about 1.5, gm1.Math.gm2 is then >1.2, this allowing the tandem to be used as focal group, as in the continuous GC-PC range. In the present case gm1.Math.gm2=0.125 in GC, 0.833 in PC and 1.25 in TPC.

(30) A second example of an imaging system according to the invention is now described with reference to FIGS. 3A, 3B and 3C, this system using a zoom with a fixed group between the two movable groups. The zoom comprises in order on the optical axis z:

(31) A fixed group Gf1 of focal length F1 with for example a single convergent lens.

(32) A divergent movable group Gm1 that plays the role of variator and that works with a variable and negative magnification gm1.

(33) A convergent fixed group Gf3 that works with a variable and negative magnification gf3.

(34) A divergent movable group Gm2 that plays the role of compensator, and that works with a variable and negative magnification gm2 of large absolute value (lower than 15, typically).

(35) A fixed group Gf2 that relays to the detector 1 the intermediate image delivered by the four groups Gf1, Gm1, Gf3 and Gm2; Gf2 works with a constant magnification gf2 of small absolute value generally lower than 0.1, this meaning that, in the intermediate space located between the groups Gm2 and Gf2, the beams are almost collimated.

(36) As in the preceding example, in the entirety of the continuous GC-PC range, the exit pupil coincides with the cold diaphragm of the detector, which therefore plays the role of GC-PC aperture diaphragm of the zoom. In order to minimize the diameter of the head lens (of the head group), it is possible to constrain the position of the entrance pupil of the PC to the vicinity thereof, optionally using a field lens 4 placed in the vicinity of the intermediate focal plane PFi in the re-imager Gf2. In GC, the entrance pupil is most often virtual.

(37) The focal length F=F1.Math.gm1.Math.gf3.Math.gm2.Math.gf2. Since the magnification gf2 is constant, the variation in focal length is obtained by varying the product gm1.Math.gf3.Math.gm2, the absolute value of which is minimum in GC and maximum in TPC. Approximately, in the present case, |gm1.Math.gf3.Math.gm2|=4.0 in GC, and 40 in TPC.

(38) Furthermore, in the GC-TPC range, gm2 varies between 17.5 and 19.4, and gf2=0.07, and hence the compensator group Gm2 may play the role of focal group. Specifically, the axial sensitivity of Gm2 is expressed by [1(gm2).sup.2] (gf2).sup.2. On account of the values of gm2 and gf2 that are in play, this sensitivity varies approximately from 1.5 to 1.8 in absolute value, which is largely compatible with an elementary movement step size of 50-micron class.

(39) In these two examples, the aperture diaphragm of the TPC is located in the head group Gf1, which is dimensioned by the needs of the PC. As a result, the useful aperture of the TPC is decreased with respect to the continuous GC-PC zoom by a factor lower than the ratio F.sub.TPC/F.sub.PC, because the head group must let the useful beams of the continuous zoom pass without vignetting under all conditions (thermal conditions and manufacturing tolerances), this requiring the components to be slightly over-dimensioned with respect to the diameter of the PC entrance pupil. In the above two examples, the diameter of the head lens was set to 60 mm, whereas the theoretical diameter of the entrance pupil of the PC is 46.9 mm. Advantage is taken of this for the TPC, which may make use of an entrance pupil of about 57 mm, considering a reasonable margin of 1.5 mm for the ray between the edge of the lens and the useful optical zone. This margin 52 is shown in FIG. 4 between the edge of the lens L1 and the useful zone.

(40) Lastly, the image in the detector space of this aperture diagram is located in the vicinity of the cold diaphragm of the cryostat, in order that, in TPC, only this diaphragm is liable to slightly vignette the useful beams. This condition guarantees slow spatial variations in the structural flux in TPC.