This disclosure relates to a compact achromatic and athermalized telephoto lens arrangement for infrared imaging.

Telephoto lenses enlarge object scenes in far distances and warrant the sharp and high- contrast images expected from modern infrared cameras. Newly developed detectors such as those with 17 μιη pixel pitch require lens assemblies with higher spatial resolution than before. The most critical parameters are the thermal drift and the wavelength drift of the system. Moreover, the overall dimensions and weight are to be minimised as such systems are most often for portable use, or are embarked on vehicles where space is at a premium.

Developing high performance infrared systems capable of operating in harsh environment, in particular in widely varying ambient temperatures, is challenging. Indeed, most commonly used IR materials exhibit a very large change in refractive index with temperature, rendering IR optical systems much more sensitive to thermal de focusing than visible optics. This is most critical for large aperture systems and long focal lengths.

Active athermalization, using an electromechanical device to adjust the position of a lens or group, offers a solution to the thermal drift problem. The positional adjustment can be calculated based on the ambient temperature as measured by a build-in sensor, or else by processing the image acquired by the imaging sensor itself. These methods however necessitate the inclusion of a servomotor, and of electronics and power sources either inside or outside of the assembly. For the sake of simplicity, passive athermalization is often preferred.

Passive athermal solutions for a single lens group are known, using an appropriate combination of 2 different materials. Combining 3 different materials makes it even possible to obtain both athermal and achromatic solutions. Graphical methods exist to achieve such goals.

Athermalization of a complete optical arrangement without resorting to the athermalization of the individual groups is susceptible to deliver superior results. Such a global optimization is however beyond what can be done with graphical methods. The effect of the expansion of the holder should then also be taken into account, preferably putting it to good use to cancel residual drift. Due to the large number of actionable parameters, one has then to resort to modelling software, and also to rely on the insights of an optical designer. This is especially true if additional boundary conditions are added, such as the need for a compact or lightweight assembly, for high resolution, and for minimal costs.

GB 2121211 describes a self-compensated IR system. The front positive group comprises 2 lenses so as to compensate for chromatic aberrations, and is said to be neutral in terms of refractive index temperature dependency. The back group is also positive, comprising 2 lenses, one of them being negative and having a temperature sensitive refractive index such as Ge. This back group is said to compensate for the thermal drift of the complete system, including front group, back group and support assembly.

US 4679891 shows different lens assemblies which are arranged in front of a scanning polygon. This type of Infrared imaging systems is very expensive. Today, the development of high resolution infrared matrix detectors restricts the application of polygon systems to very special applications.

US 5504628 divulges a front group providing passive athermalization by careful design of the comprised 2 lenses. This doublet of lenses has to be made out of materials having different refractive index temperature dependency. Materials having similar Abbe numbers are used, the chromatic aberration being compensated for by a diffractive surface. The optical arrangement has the aperture stop quite far from front group. Consequently, the diameter of front group has to be large.

In "Passively athermalized hybrid objective for a far infrared uncooled thermal imager", A. P. Wood et al, Proc. SPIE (1996), Vol. 2744, pp. 500-509, an athermalized solution is presented using a hybrid ZnSe/Ge doublet front lens and a Ge meniscus lens at the rear. Being a Petzval type of arrangement, both groups are positive. Moreover, such a setup is not particularly compact. A diffractive surface is used, resulting in an MTF (Modulation Transfer Function) of about 80% at 9 cy/mm.

In "Using new optical materials and DOE in low-cost lenses for uncooled IR cameras", J.-M. Bacchus, Proc. SPIE (2004), Vol. 5249, pp. 425-432), narrow field lens systems using 2 GASIR ^® lenses and 2 Ge lenses are compared. The GASIR ^® embodiment is shown to be superior in terms of thermal drift. Drift compensation is said to be improved by mounting thermal expansion rings in the housing.

In "The radiation tolerance of chalcogenide glasses", Masataka Naitoh et al., Proc. Spie (2010), Vol. 7660, 7660028, an athermal IR telephoto lens is described. Both chromatic aberrations and thermal defocusing are said to be compensated for. The design uses a convergent front group consisting of 2 lenses, one made of chalcogenide glass and one of Ge. This latter lens is moreover diffractive. The back group is slightly convergent and consists of 3 Ge lenses. A focal length of 78 mm is reported for an overall length of 102 mm.

Known achromatic telephoto lens arrangements with passive athermalization have a longer overall length than their effective focal length. Such assemblies are bulky and difficult to handle. Moreover, most systems comprise more than 2 lens groups. One of the primary aims of the present invention is to provide a compact assembly using only 2 lens groups. The only way to shrink the volume for a given F-number is to reduce the overall length. To this end, a closely spaced convergent front group is combined with a divergent back group. This technique is known in the visible spectrum, but has not yet been applied in IR optics.

It appears that compensation of thermal drift by proper choice of materials is very common in IR systems. In the prior art, compensation is sought within each lens groups, in particular within the front group. This typically entails the use of multiple lenses in at least this front group.

The current invention achieves thermal compensation globally for the complete assembly by using a convergent group in the front, and a divergent group in the back, the choice of the front and back materials being subject to particular conditions. The optical compensation can be supplemented with mechanical compensation, e.g. by using a relatively short thermal expansion ring, which fits easily in the compact assembly.

More particularly, the invention concerns a lens arrangement for high resolution infrared imaging with effective focal length EFL and overall length OL, comprising one convergent front (object-side) group and one divergent back (image-side) group, characterized in that: the front group comprises one or more lenses forming a convergent system having an equivalent thermal glass constant JT of less than 40.10 ^~6 K ^"1 ; the back group comprises one or more lenses forming a divergent system having an equivalent thermal glass constant jr of more than 70.10 ⁶ K ^"1 ; and, the separation between front and back groups is more than 0.45 EFL, and OL / EFL < 1.05.

OL is the distance between the object-side surface of the front lens group and the image plane. The separation between front and back groups is the distance between the image plane side of the front lens group and the object side of the back lens group. Advantageously, either the convergent front group consists of a single lens made out of a glass having a thermal glass constant JT of less than 40.10 ⁶ K ^"1 , or the divergent back group consists of a single lens made out of a glass having a thermal glass constant JT of more than 70.10 ^~6 K ^~1 . More advantageously, both the convergent front group and the divergent back group consist each of a single lens made out of a glasses having a thermal glass constant JT of less than 40.10 ⁶ K ^"1 , and of more than 70.10 ⁶ K ^"1 respectively.

When the divergent back group consists of a single lens, then Ge is an ideal material.

Any of the above lens arrangements are particularly suitable for achieving a diagonal field of view of less than 18°, or, preferably of less than 12°, the former corresponding to a telephoto lens, the latter to a super telephoto lens. Moreover, very compact assemblies may be obtained, characterized in that OL / EFL < 1, or even < 0.95.

Such an arrangement may provide for a spatial resolution, expressed by an averaged on axis MTF-value at 30 cy/mm between -40 and +80 °C, of more than 0.30. Also, the illumination in the detector plane may amount to more than 86% of the on axis value. These values correspond to design goals that can be achieved without undue burden using known simulation and optimization software. The key to success is the combination of the convergent-divergent lens configuration with the proper choice of materials, as defined above. Apart from the design goals mentioned above, the present arrangement also allows for a substantial back focal length of more than 15mm , thus ensuring adaptability to a wide range of detectors. Figure 1 illustrates the invention with an arrangement providing a passive athermalized achromatic telephoto lens arrangement according to Examples 3 or 4 below. It comprises a convergent front group (1), a divergent back group (2), a detector window (3), and a detector plane (4). Also shown are the distance between front group and back group (5), the free back focal length (BFL) (6), the overall length (OL) (7), and the field angle of view (8).

IR-optics, the thermal glass constant y _T is defined as:

1 dn wherein n is the refractive index, and <¾ the linear thermal lens expansion coefficient. The thermal drift As of the focal length of a lens is then expressed as:

where <¾ is the thermal expansion coefficient of the lens holder,/' is the focal length of the lens, and JT is the thermal glass constant. Table 1 illustrates the imposed design limits for the calculation of Examples 1 to 5 as shown in Tables 2, 3 and 4.

Table 1 : Design limits for the calculation of Examples 1 to 5.

Ex. Lenses f/# OL / WaveImage On axis Corner On axis On axis in EFL band circle MTF MTF MTF MTF groups (μ ^η ) (mm) +20 °C +20 °C -40 °C +80°C

1 1+1 1.3 0,89 8-12 21,8 >0,35 >0,30 >0,34 >0,34

2 1+1 1.3 0,86 3-5 21,8 >0,35 >0,23 >0,34 >0,34

3 1+1 1.5 0,91 8-12 14 >0,37 >0,33 >0,36 >0,36

4 1+1 1.5 0,91 8-14 14 >0,30 >0,27 >0,30 >0,30

5 2+1 1.5 0,91 8-14 14 >0,37 >0,33 >0,36 >0,36 These design limits could be reached, essentially thanks to the arrangement of a convergent front group and a divergent back group, both groups being separated by a distance of more than 0.45 EFL, yet resulting in a reduced overall length, characterized by OL / EFL < 0.95. Another result of this arrangement is to offer optical athermalization. This is realized by a suitable choice of lens materials. Indeed, of the front group has a low thermal glass constant j _\ and the back group has a high thermal glass constant γ ₂ , then the thermal drift of whole arrangement is considerable reduced. Residual thermal drift is passively compensated by well known mechanical means, using a thermal expansion ring.

In mentioned Examples 1 to 4, the convergent front group is only a single meniscus made on GASIR ^® 1. Diffractive rings are placed on concave surface of front to achromatize this group.

In Example 5, achromatization is obtained by using a doublet in the front group. A diffractive pattern is then unnecessary.

In mentioned Example, both optical surfaces of front meniscus have an aspheric profile.

Table 2 shows detailed design parameters for 5 embodiments according to the invention. Apart from variations of the geometry, the Examples illustrate the use of different detector sizes, wavebands, and lens materials. The resulting image quality is reported in terms of relative illumination of the corners, and in terms of the MTF at 30 cy/mm, which is the relevant Nyquist frequency for 17 μιη pitched detectors. Additionally, Table 2 mentions the length of the POM-ring (polyoxymethylene) mechanically compensating the residual thermal drift. This ring is mounted in an aluminium holder, so as to slightly retract the front lens group towards the image plane with rising temperature. Tables 3 and 4 mention detailed lens design parameters: Table 3 for convergent front groups, Table 4 for divergent back groups. Lens materials are GASIR 1 (Ge22As2oSe ₅ 8), which has a refractive index of about 2.5; ZnS, which has a refractive index of about 2.2; and Ge, which has a refractive index of about 4. Other materials such as other chalcogenide glasses would be suitable too.

The geometric parameters in Table 3 and 4 correspond to lens surfaces according to the formula: z(r

where c = 1 / r ₀ with ro the vertex radius, r the distance from optical axis, and z the coordinate on the optical axis, ro, r and z being expressed in mm. The surfaces are numbered from 1 to 6 starting from the object side towards the image plane.

The diffractive structures are presented by phase deformation in first diffractive order accordin to the formula: where p = r I r _\ with the normalization radius ri and the phase coefficients A,. The reference wavelength is the middle of the waveband.

Table 2: First order parameters, image quality values, and ring length for thermal drift compensation for Examples 1 to 5.

Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5

Waveband (μηι) 8-12 3-5 8-12 8-14 8-14 Image circle (mm) 21 ,8 21 ,8 14 14 14

First order

EFL (mm) 140 140 100 100 100 parameters

F-Number 1 ,3 1 ,3 1 ,5 1 ,5 1 ,5 OL / EFL 0,89 0,86 0,91 0,91 0,91

MTF on axis +20 °C 0,370 0,367 0,401 0,31 1 0,395

Image MTF corner +20 °C 0,335 0,245 0,361 0,286 0,357 quality MTF on axis -40 °C 0,373 0,373 0,406 0,314 0,397

MTF on axis +80 °C 0,370 0,363 0,399 0,312 0,384

Length of thermal expansion ring 31mm 24mm 25mm 25mm 23mm Table 3 : Detailed design parameters of convergent front group.

Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5

Material GASIR ^® 1 GASIR ^® 1 GASIR ^® 1 GASIR ^® 1 GASIR ^® 1

Thickness (mm) 14 14 10 10 10

Surface 1 r ₀ 79,54 71,62 58,61 58,61 59,67 k -0,34 -0,55 -0,56 -0,56 -0,66 cc ₄ 0,00E+00 0,00E+00 2,85E-07 2,85E-07 3,22E-07 cc ₆ 5.07E-11 8.83E-11 2.03E-11 2.03E-11 3.13E-11 cc ₈ -1.16E-14 -1.16E-14 0,00E+00 0,00E+00 4,00E-14

OCio 1.19E-18 8,24E-19 0,00E+00 0,00E+00 0,00E+00

Lens 1

Surface 2 r ₀ 129,72 112,23 97,38 97,38 135,59 k 0,00 0,00 0,00 0,00 0,53 a ₄ -9.76E-08 -2,39E-07 2,91E-08 2,91E-08 0,00E+00 6 1.21E-10 1.73E-10 -6.48E-11 -6.48E-11 0,00E+00 8 -6.01E-14 -6.01E-14 0,00E+00 0,00E+00 3.10E-14

OCio 1.25E-17 7.51E-18 0,00E+00 0,00E+00 0,00E+00

0 l2 0,00E+00 3,81E-22 0,00E+00 0,00E+00 0,00E+00

0 i ₄ -2,88E-25 -1.50E-25 0,00E+00 0,00E+00 0,00E+00

Air gap (mm) - - - - 3

Material - - - - ZnS

Thickness (mm) - - - - 5,0

Surface 3 r ₀ - - - - 264,36

Surface 4 r ₀ - - - - 142,41

Lens 2 k - - - - -3,65 a ₄ - - - - 0,00E+00 6 - - - - 0,00E+00 8 - - - - -3.36E-14

OCio - - - - 0,00E+00

Difiractive pattern on surface 2 2 2 2 none

n 1,00 1,00 1,00 1,00 -

Aj -6,36E-02 -7,82E-02 -8,80E-02 -8,80E-02 -

A ₂ -2.61E-06 -2,83E-06 0,00E+00 0,00E+00 -

A ₃ 6,22E-10 2,82E-10 0,00E+00 0,00E+00 - Table 3 : (continued)

Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5

Air gap to back group (mm) 75,8 64,6 59,0 59,0 51,5

Table 4: Detailed design parameters of divergent back group.

Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5

Material Ge Ge Ge Ge Ge

Thickness (mm) 2,5 3,7 3,0 3,0 2,5

Surface 5 r ₀ 47,30 50,30 42,67 42,67 109,07

Surface 6 r ₀ 40,30 40,79 33,52 33,52 71,88 k 0,73 1,06 2,95 2,95 7,88

Lens 3 cc ₄ 0,00E+00 0,00E+00 -2,58E-06 -2,58E-06 3,88E-06 cc ₆ 3,22E-09 -6.52E-10 2.72E-09 2.72E-09 1.27E-08 cc ₈ -7.81E-12 -7.81E-12 1.07E-11 1.07E-11 1.83E-11

O io 2,60E-14 1.28E-13 0,00E+00 0,00E+00 0,00E+00

0 l2 0,00E+00 -4.70E-16 0,00E+00 0,00E+00 0,00E+00

0 i ₄ 0,00E+00 6,39E-19 0,00E+00 0,00E+00 0,00E+00

The illustrated lens arrangements are particularly compact, are compensated over a wide temperature range, and provide a high resolution suitable for 17 μιη detectors. The key to achieve these design goals is the combination of the convergent-divergent lens configuration with the proper choice of materials, as defined above.