GB2030315A

GB2030315A - Catadioptric Infra-red Lenses

Info

Publication number: GB2030315A
Application number: GB7927047A
Authority: GB
Original assignee: Pilkington PE Ltd
Current assignee: Qioptiq Ltd
Priority date: 1978-08-24
Filing date: 1979-08-03
Publication date: 1980-04-02
Also published as: GB2030315B

Abstract

An infra-red lens which achieves good correction over a flat field of view while employing all spherical surfaces and only a small number of elements of infra-red transmitting material is a catadioptric lens consisting of a front surface primary concave mirror 1 gas spaced from a Mangin secondary mirror 2 of infra- red transmitting material and having an internally convex reflecting surface R3, and at least one positive meniscus element 3 of infrared transmitting material gas spaced from and with its convex surface R5 facing the Mangin secondary mirror and located at or near a central aperture 4 in the primary mirror 1. The lens can be used as an objective lens at infra-red wavelengths, notably 3 to 5.5 microns or 8 to 14 microns. Preferably baffles 5, 6 are provided to obstruct unwanted radiation paths. <IMAGE>

Description

SPECIFICATION Improvements in or Relating to Lenses This invention concerns improvements in or reiating to lenses and relates more particularly to catadioptric lenses for use in infra-red wavelengths, for example with infra-red radiation in the waveband 3 to 5.5 microns or 8 to 14 microns.

According to the invention there is provided a catadioptric lens consisting of a front surface primary concave mirror of spherical curvature having a central aperture, a Mangin secondary mirror of infra-red transmitting material and having an internally convex reflecting surface of spherical curvature, the Mangin secondary mirror being gas spaced from the primary mirror, and at least one positive meniscus element of infra-red transmitting material gas spaced from, and with its convex surface facing towards, the Mangin secondary mirror, located at or near the aperture in the primary mirror.

Preferably the lens is a three element lens having a single positive meniscus element at or near the aperture in the primary mirror. If desired, however, there may be more than one positive meniscus element at or near the aperture in the primary mirror. For example, there may be two such elements thus providing a four element lens.

The infra-red transmitting material of the Mangin secondary mirror and the positive meniscus element or elements is preferably germanium if one wishes to use the lens with infra-red radiation of between 8 to 14 microns. If one wishes to use the lens with infra-red radiation of between 3 to 5.5 microns then preferably silicon is used as the infra-red transmitting material. Conveniently the gas spaces between the elements are air spaces.

The surface of the Mangin secondary mirror opposite the internally convex reflecting surface may be planar or may be concave or may be convex.

With a lens as set forth above the Mangin secondary mirror compensates for the spherical aberration of the primary mirror plus positive meniscus element or elements, the Mangin secondary mirror plus the positive meniscus element or elements correct the coma of the primary mirror, and the positive meniscus element or elements correct the residual astigmatism. Field curvature can be very low by reason of the Petzval sum contributions of all the elements almost cancelling each other.

Further, the lens can be almost self-achromatised and have a relatively small shift of focus with temperature by reason of the effective refractive powers partially cancelling each other.

Preferably baffles are provided to obstruct unwanted radiation paths through the lens.

Embodiments of infra-red catadioptric objective lenses in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a schematic representation of a first embodiment, and Figure 2 is a schematic representation of a second embodiment.

The embodiment shown in Figure 1 is a three element lens consisting of a front surface primary mirror 1, a secondary Mangin mirror 2 and a positive meniscus element 3, the elements 1 and 2 being air spaced.

The primary mirror 1 has a concave mirrored front surface R1 of spherical curvature and has a central aperture 4. The substrate of the primary mirror 1 is of any suitable material which does not need to be transmissive in the infra-red since infra-red radiation is not required to pass through the substrate.

The secondary Mangin mirror 2 is an element of infra-red transmitting material, and preferably germanium, having a surface R2, shown as planar but which may be concave or convex, facing the primary mirror 1, and a concave mirrored surface R3 of spherical curvature. In use infra-red radiation is internally reflected from the mirrored surface R3 so that it presents an internal convex reflecting surface to the radiation. In the subsequent description the surface R2 is sometimes also referred to as surface R4 since it is traversed twice by the incident radiation as more fully described later.

The positive meniscus element 3 is located at or adjacent the aperture 4 in the primary mirror 1 with its convex surface R5 (which is of spherical curvature, as is the concave surface R6) facing the secondary Mangin mirror 2 and its concave surface R6 facing through the aperture 4. The element 3 is of an infra-red transmitting material and preferably germanium.

In use, infra-red radiation, and in particular radiation in the waveband of about 3 to 5.5 microns or more especially in the waveband of about 8 to 14 microns, from a scene or object, and usually a distant scene or object, is received directly (i.e. without having previously passed through other lens elements) on the surface R1 of the primary mirror 1. The radiation is reflected from the primary mirror 1 through the air space towards the secondary Mangin mirror 2, the radiation path between the primary and secondary mirrors being occupied solely by air. The radiation is refracted at the surface R2 and then travels in the element 2 to the internal convex-reflecting surface R3.The radiation internally reflected from the surface R3 travels back to the surface R4 (which, as previously mentioned is also the surface R2 but is here referred to as R4 being the fourth surface encountered by the radiation). The radiation is refracted at surface R4 and then travels through the air space to the convex surface R5 of the positive element 3, the radiation path beteween the surfaces R4 and R5 being occupied solely by air. The radiation is transmitted through the element 3, with refraction at the surfaces R5 and R6, and then emerges through the aperture 4 for receipt, for example, on an infra-red photodetector array or pyroelectric vidicon.

With such a lens, which may conveniently be referred to as a spherical mirror plus sub-aperture correctors catadioptric, good correction over a flat field of view can be achieved. The Mangin secondary mirror 2 compensates for the spherical aberration of the primary mirror 1 pius positive element 3. The Mangin secondary mirror 2 plus positive lens element 3 correct the coma of the primary mirror 1. The positive element 3 corrects the residual astigmatism of the system. Field curvature is very low as the Petzval sum contributions of all the components almost cancel each other.

The system has the major constructional advantages of employing all-spherical surfaces and only two relatively small elements of infra-red transmitting material, notably germanium. It also has the very big advantages of being almost self-achromatised and having a comparatively small shift of focus with temperature, both of these advantages resulting from the fact that the effective refractive powers partially cancel each other.

Typically, a lens having an equivalent focal length of 200, F number 1.0 and Field angle 8 degrees total can give a residual monochromatic on-axis R.M.S. aberration at best focus of about 0.037 A a typical on-axis chromatic aberration (between 8.5 and 12.5 microns wavelength) of about 0.24 A and a maximum R.M.S. aberration at 40 off-axis of about 0.496 A. The overall length of the lens, i.e. the distance between the two most extreme points in the system such as the image plane and the secondary mirror can be about 0.76 F, where F is the Equivalent Focal Length, and the back focal length, i.e. the distance of the image behind the rearmost surface in the system, can be about 0.06 F.

The obscuration area (allowing no vignetting) can be about 35%.

The aperture stop may be at the primary mirror 1 as indicated by the reference S in Figure 1.

Alternatively the aperture stop may be located in front of the primary mirror 1 in the region of the secondary Mangin mirror 2.

Preferably baffles, indicated as 5 and 6, are provided to obstruct unwanted radiation paths, and in particular to prevent incident radiation passing directly to the positive meniscus element 3, i.e. so that the aperture 4 passes only radiation which was incident on the primary mirror 1 and has travelled therefrom via the secondary Mangin mirror 2 and the positive element 3. The problem of baffling and consequent increase of obscuration would not occur if the catadioptric lens were to be used as the objective part of a high magnification afocal telescope having defined field and exit pupil stops.

Particular examples of lens in accordance with the Figure 1 embodiment have numerical data as follows, the dimensional units being millimetres but the values being relative and scaleable accordingly.

Example 1 Radius of Axial Material Surface Curvature Thickness/Spacing R1 -314.974 (Reflection) -92.853 -Air R2 Infinity -8.103 Germanium R3 -1044.907 (Reflection) 8.103 Germanium R4 Infinity 77.244 Air R5 +65.174 9.158 Germanium R6 +118.357 Equivalent focal length: 1 50.0 F. number:0.95 Field angle: 8 degrees total Stop position: on surface R1 Defocus: -0.009.

Example 2 Radius of Axial Surface Curvature Thickness/Spacing Material R1 -208.03 -60.18 -Air R2 infinity -5.334 -Germanium R3 -705.73 5.334 Germanium R4 Infinity 52.31 Air R5 +45.641 6.096 Germanium R6 +83.724 12.040 Air Equivalent focal length: 100 F. number: 1.0 Field angle: 8 degrees Stop position: 6.11 before surface R1 Defocus: +0.0549.

Example 3 Radius of Axial Surface Curvature Thickness/Spacing Material R1 -206.51 -58.30 -Air R2 -5040.4 -6.353 Germanium R3 -681.00 6.353 Germanium R4 -5040.4 52.73 Air R5 +44.511 4.512 Germanium R6 +82.047 11.893 Air Equivalent focal iength:100 F. number: 1.0 Field angle: 8 degrees Stop position: 65.28 before surface R1 Defocus: +0.0549.

The embodiment shown in Figure 2 is basically similar to that shown in Figure 1 but is a four element lens, there being two positive meniscus elements 7 and 8 in place of the single positive meniscus element 3 of Figure 1. The meniscus element 7 has a convex surface R5 facing the secondary Mangin mirror 2 and a concave surface R6 facing through the aperture 4. The meniscus element 8 has a convex surface R7 facing the secondary Mangin mirror 2 and a concave surface R8 facing through the aperture 4. Thus, the function and power of the positive meniscus element 3 of the Figure 1 embodiment is in the Figure 2 embodiment effectively split between two positive meniscus elements 7 and 8 in a manner which will be well understood by those skilled in the art.The two elements 7 and 8 are both, of course, of infra-red transmitting material and preferably germanium, and have a small air space between them.

Figure 2 also shows, for purposes of illustration, a concave surface R2 (and R4) on the Mangin secondary mirror 2, and an aperture stop S located in the region of the Mangin secondary mirror.

It will be understood that although as described above, air is the most convenient and preferred gas for the spaces between elements, other suitable gases could be employed.

A particular example of lens in accordance with the Figure 2 embodiment has numerical data as follows: Example 4 Radius of Axial Surface Curvature Thickness/Spacing Material R1 -248.287 -75.566 -Air R2 +431.922 -5.409 Germanium R3 -1668.62 +5.409 Germanium R4 +431.922 +58.128 Air R5 +148.142 +6.113 Germanium R6 +323.855 +0.291 Air R7 +55.001 +6.113 Germanium R8 +71.607 Equivalent focal length: 100.0 F. number: 0.7 Field angle: 40 total Stop position: On surface R1 Defocus: Zero.

Claims

1. A catadioptric lens consisting of a front surface primary concave mirror of spherical curvature having a central aperture, a Mangin secondary mirror of infra-red transmitting material and having an internally convex reflecting surface of spherical curvature, the Mangin secondary mirror being gas spaced from the primary mirror, and at least one positive meniscus element of infra-red transmitting material gas spaced from, and with its convex surface facing towards, the Mangin secondary mirror, located at or near the aperture in the primary mirror.

2. A three element lens according to Claim 1 having a single positive meniscus element at or near the aperture in the primary mirror.

3. A four element lens according to Claim 1 having two positive meniscus elements at or near the aperture in the primary mirror.

4. A lens according to any preceding claim for use with infra-red radiation of between 8 to 14 microns wavelength wherein the material of the Mangin secondary mirror and the positive meniscus element or elements is germanium.

5. A lens according to any of Claims 1 to 3 for use with infra-red radiation of between 3 to 5.5 microns wavelength wherein the material of the Mangin secondary mirror and the positive meniscus element or elements is silicon.

6. A lens according to any preceding claim wherein the gas spaces between elements are air spaces.

7. A lens according to any preceding claim with baffles provided to obstruct unwanted radiation paths through the lens.

8. An infra-red catadioptric objective lens substantially as described herein with reference to Figure 1 or with reference to Figure 2 of the accompanying drawing.

9. An infra-red catadioptric objective lens substantially in accordance with any of Examples 1 to 4 set forth herein.