GB2071353A - Telescope objective system - Google Patents
Telescope objective system Download PDFInfo
- Publication number
- GB2071353A GB2071353A GB8105761A GB8105761A GB2071353A GB 2071353 A GB2071353 A GB 2071353A GB 8105761 A GB8105761 A GB 8105761A GB 8105761 A GB8105761 A GB 8105761A GB 2071353 A GB2071353 A GB 2071353A
- Authority
- GB
- United Kingdom
- Prior art keywords
- lens
- lens element
- image surface
- chalcogenide glass
- lens elements
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Granted
Links
- 239000005387 chalcogenide glass Substances 0.000 claims abstract description 29
- 239000000463 material Substances 0.000 claims abstract description 24
- 229910052732 germanium Inorganic materials 0.000 claims abstract description 20
- GNPVGFCGXDBREM-UHFFFAOYSA-N germanium atom Chemical compound [Ge] GNPVGFCGXDBREM-UHFFFAOYSA-N 0.000 claims abstract description 19
- 230000003287 optical effect Effects 0.000 claims abstract description 11
- 230000004075 alteration Effects 0.000 claims abstract description 7
- 230000003595 spectral effect Effects 0.000 claims abstract description 4
- 239000006185 dispersion Substances 0.000 claims description 8
- 239000013078 crystal Substances 0.000 claims description 7
- 150000004820 halides Chemical class 0.000 claims description 7
- 238000000926 separation method Methods 0.000 description 10
- 239000011521 glass Substances 0.000 description 4
- 230000005855 radiation Effects 0.000 description 3
- 230000015556 catabolic process Effects 0.000 description 2
- 238000006731 degradation reaction Methods 0.000 description 2
- 125000006850 spacer group Chemical group 0.000 description 2
- 241001502050 Acis Species 0.000 description 1
- 241000428533 Rhis Species 0.000 description 1
- 238000001514 detection method Methods 0.000 description 1
- 230000006866 deterioration Effects 0.000 description 1
- 238000003384 imaging method Methods 0.000 description 1
- 238000001429 visible spectrum Methods 0.000 description 1
Classifications
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B1/00—Optical elements characterised by the material of which they are made; Optical coatings for optical elements
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B13/00—Optical objectives specially designed for the purposes specified below
- G02B13/14—Optical objectives specially designed for the purposes specified below for use with infrared or ultraviolet radiation
Abstract
A telescope objective system (10) is formed by a primary lens having a single lens element (A) aligned on a common optical axis (11) with a secondary lens having two lens elements (B, C) the shape and power distribution of lens elements (A, B, C) being such as to compensate for monochromatic aberration of the system, each lens element (A, B, C) being made of a material which has a useful spectral bandpass in the infrared wavelength region and lens elements (B, C) having refractive surfaces (16, 15, 14, 13) intercepting the optical axis (11) which are substantially spherical, the system having a planar image surface (11) and an effective focal length greater than the axial distance between the image surface (11) and the distal refractive surface (18) formed by the primary lens element (A). Primary lens element (A) is positively powered and made of germanium and at least one of the secondary lens elements (B, C) is made of a Chalcogenide glass such that the system (10) is achromatic in the infrared wavelength region. Conveniently lens element (B) is made of Chalcogenide glass and is negatively powered. Lens element (C) may be negatively powered and made of germanium, or positively powered and made of a Chalcogenide glass. <IMAGE>
Description
SPECIFICATION
Telescope objective system
This invention relates to telescope objective systems having a useful spectral bandpass in the infrared wavelength region.
The detection and recognition of objects at long ranges using thermal imagining equipment ultimately depends upon the quality of the telescope objective system which for practical purposes requires to be compact and economic to produce. Previously known objective systems for this purpose have used lens elements made of optical-grade germanium which has a high refractive index and, in comparison with materials used for the visible spectrum, low dispersion coefficient. The shape, and power distribution of the lens elements has been selected, in known manner, to compensate for monochromatic aberration. The telephoto form of the objective system which is particularly desirable because of its short overall length is found to accentuate the chromatic aberration of the system thereby restricting the waveband of infrared radiation that can be successfully transmitted.Chromatic aberration arises because of the small but significant dispersion of the germanium material. In addition to the chromatic aberration problem an image-surface shift problem arises due to the high thermal coefficient of the refractive index of germanium. That is, significant variations in ambient temperature cause the image surface to shift from its design position and there is a need to provide a mechanism which can compensate for this shift if it causes deterioration of imaging performance under operating conditions.
It is an object of the present invention to provide an improved telescope objective system which is of telephoto form and achromatic in the infrared wavelength region.
According to the present invention there is provided a telescope objective system formed by a primary lens having a single lens element aligned on a common optical axis with a secondary lens having two lens elements the shape and power distribution of the lens elements being such at to compensate for monochromatic abberation of the system, each lens element being made of a material which has a useful spectrai bandpass in the infrared wavelength region and the lens elements of the secondary lens having refractive surfaces intercepting said optical axis which are substantially spherical, the system having a planar image surface and an effective focallength greater than the axial distance between the image surface and the distal one of said refractive surfaces, said distal refractive surface being formed by said primary lens element which is positively powered and made of germanium, at least one of the two lens elements of the secondary lens being made of a chalcogenide glass such that the system is achromatic in the infrared wavelength region.
It will be noted that because the effective focal length of the system is greater than the axial distance between the image surface and the refractive surface on which the radiation is initially incident the system is of telephoto form; with only three lens elements the system is optically and mechanically simple; and because of the compensation for both monochromatic and chromatic aberrations of the performance can approach the diffraction limit over an appreciable waveband.
Conveniently the middle one of the three lens elements is negatively powered and made of a chalcogenide glass. The lens element adjacent the image surface may be positively or negatively powered and made of germanium, or a chalocegenide glass. The two lens elements of the secondary lens may each be made of the same chalcogenide glass. Alternatively, the secondary lens may be formed with differing chalcogenide glasses the middle lens element of the three having a lower dispersion coefficient than the lens element adjacent the image surface.
The achromatizing lens element may be a chalcogenide glass such as that sold by Barr and Stroud
Limited under their designations BSA, BS 1 or BS2; or that sold by Texas Instruments Inc. of U.S.A.
under their designation Tl20 or TI1 1 73; or that sold by Amorphous Materials Inc., of Garland, Texas,
U.S.A. under their designation AMTIR-1.
Where the secondary lens has a positively powered lens element this may conveniently be made of a halide crystal material, which has a positive thermal coefficient in order to render the system athermal.
Convenient halide crystal materials with a bandpass in the infrared wavelength region are KRS5 and KRS6 both of which are sold by the Harshaw Chemical Co., 6801 Cochran Road, Selon, Ohio U.S.A.
With this arrangement the lens elements are preferably fixedly mounted with respect to each other with inter element mount spacers of the correct thermal expansion coefficients, e.g. dural mounts to prevent movement of image position so that image degradation does not occur for temperature variations in the --300 to +500C range.
Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which:
Fig. 1 shows a system in which the middle lens element is relatively flat;
Fig. 2 shows a system in which the lens element adjacent the image surface is relatively flat; and
Fig. 3 shows a system in which the two lens elements adjacent the image surface are each relatively flat.
In each of the drawings the objective system 1 0 comprises three air-spaced lens elements A, B, C which are aligned on a common optical axis 11. A planar image surface is formed at 12. Element A constitutes the primary lens and elements B and C together constitute the secondary lens of the system.
Each secondary lens element B, C has spherical refractive surfaces intercepting the acis 11, the refractive surfaces being 13, 14, 1 5, 1 6, ordered sequentially in the direction away from the image surface 12. The refractive surfaces 17, 1 8 or the primary lens element A may be spheric or aspheric.
Thus, radiation from object space 0 is incident on surface 18 and is refracted by lens elements A, B and C to form an image at surface 12.
In accordance with the present invention the lens elements A, B and C are each made of a material which has a useful spectral bandpass in the infrared wavelength region, particularly the 8-1 3 micron (,us) range and the effective focal length of the system 10 is greater than the axial distance between
refractive surface 1 8 and image surface 12, i.e. the system 10 has a telephoto form.
In each embodiment lens element A is positively powered (i.e. convergent) and is made of optical
grade germanium. In Fig. 1, lens element C is also made of germanium but is negatively powered (i.e.
divergent) and element B is made of a chalcogenide glass and is negatively powered. In Fig. 2 lens
element B is made of germanium and is negatively powered and lens element C is made of a
chalcogenide glass and is negatively powered. In Fig. 3 lens element B is made of a chalcogenide glass
and is negatively powered and element C is positively powered and may be either a chalcogenide glass
or a halide crystal material.
In the case of the Fig. 1 and Fig. 2 embodiments achromatisation of the system 10 is achieved by the lens element which is chalcogenide glass, these glasses being more dispersive than germanium. In the case of the Fig. 3 embodiment where two chalcogenide glass elements are employed these are most effective when the dispersions of the two glasses are matched to ensure minimum overall length of the system 10 simultaneously with the achievement of full achromatisation. It will also be noted that in each embodiment the lens element made of chalcogenide glass is relatively small and relatively flat (i.e. the refractive surfaces are of large radius of curvature). This is advantageous in that known chalcogenide glasses do not display optical homogeneity of refractive index over very large surface areas.
Four specific examples of objective systems will now be given relating to the three embodiments described and in which the lens elements are either germanium or chaicogenide glass and all refractive surfaces are spheric.
EXAMPLE 1 relates to the embodiment of Fig. 1; lens elements A and C are each germanium and lens element B is chalcogenide glass made by Barr and Stroud Limited and designated BS1 The effective focal length is 375 mm, the back focal length is 157.4 mm and the separation and radius of curvature of the refractive surfaces is given in the table reading from surface 18 in the direction of the image surface 12 (dimensions being in mm): :
Separation Radius - 208.2 14.1 283.7 Q835 1262.3 8.0 482.8 .5 59.9 5.6 53.1 It will be understood that when the radius of curvature is positive the centre of curvature lies on the image plane side of the refractive surface. Negative signs would indicate the opposite
EXAMPLE 2 relates to the embodiment of Fig. 2; lens elements A and B are each germanium and lens element C BS1 chalcogenide glass.The effective focal length is 1.00, the back focal length is 0.393, and the separation and radius of curvature of the refractive surfaces is given in the table reading from surface 18 in the direction of the image surface 12 (dimensions being normalized):
Separation Radius 0.5011 0.0400 0.6291 0.2538 0.1549 0.0150 0.1382 0.1000 -0.4366 0.0200 -0.5156 EXAMPLE 3 relates to the embodiment of Fig. 3; lens element A is germanium and lens elements
B and C are each BS1 chalcogenide glass.The effective focal length is normalised to 1.00, the back focal length is 0.3994 and the separation and radius of curvature of the refractive surfaces is given in the table reading from surface 18 in the direction of the image surface 12 (dimensions being normalised):
Separation Radius 0.5926 .0427 0.7692 0.3948 -3.2968 .0160 0.3825 .0287 0.6204 .0267 -1.5895 EXAMPLE 4 relates to the embodiment of Fig. 3; lens element A is germanium, lens element B is
BSA chalcogenide glass and lens element C is BS1 chalcogenide glass.The effective focal length is 375 mm, the back focal length is 138.178 mm and the separation and radius of curvature of the refractive surface is given in the table reading from surface 1 8 in the direction of the image surface 1 2 (dimensions in mm);
Separation Radius 195.33 15.65 268.99 113.37 825.5 6.05 109.22 10.98 199.06 10.00 -2583.9 The examples described above in common with previously known all-germanium designs can only be maintained in focus over a range of temperatures by appreciable axial movement of the primary or secondary lens.However a substantial improvement in thermal stability of image surface 12 can be
achieved in the embodiment of Fig. 3 by making lens element C of halide crystal material (which is a commercially available infrared transmitting material).rhis permits the lens elements to be fixedly
mounted and spaced apart with a material or normal thermal expansion. Thus, in examples 3 and 4
KRS5 material replaces BS1 material and because KRS5 is less dispersive than BS1 lens element B also
requires to have reduced dispersion and conveniently is BS2 chalcogenide glass.
A specific example, EXAMPLE 5, is now given, compensatal for dural spacers, the effective focal
length being normalised to 1.00, the back focal length being 0.4934 and the separation and radius of
curvature of the refractive surfaces is detailed in the table reading from surface 1 8 in the direction of the
focal surface 12 (normalised dimensions)::
Separation Radius - .5117 s0427 6424 3028 7.6224 .016 .3796 .0267 .5813 .0267 -1.2106 In each of the five examples given above the dimensions may be scaled within limits determined
by the diffraction limit for the 10 micron (um) wavelength and the homogeneity of the optical materials
The examples have been optimised for relative apertures between f/2 and f/3 and a field of view of
about 60 and are conveniently used with aperture diameters of up to 250 mm when performance is
close to diffraction limited.As is usual with lens design the relative aperture can be improved with the
use of an aspheric surface on the primary lens element A in which case a relative aperture exceeding
f/1.5 and aperture diameters up to 500 mm are feasible without performance degradation.
Characteristics of the germanium and halide crystal materials referred to herein are detailed in
Table 1. Table 2 details the corresponding characteristics of various chalcogenide glasses which are commercially available.
TABLE 1 - Characteristics of Optical Materials at 10 microns
Refractive Dispersion Thermal Material index coeff icient coefficient 4,0032 .00085 -.00474 2,3704 1 .00385 +.00621 2.1768 0.1054 +.00489* TABLE 2 - Characteristics of Chalcogenide glasses at 10 microns
Refractive - Dispersion Material index coefficient Thermal coefficient Tl 1173 2*6001 .00705 -.00171 TI 20 2A919 0.00696 -0.00174 AMTIR-1 2.4975 0.00592 0.00170 BSA 2.7792 .00479 1 -.00128 BS1 2.4916 .00660 -.00171 BS2 2.8563 .00404 - .00171 * Estimated values.
Claims (9)
1. A telescope objective system formed by a primary lens having a single lens element aligned on a common optical axis with a secondary lems having two lens elements the shape and power distribution of the lens elements being such as to compensate for monochromatic aberration of the system, each lens element being made of a material which has a useful spectral bandpass in the infrared wavelength region and the lens elements of the secondary lens having refractive surfaces intercepting said optical axis which are substantially spherical, the system having a planar image surface and an effective focal length greater than the axial distance between the image surface and the distal refractive surface, said distal refractive surface being formed by said primary lens element which is positively powered and made of germanium, at least one of the two lens elements of the secondary lens being made of a chalcogenide glass such that the system is achromatic in the infrared wavelength region.
2. A system as claimed in claim 1, wherein the middle one of said three lens elements is negatively powered and made of a chalcogenide glass.
3. A system as claimed in claim 1 or 2, wherein the lens element proximal said image surface is negatively powered and made of germanium.
4. A system as claimed in claim 1 or 2, wherein the lens element proximal said image surface is positively powered and made of a chalcogenide glass.
5. A system as claimed in claim 4 when appendant to claim 2, wherein the two chalcogenide glass lens elements each have the same material specification.
6. A system as claimed in claim 4 when appendant to claim 2, wherein the two chalcogenide glass lens elements have differing material specifications, the middle lens element having a lower dispersion coefficient than the lens element which is proximal said image surface.
7. A system as claimed in claiml or 2, wherein the lens element proximal said image surface is positively powered and made of halide crystal material such that the image surface is maintained stationary with temperature change.
8. A system as claimed in claim 7, wherein the halide crystal material is KRS5.
9. A system as claimed in claim 1, and substantially as hereinbefore described with reference to any one of the examples.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB8105761A GB2071353B (en) | 1980-03-05 | 1981-02-24 | Telescope objective system |
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB8007406 | 1980-03-05 | ||
| GB8105761A GB2071353B (en) | 1980-03-05 | 1981-02-24 | Telescope objective system |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB2071353A true GB2071353A (en) | 1981-09-16 |
| GB2071353B GB2071353B (en) | 1984-02-08 |
Family
ID=26274698
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB8105761A Expired GB2071353B (en) | 1980-03-05 | 1981-02-24 | Telescope objective system |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2071353B (en) |
Cited By (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2121211A (en) * | 1982-06-02 | 1983-12-14 | Barr & Stroud Ltd | Athermal infrared objective lens systems |
| GB2161616A (en) * | 1984-07-14 | 1986-01-15 | Pilkington Perkin Elmer Ltd | Optically athermal infra-red lenses |
| GB2194072A (en) * | 1986-04-03 | 1988-02-24 | Pilkington Perkin Elmer Ltd | Athermalised optical beam expander |
| FR2667695A1 (en) * | 1990-10-09 | 1992-04-10 | Thomson Trt Defense | OPTICAL ATHERMALIZATION OBJECTIVE SYSTEM. |
| EP0644445A2 (en) * | 1993-09-17 | 1995-03-22 | STEINHEIL OPTRONIK GmbH | Aspherical lens |
| EP3191810A4 (en) * | 2014-09-09 | 2018-03-21 | The Government of the United States of America as represented by the Secretary of the Navy | Multispectral imaging system comprising new multispectral optics |
-
1981
- 1981-02-24 GB GB8105761A patent/GB2071353B/en not_active Expired
Cited By (13)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2121211A (en) * | 1982-06-02 | 1983-12-14 | Barr & Stroud Ltd | Athermal infrared objective lens systems |
| GB2161616A (en) * | 1984-07-14 | 1986-01-15 | Pilkington Perkin Elmer Ltd | Optically athermal infra-red lenses |
| EP0171903A1 (en) * | 1984-07-14 | 1986-02-19 | Pilkington P.E. Limited | Improvements in or relating to infra-red lenses |
| GB2194072A (en) * | 1986-04-03 | 1988-02-24 | Pilkington Perkin Elmer Ltd | Athermalised optical beam expander |
| US4834472A (en) * | 1986-04-03 | 1989-05-30 | Pilkington P.E. Limited | Optical beam expanders with materials chosen to effect athermalization |
| GB2194072B (en) * | 1986-04-03 | 1990-03-21 | Pilkington Perkin Elmer Ltd | Athermalised optical beam expander |
| FR2667695A1 (en) * | 1990-10-09 | 1992-04-10 | Thomson Trt Defense | OPTICAL ATHERMALIZATION OBJECTIVE SYSTEM. |
| EP0480805A1 (en) * | 1990-10-09 | 1992-04-15 | Thomson-Trt Defense | Objective systems with optical temperature compensation |
| US5202792A (en) * | 1990-10-09 | 1993-04-13 | Thomson Trt Defense | Systems of objectives with optical athermalization |
| EP0644445A2 (en) * | 1993-09-17 | 1995-03-22 | STEINHEIL OPTRONIK GmbH | Aspherical lens |
| EP0644445A3 (en) * | 1993-09-17 | 1995-08-16 | Steinheil Optronik Gmbh | Aspherical lens. |
| US5668671A (en) * | 1993-09-17 | 1997-09-16 | British Aerospace Public Limited Co. | Dioptric lens system |
| EP3191810A4 (en) * | 2014-09-09 | 2018-03-21 | The Government of the United States of America as represented by the Secretary of the Navy | Multispectral imaging system comprising new multispectral optics |
Also Published As
| Publication number | Publication date |
|---|---|
| GB2071353B (en) | 1984-02-08 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| PCNP | Patent ceased through non-payment of renewal fee |
Effective date: 20000224 |