High magnification MWIR continuous zoom system

Abstract

A high magnification MWIR continuous zoom optical system is described herein that consists of the following components: a front detachable extender group, a fixed group for focusing incoming radiation, three moving groups for zooming and generating an intermediate image and a relay group. The mentioned optical system has the ability to work with MWIR radiation (3-5 μm) and generate a thermal image from the gathered radiation. The system also has the ability to zoom continuously in a wide variable focal length range with a high magnification ratio of 20×. With the use of a cooled detector, the combined system allows its user to be able to receive high quality thermal images in all FOV configurations.

Claims

1. A high magnification Mid-wave infrared (MWIR) continuous zoom optical system consisting of two parts with 6 main groups with the following arrangement from the object plane to the image plane, therein: a front part includes: an extender group (G1) which comprises 3 optical elements in a negative-positive-negative (N-P-N) power configuration, and that expands a diameter of the incoming radiation and a variable focal length range of the optical system; a rear part includes: a fixed group (G2) which comprises 2 optical elements, one of which is made from Silicon, has positive power and two spherical surfaces with a convex one heading towards an object plane while the other is made from Germanium, has negative power, a spherical surface and an aspheric surface; a first moving group (G3) which moves for zooming, and that comprises a single Germanium element having negative power, a concave aspheric surface and a concave spherical surface; a second moving group (G4) which moves for zooming, and that comprises a single Zinc Selenide element having positive power and two convex aspheric surfaces; a third moving group (G5) which moves for zooming, and that comprises a single Germanium element having positive power, an aspheric surface and a hybrid aspheric-diffractive surface; a relay group (G6) which comprises 3 optical elements in a positive-negative-positive (P-N-P) power configuration, and that is optimized in order to be movable within a small range of ±0.5 mm.

2. The high magnification MWIR continuous zoom optical system of claim 1, wherein the three optical elements which belong to the extender group also have these following properties: a first front element is made from Germanium, has negative power, a convex spherical surface and a concave aspheric surface, with a convex surface heading towards the object plane; a following second element is made from Silicon, has positive power and two spherical surfaces; a following third element is made from Silicon, has negative power and two spherical surfaces.

3. The high magnification MWIR continuous zoom optical system of claim 2, wherein the three optical elements which belong to the relay group also have these following properties: the first front element is made from Germanium, has positive power, a convex aspheric surface and a concave spherical surface; the following second element is made from Germanium, has negative power, an aspheric surface and a hybrid aspheric-diffractive surface; the following third element is made from Silicon, has positive power, a concave spherical surface and a convex spherical surface with the convex surface heading towards the image plane.

4. The high magnification MWIR continuous zoom optical system of claim 3, wherein the simultaneous changes in position of (G3), (G4) and (G5) leads to change of the system focal length, with some following limits: the optical system achieves its minimum focal length value when (G3) and (G4) are at most distant positions from each other and (G5) is at a closest position to (G4); the optical system achieves its maximum focal length value when (G3) and (G4) are at closest positions to each other and (G5) is at a most distant position from (G4).

5. The high magnification MWIR continuous zoom optical system of claim 3, wherein the relay group is responsible for magnifying an intermediate image and focusing that image onto a detector image plane, while its movement within a small range of ±0.5 mm helps maintaining the focus property of incoming radiation at different temperatures and object distances.

6. The high magnification MWIR continuous zoom optical system of claim 2, wherein the simultaneous changes in position of (G3), (G4) and (G5) leads to change of the system focal length, with some following limits: the optical system achieves its minimum focal length value when (G3) and (G4) are at most distant positions from each other and (G5) is at a closest position to (G4); the optical system achieves its maximum focal length value when (G3) and (G4) are at closest positions to each other and (G5) is at a most distant position from (G4).

7. The high magnification MWIR continuous zoom optical system of claim 6, wherein the relay group is responsible for magnifying an intermediate image and focusing that image onto a detector image plane, while its movement within a small range of ±0.5 mm helps maintaining the focus property of incoming radiation at different temperatures and object distances.

8. The high magnification MWIR continuous zoom optical system of claim 2, wherein the relay group is responsible for magnifying an intermediate image and focusing that image onto a detector image plane, while its movement within a small range of ±0.5 mm helps maintaining the focus property of incoming radiation at different temperatures and object distances.

9. The high magnification MWIR continuous zoom optical system of claim 1, wherein when the extender group is detached, the remaining part of the system still maintains good optical quality and operates as an independent high magnification middle wave infrared continuous zoom lens.

10. The high magnification MWIR continuous zoom optical system of claim 9, wherein the simultaneous changes in position of (G3), (G4) and (G5) leads to change of the system focal length, with some following limits: the optical system achieves its minimum focal length value when (G3) and (G4) are at most distant positions from each other and (G5) is at a closest position to (G4); the optical system achieves its maximum focal length value when (G3) and (G4) are at closest positions to each other and (G5) is at a most distant position from (G4).

11. The high magnification MWIR continuous zoom optical system of claim 10, wherein the relay group is responsible for magnifying an intermediate image and focusing that image onto a detector image plane, while its movement within a small range of ±0.5 mm helps maintaining the focus property of incoming radiation at different temperatures and object distances.

12. The high magnification MWIR continuous zoom optical system of claim 9, wherein the relay group is responsible for magnifying an intermediate image and focusing that image onto a detector image plane, while its movement within a small range of ±0.5 mm helps maintaining the focus property of incoming radiation at different temperatures and object distances.

13. The high magnification MWIR continuous zoom optical system of claim 1, wherein the three optical elements which belong to the relay group also have these following properties: a first front element is made from Germanium, has positive power, a convex aspheric surface and a concave spherical surface; a second following element is made from Germanium, has negative power, an aspheric surface and a hybrid aspheric-diffractive surface; a third following element is made from Silicon, has positive power, a concave spherical surface and a convex spherical surface with a convex surface heading towards the image plane.

14. The high magnification MWIR continuous zoom optical system of claim 13, wherein the simultaneous changes in position of (G3), (G4) and (G5) leads to change of the system focal length, with some following limits: the optical system achieves its minimum focal length value when (G3) and (G4) are at most distant positions from each other and (G5) is at a closest position to (G4); the optical system achieves its maximum focal length value when (G3) and (G4) are at closest positions to each other and (G5) is at a most distant position from (G4).

15. The high magnification MWIR continuous zoom optical system of claim 14, wherein the relay group is responsible for magnifying an intermediate image and focusing that image onto a detector image plane, while its movement within a small range of ±0.5 mm helps maintaining the focus property of incoming radiation at different temperatures and object distances.

16. The high magnification MWIR continuous zoom optical system of claim 13, wherein the relay group is responsible for magnifying an intermediate image and focusing that image onto a detector image plane, while its movement within a small range of ±0.5 mm helps maintaining the focus property of incoming radiation at different temperatures and object distances.

17. The high magnification MWIR continuous zoom optical system of claim 1, wherein the simultaneous changes in position of (G3), (G4) and (G5) leads to change of the system focal length, with some following limits: the optical system achieves its minimum focal length value when (G3) and (G4) are at most distant positions from each other and (G5) is at a closest position to (G4); the optical system achieves its maximum focal length value when (G3) and (G4) are at closest positions to each other and (G5) is at a most distant position from (G4).

18. The high magnification MWIR continuous zoom optical system of claim 1, wherein the relay group is responsible for magnifying an intermediate image and focusing that image onto a detector image plane, while its movement within a small range of ±0.5 mm helps maintaining the focus property of incoming radiation at different temperatures and object distances.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates the general structure of the optical system of the present invention;

(2) FIG. 2 illustrates the structure of the optical system at NFOV configuration;

(3) FIG. 3 illustrates the spot diagram on the detector image plane at NFOV configuration;

(4) FIG. 4 illustrates the MTF diagram of the optical system at NFOV configuration;

(5) FIG. 5 illustrates the structure of the optical system at MFOV configuration;

(6) FIG. 6 illustrates the spot diagram on the detector image plane at MFOV configuration;

(7) FIG. 7 illustrates the MTF diagram of the optical system at MFOV configuration;

(8) FIG. 8 illustrates the structure of the optical system at WFOV configuration;

(9) FIG. 9 illustrates the spot diagram on the detector image plane at WFOV configuration;

(10) FIG. 10 illustrates the MTF diagram of the optical system at WFOV configuration;

(11) FIG. 11 illustrates the Seidel diagram which shows the distribution of optical aberrations at each optical surface of lenses in the fix group (G2).

(12) FIG. 12 illustrates the structure of the optical system at NFOV configuration when the extender is detached;

(13) FIG. 13 illustrates the spot diagram on the detector image plane at NFOV configuration when the extender is detached;

(14) FIG. 14 illustrates the MTF diagram of the optical system at NFOV configuration when the extender is detached;

(15) FIG. 15 illustrates the structure of the optical system at MFOV configuration when the extender is detached;

(16) FIG. 16 illustrates the spot diagram on the detector image plane at MFOV configuration when the extender is detached;

(17) FIG. 17 illustrates the MTF diagram of the optical system at MFOV configuration when the extender is detached;

(18) FIG. 18 illustrates the structure of the optical system at WFOV configuration when the extender is detached;

(19) FIG. 19 illustrates the spot diagram on the detector image plane at WFOV configuration when the extender is detached; and

(20) FIG. 20 illustrates the MTF diagram of the optical system at WFOV configuration when the extender is detached;

DETAILED DESCRIPTION

(21) Referring to FIG. 1, the high magnification MWIR continuous zoom optical system consists of two parts with 6 main groups which follow this particular arrangement from the object plane to the final image plane:

(22) the extender group (G1), also the closest group to the object plane;

(23) the fixed group (G2);

(24) the first moving group (G3);

(25) the second moving group (G4);

(26) the third moving group (G5);

(27) the fourth moving group, or the relay group (G6), which stands right in front of the image plane (FPA);

(28) Referring to FIG. 1 and FIG. 2, the extender group (G1) comprises three optical elements, with the first front element (L1) made from Germanium, having negative power, a convex spherical surface (S1) heading towards the object plane and a concave spherical surface (S2). The following second element (L2) is made from Silicon, has positive power and two spherical surfaces, which are (S3) and (S4). The following third element (L3) is made from Silicon, has negative power and also two spherical surfaces, which are (S5) and (S6). Overall, the extender group has an N-P-N power configuration and is responsible for the expansion of the diameter of the incoming radiation and also the expansion of the variable focal length range of the optical system. To be specific, according to the proposed structure, the extender group might be able to expand the diameter of the incoming radiation from 1.4× up to 2.5× of the original system (without the extender group), which translates to the variable focal length range to be expanded from at least 21-420 mm up to 37.5-750 mm.

(29) Referring to FIG. 1 and FIG. 2, the fixed group (G2) comprises two optical elements. The first front element (L4) is made from Silicon, has positive power and two spherical surfaces, which are (S7) and (S8), with the convex surface (S7) heading towards the object plane. The second following element (L5) is made from Germanium, has negative power, a spherical surface (S9) and an aspheric surface (S10).

(30) The diameter value of the first front element (L4) in the fixed group (G2) is equivalent to the entrance pupil diameter of the optical system at NFOV configuration.

(31) Referring to FIG. 1 and FIG. 2, (G3), (G4) and (G5) are moving zoom groups that can move simultaneously within the optical system parameter in order to change the focal length of the system and generate an intermediate image plane. Group (G3) comprises one single Germanium element (L6) that has negative power, a concave aspheric surface (S11) and a concave spherical surface (S12). Group (G4) comprises one single Zinc Selenide element (L7) that has two convex aspheric surfaces, which are (S13) and (S14). Group (G5) comprises one single Germanium element (L8) that has positive power, an aspheric surface (S15) and a hybrid aspheric-diffractive surface (S16) for optimization of optical aberrations of the optical system, especially chromatic aberration.

(32) Referring to FIG. 1 and FIG. 2, a fourth moving group or relay group (G6) comprises three optical elements with a P-N-P power configuration. The first positive front element (L9) is made from Germanium, has a convex aspheric surface (S17) and a concave spherical surface (S18). The second negative following element (L10) is also made from Germanium, has an aspheric surface (S19) and a hybrid aspheric-diffractive surface (S20). The third positive following element (L11) is made from Silicon, has a concave surface (S21) and a convex surface (S22) which heads towards the final image plane. According to the proposed structure, group (G6) is designed so that it can move within a small range of ±0.5 mm for the ability to work in a wide range of temperature (−20° C.˜60° C.) and having a minimum focus range of 10 m at WFOV configuration and 100 m at NFOV configuration.

(33) According to the proposed structure and referring to FIG. 3, FIG. 4, FIG. 6 and

(34) FIG. 7, FIG. 9, FIG. 10, the optical system is optimized in order to gather MWIR radiation (3-5 μm) coming from objects that lie within the FOV of the system and focus the radiation onto the detector image plane, which gives crisp images of the mentioned objects.

(35) According to the proposed structure, the optical system is optimized so that it can have the ability to zoom continuously within a magnification ratio of 20× while maintaining the high quality of the final image in every configurations.

(36) Referring to FIG. 2, FIG. 5 and FIG. 8, the simultaneous change in positions of (G3), (G4) and (G5) groups along the optical axis leads to the change in the focal length of the optical system. For example, when the extender group is added to the optical system making the focal length range expand by 1.4× while (G3) and (G4) are at the most distant positions from each other and (G5) is at the closest position to (G4), the optical system is at its smallest focal length value, which is 21 mm and corresponds to a wide FOV of 38.34°. In return, when (G3) and (G4) are at the closest positions to each other and (G5) is at the most distant position from (G4), the optical system is at its largest focal length value, which is 420 mm and corresponds to a narrow FOV of 1.67°.

(37) Referring to FIG. 11, the material combination between a positive Silicon element (L4) and a negative Germanium element (L5), together with an aspheric surface (S10) allow the fixed group (G2) to effectively balance a number of optical aberrations including spherical aberration, coma, and astigmatism.

(38) Referring to figures from FIG. 12 to FIG. 20, when the extender is detached, the remaining part of the system still maintains good optical quality and operates as an independent high magnification middle wave infrared continuous zoom lens.

(39) The invention of a high magnification MWIR continuous zoom optical system is designed in a way that must satisfy these conditions from its original system (without the extender group):

(40) $\hspace{1em}\{\begin{array}{c}50\le f_1\le 120\\ -60\le f_2\le -10\\ 15\le f_3\le 60\\ 100\le f_4\le 300\end{array}$

(41) where:

(42) f.sub.1 (mm) is the focal length of the fixed group (G2);

(43) f.sub.2 (mm) is the focal length of the first moving group (G3);

(44) f.sub.3 (mm) is the focal length of the second moving group (G4);

(45) f.sub.4 (mm) is the focal length of the third moving group (G5);

(46) Furthermore, according to the present invention, the sag values of every non-spherical surfaces in the optical system are defined by the following equation:

(47) $z=\frac{\frac{1}{R}{y}^2}{1+\sqrt{1-\left(1+k\right)\frac{1}{R^2}{y}^2}}+\underset{i=2}{\overset{n}{.Math.}}{A}_{2i}{y}^{2i}$

(48) where:

(49) R—Surface radius of curvature;

(50) y—Aperture height, measured perpendicular to optical axis;

(51) k—Conic coefficient;

(52) A—Even aspheric coefficients (A.sub.4, A.sub.6, A.sub.8, A.sub.10, etc.);

(53) Table 1 below presents the aspheric data of non-spherical surfaces included in the proposed optical system with the X, Y and Z values must stay within the following ranges:

(54) $\hspace{1em}\{\begin{array}{c}3\times 10^{-7}\le X\le 3\times 10^{-5}\\ 5\times 10^{-12}\le Y\le 5\times 10^{-11}\\ -1\times 10^{-5}\le Z\le -1\times 10^{-7}\end{array}$

(55) TABLE-US-00001 TABLE 1 Surface Conic (k) A.sub.4 A.sub.6 A.sub.8 A.sub.10 2 0.11 −8.19E−09 −5.88E−13 −9.52E−17  0.00E+00 10 0  3.38E−08  2.95E−12 2.03E−15 0.00E+00 11 0 X −6.57E−09 5.50E−11 −1.65E−13  13 0 −3.26E−06 −6.10E−09 Y 6.96E−14 14 0  2.57E−06 −9.03E−09 1.24E−11 6.29E−14 15 0 −1.29E−05 −6.19E−07 0.00E+00 0.00E+00 16 0 −2.06E−05 Z 2.31E−09 3.12E−11 17 0 −6.84E−05  3.03E−06 −1.75E−07  2.64E−09 19 0 −8.96E−04 −4.51E−06 2.44E−07 1.95E−08 20 0  2.79E−06 6.38E−08 4.69E−10

(56) Meanwhile, the diffractive surfaces used in the optical system are defined by the following equation:

(57) $\Phi =M\underset{i=1}{\overset{n}{.Math.}}{A}_i{\rho }^{2i}$

(58) where:

(59) A—Diffractive coefficients (A.sub.1, A.sub.2, etc.);

(60) ρ—Normalized radial coordinate;

(61) Φ—Added phase at coordinate ρ;

(62) Table 2 presents diffractive data (in phase) of all diffractive surfaces used in the proposed optical system.

(63) TABLE-US-00002 TABLE 2 Surface A.sub.1 A.sub.2 16 −55.075 4.74 20 −40.12 4.064

(64) According to the present invention, the following Table 3 will present the prescription data (in millimeters) and materials that could be used to manufacture a high magnification MWIR continuous zoom optical system. The example is illustrative only and does not impose any limitations on the scope of the present invention. For a MWIR continuous zoom optical system with a focal length range of 21-420 mm and an aperture of F/#4.0, the specific parameters of every elements in the optical system could be as follow:

(65) TABLE-US-00003 TABLE 3 Radius of Surface Surface type curvature Thickness Material 1 Spherical 130.04 8.60 GERMANIUM 2 Aspheric 112.43 3.37 3 Spherical 116.22 10.70  SILICON 4 Spherical 257.47 16.38  5 Spherical 247.24 7.90 SILICON 6 Spherical 104.58 10.00  7 Spherical 92.22 9.00 SILICON 8 Spherical 277.75 2.45 9 Spherical 238.67 3.50 GERMANIUM 10 Aspheric 145.83 V1 11 Aspheric −135.54 2.00 GERMANIUM 12 Spherical 56.54 V2 13 Aspheric 63.64 5.00 ZNSE 14 Aspheric −55.46 V3 15 Aspheric 12.80 2.30 GERMANIUM 16 Diffractive 11.19 7.00 17 Aspheric 13.00 2.20 GERMANIUM 18 Spherical 15.05 3.30 19 Aspheric −8.00 2.50 GERMANIUM 20 Diffractive −11.06 0.20 21 Spherical −30.56 4.00 SILICON 22 Spherical −12.74 5.51

(66) where V1, V2 and V3 are the distances between moving groups or between a moving group and a fixed one. These values will change when the moving groups change their positions along pre-defined routes, called CAM curves. Table 4 presents V1, V2 and V3 values in some FOV configurations.

(67) TABLE-US-00004 TABLE 4 Value (mm) Wide FOV Middle FOV Narrow FOV V1 11.50 48.79 53.00 V2 66.598 16.84 6.00 V3 9.50 23.42 37.56

(68) In the present invention of a high magnification MWIR continuous zoom optical system, the authors made use of previous knowledge and experience in optical design, Gaussian optics analysis and Seidel forms of optical aberrations in order to build a standard design process that could be used to search for high quality on-axis starting points. Also in the present invention, the authors noticed and analyzed the intensity of the Narcissus effect right from the design stage of the optical system and also made use of different optimization techniques so that the intensity of this particular effect was reduced. A light and compact structure was also one of the important factors which the present invention achieved by using the extender—zoom—relay structure, with the extender group allowed the original system to achieve higher focal length value while also being detachable if not needed and the remaining components could work as an independent continuous zoom system. Meanwhile, the relay group helped optimizing the size of the optical elements and focus all infrared radiation onto the detector image plane with 100% cold stop efficiency. All elements within the optical system had reasonable sizes and were made from common infrared materials including Germanium, Zinc Selenide and Silicon which helped driving down the overall cost of the system as a whole. Also in the present invention, in order to achieve a magnification ratio of 20λ, the three moving groups must be moved simultaneously in order to change the focal length of the system while maintaining the focus property on the image plane at all time. Besides, another fourth moving group was also used in order to maintain optical quality in a wide range of temperature (from −20° C. to 60° C.) and a wide observation range (for objects from 10 m away from the camera up to 20 km). The proposed high magnification MWIR continuous zoom optical system is compatible with F/#4.0 detectors that has a focal plane array (FPA) of 640×512 pixels with 15 μm pixel pitch or a 320×240-pixel FPA with 30 μm pixel pitch.

(69) Although the structure of the optical system in the present invention has been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it should be noted that the invention is not limited to the described optical system, but is capable of different rearrangements, modifications or substitutions without departing from the invention as set forth and defined by the following claims.