Login| Sign Up| Help| Contact|

Patent Searching and Data


Title:
OPTICAL SYSTEM AND INFRARED CAMERA WITH A NON-UNIFORMITY CORRECTION SHUTTER INTEGRATED WITHIN THE OPTICAL SYSTEM
Document Type and Number:
WIPO Patent Application WO/2015/136525
Kind Code:
A1
Abstract:
Optical systems and respective cameras are provided, which have a NUC (non-uniformity correction) -shutter incorporated internally in the optical system, between its optical elements. The NUC-shutter is configured to be temporarily inserted between two of the optical elements to enable performing NUC of a detector array in the camera. Camera designs, in particular infrared camera designs, achieve wide fields of view and very low distortions by exploiting the possibility to minimize the back focal length of the optical system which is enabled by the configuration of the NUC-shutter.

Inventors:
ABURMAD SHIMON (IL)
GRIMBERG ERNEST (IL)
YANAI OMER (IL)
SEGEV YARON (IL)
Application Number:
PCT/IL2015/050243
Publication Date:
September 17, 2015
Filing Date:
March 09, 2015
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
OPGAL OPTRONIC IND LTD (IL)
International Classes:
H04N5/33; G01J5/08
Domestic Patent References:
WO2013055274A22013-04-18
Foreign References:
CN202928696U2013-05-08
US20130301119A12013-11-14
Attorney, Agent or Firm:
TAL, Ophir et al. (P.O. Box 12704, 49 Herzliya, IL)
Download PDF:
Claims:
CLAIMS

What is claimed is:

1. An optical system comprising a plurality of optical elements and a NUC (non- uniformity correction) -shutter, the NUC-shutter configured to be temporarily inserted between two of the optical elements to enable performing NUC of a detector array associated with the optical system in a camera.

2. The optical system of claim 1, further configured to have a Back Focal Length (BFL) smaller than twice a width of an external NUC shutter assembly.

3. The optical system of claim 1, further configured to have a Back Focal Length (BFL) smaller than 7mm.

4. The optical system of claim 1, further configured to have a wide field of view (WFOV).

5. The optical system of claim 1, further configured to have a field of view of at least 110°.

6. The optical system of claim 1, wherein the NUC-shutter is positioned behind a front optical element.

7. The optical system of claim 1, wherein the NUC-shutter is positioned adjacent to an intermediate optical element in the optical system.

8. The optical system of claim 1, wherein the NUC-shutter is positioned before a rear optical element.

9. A camera comprising a detector array, at least partly operable in the infrared, and the optical system of any one of claims 1-8.

10. An infrared camera comprising an infrared detector array, a plurality of optical elements and a shutter configured to carry out non-uniformity correction of said detector array by temporality being inserted between two of said plurality of optical elements.

11. A method comprising configuring an optical system to enable performing NUC of a detector array associated with the optical system in a camera, by positioning a NUC- shutter between optical elements of the optical system.

12. The method of claim 11, further comprising configuring the optical system to have a Back Focal Length (BFL) smaller than twice a width of an external NUC shutter assembly or smaller than 7mm.

13. The method of claim 11, further comprising configuring the optical system to have a field of view of at least 110°.

14. The method of claim 11, further comprising positioning the NUC-shutter behind a front optical element in the optical system.

15. The method of claim 11, further comprising positioning the NUC-shutter adjacent to an intermediate optical element in the optical system.

16. The method of claim 11, further comprising positioning the NUC-shutter before a rear optical element in the optical system.

7. The method of claim 11, further comprising setting the optical system in a camera having a detector array which is at least partly operable in the infrared.

Description:
OPTICAL SYSTEM AND INFRARED CAMERA WITH A NON-UNIFORMITY CORRECTION SHUTTER INTEGRATED WITHIN THE OPTICAL SYSTEM

BACKGROUND OF THE INVENTION

1. TECHNICAL FIELD

[0001] The present invention relates to the field of optical systems, and more particularly, to non-uniformity corrections (NUC) thereof.

2. DISCUSSION OF RELATED ART

[0002] Infrared cameras collect radiation from the outside world and turn it into an image. The two most important components in an infrared imaging camera are the detector and the lens. Modern infrared detectors contain arrays of infrared sensitive pixels. The complex design and manufacturing processes of the detector introduce an inherent Non-Uniformity between elements of the FPA (Focal Plane Array). These differences are affecting the pixel's well capacity, responsiveness and spectral response. These non-uniformity factors have a crucial influence on the image quality of infra-red thermal cameras. A lot of effort has been invested in solving the non-uniformity problem and a very common approach is based on measuring and correcting the detector's non- uniformity by temporarily inserting a shutter between the lens and the detector that is imaged as an ideal black body and serves as a reference for applying a Non-Uniformity Correction (NUC).

[0003] Since uncooled detectors typically do not contain radiometric shields or spectral band filters, the spectral band and F number of uncooled thermal cameras are defined by the (lens) optics properties. Performing non-uniformity correction (NUC) based on a shutter which is inserted between the detector and the optics is not an optimal method, as it is usually performed on different F number and spectral band than the ones actually in use by the camera, and it ignores the radiation emitted by the lens itself. Moreover, the radiation emitted by the lens itself is not homogenously distributed on the detector face thus becoming an additional cause of non-uniformity.

[0004] Another important factor that is impacting image quahty is the optics' design. In order to mechanically insert a shutter between the lens and the FPA camera, designers are forced to use lenses with a large BFL (Back Focal Length). However, increasing the BFL degrades the MTF (Modulation Transfer Function) and the Distortion of the lens. Furthermore, there are additional trade-offs between several optical parameters. For example, when designing long focal length optics, large BFL is easier to achieve. However, large BFL for short focal length lens is almost impossible to achieve using a small number of optical elements, and increasing the number of optical elements causes the optics' transmittance and thus the performance of the entire assembly to decrease drastically. Furthermore, due to the high cost of optical elements in the infrared world, one of the important goals for the optics' designer is to decrease the number of optical elements to a minimum.

SUMMARY OF THE INVENTION

[0005] One aspect of the present invention provides an optical system comprising a plurality of optical elements and a NUC (non-uniformity correction) -shutter, the NUC- shutter configured to be temporarily inserted between two of the optical elements to enable performing NUC of a detector array associated with the optical system in a camera.

[0006] These, additional, and/or other aspects and/or advantages of the present invention are set forth in the detailed description which follows; possibly inferable from the detailed description; and/or learnable by practice of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] For a better understanding of embodiments of the invention and to show how the same may be carried into effect, reference will now be made, purely by way of example, to the accompanying drawings in which like numerals designate corresponding elements or sections throughout.

[0008] In the accompanying drawings:

[0009] Figure 1 is a high level schematic illustration of an optical system in a camera, according to some embodiments of the invention. [0010] Figure 2 is a high level schematic illustration of an exemplary non-limiting optical design of the optical system, with beam tracing, within the camera, according to some embodiments of the invention.

[0011] Figures 3A and 3B schematically illustrate an exemplary optical system constructed according to the disclosed principles, in cross section and in perspective view, respectively, according to some embodiments of the invention.

[0012] Figures 4A and 4B present comparative experimental results for a prior art system and a system according to some embodiments of the invention, respectively, after application of NUC.

[0013] Figure 5 is a high level flowchart illustrating a method, according to some embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0014] With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.

[0015] Before at least one embodiment of the invention is explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is applicable to other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting.

[0016] The present invention, in embodiments thereof, enables high MTF (Modulation Transfer Function) and low distortion on one hand, and very short on the other hand. According to embodiments of the present invention, a shutter is inserted between the optical elements to create an integrated shutter within the optical system. The shutter itself is inserted temporarily using a relevant motion mechanism to allow, when inserted, non-uniformity measurement and correction (while not affecting the optical performance when not inserted).

[0017] A numerical non-limiting example, it is noted that that for typical infrared pixel size of about 17 μηι, and a typical FPA (Focal Plane Array) size of about 8.16x10.88 mm (for a 640x480 format) the distance between the detector's external surface and the FPA face plate is about 3 mm. Designing a WFOV (Wide Field Of View) optical system implies a very short focal length (e.g., around 3 mm). In such case, the BFL (Back Focal Length) of the optical system needs to be as short as the focal length. Considering the short BFL and the detector window-to-FPA distance, there is not enough space left for inserting NUC-shutter assemblies, which are several mm wide. As increasing the BFL results in a dramatic decrease in the MTF and as improving the MTF comes at the expense of distortions, current WFOV cameras inherently have to compromise at least one of their parameters.

[0018] It is noted that several solutions in the prior art attempt to overcome this necessity by performing NUC (Non-Uniformity Corrections) without inserting a shutter between the detector and the lens are by either placing a shutter (or a cap) in front of the front lens, or by using a de-focused position of the lens. The former approach enables good imaging performance but imposes mechanical difficulties which impact the overall system cost, reliability, size, and weight; while the latter approach requires a motorized lens which is obviously more expensive and less reliable than a fixed FOV lens.

[0019] The disclosed invention solves the problem differently, by configurin the NUC- shutter to be temporarily inserted between two of the optical elements within the optical system, to enable performing NUC of the detector array while maintaining a short BFL and avoiding accessorial elements. The inventors have found out that this approach is an advantageous solution for a wider range of optical system, beyond WFOV systems with short BFL.

[0020] Optical systems and respective cameras are provided, which have a NUC (non- uniformity correction) -shutter incorporated internally in the optical system, between its optical elements. The NUC-shutter is configured to be temporarily inserted between two of the optical elements to enable performing NUC of a detector array in the camera. Camera designs, in particular infrared camera designs, achieve wide fields of view and very low distortions by exploiting the possibility to minimize the back focal length of the optical system which is enabled by the configuration of the NUC-shutter.

[0021] Figure 1 is a high level schematic illustration of an optical system 101 in a camera 100, according to some embodiments of the invention. Optical system 101 comprises optical elements, e.g., a front optical element(s) 91, a rear optical element(s) 93 and intermediate optical element(s) 92, as well as a NUC-shutter 110 within optical system 101, optionally positioned either behind front optical element(s) 91 at position 110(A) or before rear optical element(s) 93 at position 110(B). In certain embodiments, NUC-shutter 110 may be positioned adjacent to intermediate optical element(s) 92 in optical system 101. Optical system 101 may be part of camera 100, operable at least partly in the infrared range and further comprising detector array 80.

[0022] In certain embodiments, optical system 101 is characterized by providing a wider field of view (FOV) 85 alongside a low distortion at detector array 80, thus possibly having a small BFL (Back Focal Length) 95, e.g., 120° FOV (+10° or larger than 110°), distortion smaller than 30% or even smaller than 10% or smaller than 5%, and corresponding BFL smaller than 7mm, 6mm or even 3mm. BFL 95 compared with typical NUC-shutter 110 assembly widths of several nun's (schematically represented by width w 96 in Figure 1), prohibits positioning NUC shutter between rear optical element(s) 93 and detector array 80, i.e., externally to optical system 101. Hence, disclosed solutions of integrating NUC-shutter 110 within optical system 101 enable performing NUC even for cameras 100 with small BFL 95 and without the complexity involved in positioning NUC-shutter 110 in front of front optical element(s) 91, i.e., externally to optical system 101, where its size and mass impose mechanical and operational difficulties.

[0023] Figure 2 is a high level schematic illustration of an exemplary non-limiting optical design of system 101 with beam tracing within camera 100, according to some embodiments of the invention. Figure 2 schematically illustrates front optical element(s) 91, intermediate optical element(s) 92, rear optical element(s) 93, and detector front optical element(s) 94 with detector array 80 having short BEL 95 between rear optical element(s) 93 and detector array 80. Figure 2 further schematically illustrates NUC- shutter 110, which is positioned in the illustrated case positioned behind front optical element 91 and adjacent to intermediate optical element 92. Figure 2 illustrates the advantage of having NUC-shutter 110 placed within optical system 101, in that it enables to carry out the NUC beyond optical elements 92, 93, 94 resulting in more accurate NUC, while maintaining small BFL 95 as well as small dimension and mechanical complexity of NUC shutter 110 itself, in comparison to prior art embodiments placing it in front of front optical element(s) 91 (requiring a much larger and more cumbersome NUC shutter and operation).

[0024] Optical system 101 illustrated in Figure 2 as a non-limiting example has an optical design of a Wide FOV (120°) with a short BFL (3mm), a design which maintains excellent MTF and low distortion, while still allowing non-uniformity correction using an integrated shutter, but placing NUC shutter 110 within the optical elements of optical system 101. When NUC shutter 110 is inserted between elements 91, 92, as illustrated e.g., in Figures 1 and 2, not only does the NUC process take into account the optics, but the performance is very high, and the diameter of element 92 is much smaller than the dimensions of the FPA thus improving size and weight attributes. It should be noted that embodiments of the invention relate to positioning shutter 110 between any two optical elements instead of before or after the entire optics 101. The dotted rectangles in Figure 1 suggest two options of many design alternatives in a schematically illustrated case having three optical elements.

[0025] In addition to the above, placing the mechanical NUC shutter 110 inside optical assembly 101 presents a much more sturdy design and minimizes the vulnerability of the shutter assembly to harsh environments conditions (which are sometimes applicable when placing the shutter in front of the entire lens assembly).

[0026] Preliminary testing of the non-uniformity correction process using the proposed integrated shutter approach in comparison to the conventional concept having the shutter placed between detector array 80 and optical system 101 shows a . significant improvement. The testing was performed using a uniform extended area blackbody source. [0027] Certain embodiments comprise infrared camera 100 comprising infrared detector array 80, optical elements 91, 92, 93, 94 etc. and shutter 110 configured to carry out non- uniformity correction of detector array 80 by temporality being inserted between two of optical elements 91, 92, 93, 94 etc.

[0028] Figures 3A and 3B schematically illustrate an exemplary optical system constructed according to the disclosed principles, in cross section and in perspective view, respectively, according to some embodiments of the invention. In the cross sectional view presented in Figure 3A, shutter 110 with shutter assembly 111 is positioned between front optical element 91 and intermediate optical element 92 and occupies width w 96. The perspective view in Figure 3B illustrates two alternative shutter positions - HOC with shutter outside the optical path during operation of camera 100 and HOD with shutter 110 inserted into the optical path for performing NUC correction. Shutter assembly 111 controls the motion of shutter 110.

[0029] Figures 4A and 4B present comparative experimental results for a prior art system (Figure 4A) and a system according to some embodiments of the invention (Figure 4B), after application of NUC. The results are depicted as histograms showing the pixel count at each grey level after the non-uniformity correction (NUC). In Figure 4B the spread of the histogram is significantly smaller (ca. 25 grey levels in Figure 4B versus ca. 140 grey levels in prior art Figure 4A) and the overall histogram shape lends itself better for statistical analysis (exhibiting a single peak in Figure 4B versus multiple peaks in prior art Figure 4A). It is noted that the Figure 4B is much closer than Figure 4A to the histogram for an ideal black body, namely having all pixels at a single grey level. The experimental results illustrate hence the higher efficiency of disclosed configurations of optical system 101 in camera 100.

[0030] Figure 5 is a high level flowchart illustrating a method 200, according to some embodiments of the invention. Method 200 comprises configuring an optical system to enable performing NUC of a detector array associated therewith (stage 205) by positioning a NUC-shutter between optical elements of the optical system (stage 210). For example, method 200 may comprise positioning the NUC-shutter behind a front optical element (stage 212), positioning the NUC-shutter adjacent to an intermediate optical element (stage 214) and/or positioning the NUC-shutter before a rear optical element (stage 216).

[0031] Method 200 may further comprise setting the optical system in a camera with the detector array (stage 220). Method 200 may comprise operating the optical system and/or the camera at least partially in infrared (stage 230). In certain embodiments, method 200 may comprise configuring the optical system to have a small Back Focal Length (stage 240) and/or configuring the optical system to have a wide field of view and the detector to exhibit a very low distortion (stage 250).

[0032] It is noted that the disclosed optical configurations may combine different NUC shutter locations with different optical elements to yield any of the combinations of the NUC-shutter positioned behind a front optical element, adjacent to an intermediate optical element or before a rear optical element with optical configurations having a Back Focal Length (BFL) smaller than twice a width of an external NUC shutter assembly and/or smaller than 7mm, and/or optical configurations having a wide field of view (WFOV), e.g., of at least 110°. Correspondingly, various method stage may be combined.

[0033] In the above description, an embodiment is an example or implementation of the invention. The various appearances of "one embodiment", "an embodiment", "certain embodiments" or "some embodiments" do not necessarily all refer to the same embodiments.

[0034] Although various features of the invention may be described in the context of a single embodiment, the features may also be provided separately or in any suitable combination. Conversely, although the invention may be described herein in the context of separate embodiments for clarity, the invention may also be implemented in a single embodiment.

[0035] Certain embodiments of the invention may include features from different embodiments disclosed above, and certain embodiments may incorporate elements from other embodiments disclosed above. The disclosure of elements of the invention in the context of a specific embodiment is not to be taken as limiting their used in the specific embodiment alone. [0036] Furthermore, it is to be understood that the invention can be carried out or practiced in various ways and that the invention can be implemented in certain embodiments other than the ones outlined in the description above.

[0037] The invention is not limited to those diagrams or to the corresponding descriptions. For example, flow need not move through each illustrated box or state, or in exactly the same order as illustrated and described.

[0038] Meanings of technical and scientific terms used herein are to be commonly understood as by one of ordinary skill in the art to which the invention belongs, unless otherwise defined.

[0039] While the invention has been described with respect to a limited number of embodiments, these should not be construed as hmitations on the scope of the invention, but rather as exemplifications of some of the preferred embodiments. Other possible variations, modifications, and applications are also within the scope of the invention. Accordingly, the scope of the invention should not be limited by what has thus far been described, but by the appended claims and their legal equivalents.