System and method for changing spectral range of a cryogenically cooled detector

The provided optical system allows selective spectral transfer of radiation, and provides a reflecting field of view at an undesired range directed towards cold surfaces. A removable spectral filter having a high transmittance at a first spectral range and a low transmittance at a second spectral range is disposed outside a cold shield. A reflective surface faces the detecting device and provides the detector a reflecting field of view at the second spectral range directed back towards the cold shield, and a blackened cold skirt thereof. Alternatively, a dichroic mirror is disposed inside the cold shield and has a high reflectance at a first spectral range and a high transmittance at a second range. The detecting device includes a first and a second arm of the cold shield to accommodate respective optical channels.

CROSS REFERENCE

The current application claims the priority rights of Israeli patent application No. 204,025 entitled “CHANGING SPECTRAL RANGE OF CRYOGENICALLY COOLED DETECTOR” filed Feb. 18, 2010, by the current inventor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is in the field of infra-red cryogenically cooled detectors, used to image thermal objects disposed in certain field of view. In special, the current application deals with changing detected spectral range by actions done outside cold shield of the detector.

2. Description of Related Art

An infra-red cryogenically cooled detecting device17of the prior art is illustrated inFIG. 1. An infra-red detector5is disposed on a cold finger30, and placed within a cold shield1. A cold filter14is thermally coupled to cold shield1, and filters the radiation outside the detecting range of detector5to prevent undesired heating of the detector. Cold filter14may have even a smaller spectral range as needed by a specific task.

Cold shield1is located within a vacuum cell or Dewar Detector Cooler (hereafter DDC)9. The incoming radiation from a certain field of view (FOV), which includes the object desired for imaging, enters vacuum cell9through vacuum tight window32.

Sometimes, there is a demand to operate in two different spectral ranges. Implementation of this demand entails difficulty because the spectral range is determined by filter14which is cooled by rigid attachment, by welding for example, to cold shield1situated within DDC9. Dewar9itself is drawn to high vacuum to reduce heat flow. It is also problematic to employ moving parts within the vacuum due to generation of dust and problems of lubrication. It is therefore not simple to exchange the filter within vacuum cell9.

According to the conventional approach, it is assumed that any location of the spectral filter outside Dewar9would lead to problematic noise and degradation of the images due to thermal emissions from the filter itself and/or thermal energy from the environment incident on the detector side of the filter and reflected onto a detector or a detector array5.

The present invention describes several devices and corresponding methods to operate a thermal imaging system switchably in several spectral ranges using a single cooled detector, whereas switching between spectral ranges is done completely outside the Dewar with no particular difficulty, and without significant loss of sensitivity or significant addition of undesired radiation.

BRIEF SUMMARY OF THE INVENTION

It is provided according to some embodiments of the current invention, an optical system for changing an operational spectral range of a cryogenically cooled infra-red detecting device. The device has an infra-red detector disposed within a cold shield and adapted for receiving incoming radiation. The optical system allows transfer to the detector of incoming radiation at a desired spectral range while preventing transfer thereof at an undesired spectral range, and provides a reflecting field of view at the undesired spectral range directed towards cold surfaces.

In some embodiments, the system includes a removable spectral filter disposed outside the cold shield. The spectral filter has a high transmittance at a first spectral range and a substantially lower transmittance for a second spectral range. Preferably the high transmittance is higher than 70% and the low transmittance is lower than 20%.

In some embodiments, a reflective surface faces the detecting device and providing the detector a reflecting field of view at the second spectral range directed towards the cold shield. Preferably, a blackened cold skirt surrounds a coupling window of the cold shield, and the reflective surface provides the detector a reflecting field of view directed towards the cold shield and the blackened cold skirt.

Alternatively, the reflective surface is a concave reflecting surface facing the detecting device.

In some embodiments, a dichroic mirror is disposed inside the cold shield in front of an infra-red detector. The dichroic mirror has a high reflectance at a first spectral range and a high transmittance at a second spectral range. The detecting device includes a first arm and a second arm of the cold shield to accommodate respective optical channels for the first spectral range and for the second spectral range. Preferably, a removable selective mirror having a high reflectance at the first spectral range is disposed outside the cold shield. Most preferably, the removable selective mirror prevents reception by the detecting device of incoming radiation at the second spectral range.

In some embodiments, an optical train provides a reflecting field of view at the undesired spectral range directed towards cold surfaces. Preferably, the cold surface is a blackened surface of the cold shield. Alternatively, the cold surface is a thermo-electric cooler.

It is provided according to some embodiments of the current invention, a method for changing an operational spectral range of a cryogenically cooled infra-red detecting device. The method includes a step of allowing transfer to the detector of incoming radiation at a desired spectral range, a step of preventing transfer thereof at an undesired spectral range, and a step of providing a reflecting field of view at the undesired spectral range directed towards at least one cold surface.

In some embodiments, the method includes placing and removing spectral filter outside the cold shield. The spectral filter has a high transmittance at a first spectral range and a substantially lower transmittance for a second spectral range.

In some embodiments, the method includes placing and removing a reflective surface facing the detecting device for providing the detector a reflecting field of view at the second spectral range directed towards the cold shield.

In some embodiments, the method includes disposing a dichroic mirror inside the cold shield in front of an infra-red detector. The dichroic mirror has a high reflectance at a first spectral range and a high transmittance at a second spectral range. The detecting device includes a first arm and a second arm of the cold shield to accommodate respective optical channels for the first spectral range and for the second spectral range. Preferably, the method further includes placing and removing a selective mirror outside the cold shield, whereas the removable selective mirror has a high reflectance at the first spectral range. Preferably, the method includes a step of providing an optical train for providing a reflecting field of view at the undesired spectral range directed towards cold surfaces.

It is provided an optical system for controlling radiation provided to a cryogenically cooled detecting device equipped with a cold shield. The detecting device being adapted to detect the incoming radiation from a certain field of view. The optical system blocks portion of the field of view of the incoming radiation from arriving the detecting device, and provides a reflecting field of view compatible with the blocked portion directed towards cold surfaces. Preferably, a substantially circular aperture is used for blocking portion of the incoming radiation, and a reflecting surface is used for reflecting field of view compatible with the blocked portion directed towards at least one cold surface. Most preferably, a blackened cold skirt surrounds a coupling window of the cold shield and the reflective surface, and the reflecting surface provides the detector a reflecting field of view directed towards the cold shield and the blackened cold skirt.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described in terms of specific example embodiments. It is to be understood that the invention is not limited to the example embodiments disclosed. It should also be understood that not every feature of the methods and systems handling the described device is necessary to implement the invention as claimed in any particular one of the appended claims. Various elements and features of devices are described to fully enable the invention. It should also be understood that throughout this disclosure, where a method is shown or described, the steps of the method may be performed in any order or simultaneously, unless it is clear from the context that one step depends on another being performed first.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The systems, methods, and examples provided herein are illustrative only and not intended to be limiting.

In the following, several embodiments are described.

FIG. 2aillustrates a system for controlling radiation provided to a cryogenically cooled detecting device equipped with a cold shield1. The detecting device detects the incoming radiation from a certain field of view, spanning at least the sum total of the spectral ranges to be imaged in the different modes of operation. A removable interference filter12is disposed in proximity to DDC9. It has a high transmittance for a first spectral range around a wavelength λ1and a substantially lower transmittance for a second spectral range, around a wavelength λ2. Preferably, it highly reflects at the second spectral range. Anyhow, interference filter12prevents radiation at the second spectral range from arriving the detecting device. In one example, the high transmittance at λ1is higher than 70% and the low transmittance at λ2is lower than 20%.

Note that the blocked radiation may be defined by two parameters, the central wavelength λ2, and a half bandwidth Δλ. It also may be defined by other pairs of parameters like lower and upper wavelengths defining a spectral range in between, or a central wavelength λ2, and a bandwidth expressed in percent λ2.

A blackened cooled skirt or crown13surrounds a coupling filtering window14of the cold shield. Surface12aof filter12is a high reflector for the second spectral range. It reflects the field of view of cold shield1, filtering window14and a cold skirt13back to the detecting device and into detector5.

When removing or replacing filter12, a window can be inserted or the focus of lenses shifted in order to accommodate the change in optical length.

In the embodiment ofFIG. 2b, the blocking filter is a lens15located in front of DDC9. It has a concave surface15areflecting at the second spectral range. The focal length of the reflecting surface ensures that the cold shield is reflected back to detector5at. Thus, in this embodiment, skirt13may be smaller or absent.

Rather than blocking some spectral range, skirt13is useful when reduction of the field of view is desired. In the example ofFIG. 3a, detector5gets incoming radiation from a broad field of view defined by boundary34. To reduce the field of view from outside DDC9, an aperture element16is disposed in front of the detecting device, as shown inFIG. 3b. Outside a central pupil, aperture element16has a blocking coating16aon the side facing the incoming radiation, and a reflecting surface16bfacing the detecting device. Blocking coating16amay be either a highly absorbing black coating or a highly reflecting surface. Consequently, the field of view is reduced and is limited by boundary38. In other words, the effective F# of the detecting system is changed.

Radiation which is angularly compatible with the blocked radiation is reflected by surface16bfrom cold shield1, filtering window14and cold skirt13, and arrives detector5.

Note that rather than a plane aperture element, a concave aperture mirror, analogous to the one illustrated inFIG. 2b, may be placed in front of the Dewar9.

Internal Dichroic Minor Inside a Double Channel Detecting Device (FIGS. 4a,4b,5)

In the embodiments ofFIGS. 4a,4band5, a dichroic filter or mirror2is deployed within the cold shield, thereby defining two optical paths or channels to the detector with high sensitivity in different spectral ranges. This is combined with an optical path switching arrangement outside DDC9which ensures that, at any given time, one optical path to the detector sees the region to be imaged while the other receives a much smaller amount of radiation. Switching between the optical paths is typically achieved by inserting and removing a selective mirror7.

An advantage of this approach is that in the optical path along which the detector is exposed to a spectral range it is not currently supposed to see, sees a blackened body with a very cold temperature, thereby reducing emitted radiation to insignificant levels. In order to maintain this advantage, it simulates the cold shield hatch on a blackened side panel8aof cold shield1. Likewise, in order to equalize the two paths, the right cold shield arm4should be at a similar distance from selective mirror7as the upper cold shield arm3.

In more detail, dichroic mirror2is disposed inside cold shield1in front of an infra-red detector5. Dichroic mirror2has a high reflectance at a first spectral range around λ1and a high transmittance at a second spectral range around λ2. In the embodiment ofFIG. 4a, radiation at the second spectral range is transmitted through dichroic mirror2to detector5. At the same time, at the first spectral range, blackened side panel8ais imaged onto detector5. For that sake, panel8ais imaged at the first spectral range through window8bof vacuum cell9, through lenses51,11,6and52, mirrors45,46,47and49, through windows32band14, and through arm4of cold shield1and dichroic mirror2towards filter5. Along the optical train, the aperture on arm4of the cold shield is imaged at10.

In the embodiment ofFIG. 4b, a removable mirror7is disposed outside cold shield1and vacuum cell9. Mirror7has two reflecting coatings7aand7bof different spectral specifications disposed on its respective two sides. Coating7ahas a high reflectance at the first spectral range around wavelength λ1, while it highly absorbs at the second spectral range around wavelength λ2. Coating7bhas high reflectance at the second spectral range, and may absorb at the first spectral range.

To switch operational spectral range, mirror7is disposed in front of arm3at an angle of 45° to incoming radiation. As a result, the radiation at the first spectral range is reflected towards lens6and mirror47, and is transmitted along an optical train which includes lens6, mirror47, mirror48, lens52and windows32band14. Finally it is reflected by dichroic mirror2onto detector5.

Note that while the optical train between mirror7and arm4may be the same optical train used whenever mirror7is removed, as inFIG. 4a, it is possible to modify the optical train upon removal of mirror7to account for a different optical length. Such a modification is possible as it is done outside DDC9.

At the second spectral range, cold panel8ais imaged onto detector5. To that aim, panel8ais imaged at the second spectral range through lenses51and11, mirrors45,46and7, windows32aand14, and is transmitted by dichroic mirror2onto detector5.

Rather than imaging cold panel8a, a thermo-electric cooler (TEC)59is imaged in the embodiment ofFIG. 5onto detector5at the second spectral range. Upon removal of mirror7, TEC59is imaged at the first spectral range by lenses6and52onto detector5.

Note that the detecting device ofFIGS. 4 and 5, having an internal dichroic mirror and two arms of the cold shield for two respective optical channels, may be operated at switching spectral ranges by directing the appropriate arm towards the object desired for imaging or detection. In such an operational changeable mode, special care should be paid to decease substantially the entered radiation at the undesired spectral range. Rather than reflecting field of view at the undesired spectral range towards cold surfaces as described in the embodiments of FIGS.2,4and5, one may achieve the decrease by other means.

A Method Embodiment for Changing an Operational Spectral Range (FIG. 6)

FIG. 6is a flow chart of a method60for changing an operational spectral range of a cryogenically cooled infra-red detecting device. The method includes a step61of allowing transfer to the detector of incoming radiation at a desired spectral range, a step62of preventing transfer thereof at an undesired spectral range, and a step63of providing a reflecting field of view at the undesired spectral range directed towards at least one cold surface.

In some embodiments, the method includes a step64of placing outside the cold shield and a step65of removing a spectral filter. The spectral filter has a high transmittance at a first spectral range and a substantially lower transmittance for a second spectral range.

In some embodiments, the method includes a step65of placing/removing a reflective surface facing the detecting device for providing the detector a reflecting field of view at the second spectral range directed towards the cold shield.

In some embodiments, the method includes a step66of disposing a dichroic mirror inside the cold shield in front of an infra-red detector. The dichroic mirror has a high reflectance at a first spectral range and a high transmittance at a second spectral range. The detecting device includes a first arm and a second arm of the cold shield to accommodate respective optical channels for the first spectral range and for the second spectral range. Preferably, the method further includes a step67of placing outside the cold shield and a step68of removing a selective mirror, whereas the removable selective mirror has a high reflectance at the first spectral range. Preferably, the method includes a step69of furnishing an optical train for providing a reflecting field of view at the undesired spectral range directed towards cold surfaces.

It should also be understood that the steps of method60may be performed in any order or simultaneously, unless it is clear from the context that one step depends on another being performed first.