INFRARED DETECTORS

Abstract: An infrared detector has an infrared detector element (2) mounted at a first part (11) of a housing (1), a Joule-Thomson cooling element (3) accommodated in a second part (12), and an infrared transmissive window (15) at the front of the housing (1). A compact, space-saving and light weight structure with improved optical efficiency is obtained. The core of the Joule-Thomson cooling element (3) forms a third part (13) of the housing (1), and these second and third parts (12 and 13) of the housing extend towards the front of the detector. The cooler core (13) which may carry a cooled filter (25) has a hollow shape or is otherwise infrared transmissive in front of the detector element (2) and can direct incident radiation (50) towards the detector element (2). For this purpose the inner surface of the hollow core (13) may carry a reflective conductor pattern (18,19) which also provides electrical connections for the detector element (2). This core part (13) may seal a channel in a grooved intermediate member (48 in Figure 3) between the second and third housing parts (12 and 13) so as to form the Joule-Thomson cooling element (3) without requiring conventional finned metal tubing (31).

Description: This invention relates to infrared detectors comprising a housing and a Joule-Thomson cooling element, particularly but not exclusively detectors having fast cool-down characteristics as well as a compact size and light weight and suitable for detecting infrared radiation having a wavelength in the 3 to 5 or 8 to 14 µm (micrometre) wavebands. Published United Kingdom Patent Application (GB-A) 2147739 describes an infrared detector comprising a housing. At least one infrared detector element is mounted at a first part of the housing. A Joule-Thomson cooling element formed around a core is accommodated in a second part of the housing to effect cooling of the detector element during operation of the detector by expansion of coolant into a space adjacent the first part of the housing. Infrared transmissive means at a front of the housing permit transmission of infrared radiation to the detector element. The second part accommodating the cooling element is of low thermal conductivity and capacity and is, in this device, situated behind the first part where the detector element and infrared transmissive window are mounted. Such an arrangement is conventional practice. However, the design of the housing is such that the detector element can be cooled down very rapidly, for example within a few seconds, and the detector is used in situations where the cooled operating state need not be maintained for a very long time, for example at most 5 minutes. According to the present invention there is provided an infrared detector comprising a housing in which at least one infrared detector element is mounted at a first part of the housing, a Joule-Thomson cooling element formed around a core and accommodated in a second part of the housing, which cooling element serves to effect cooling of the detector element during operation of the detector by expansion of coolant into a space adjacent the first part of the housing, and infrared transmissive means at a front of the housing to permit transmission of infrared radiation to the detector element, characterised in that the core of the Joule-Thomson cooling elenent forms a third part of the housing, that the second and third parts of the housing extend between the first part of the housing and said front of the housing, and that the third part of the housing is infrared transmissive in front of the detector element to permit transmission of the infrared radiation through the core of the Joule-Thomson cooling element, from the front of the housing to the infrared detector element. The invention is based on a recognition by the present inventors that the arrangement of the Joule-Thomson cooling element around a hollow or otherwise infrared transmissive core forming a third part of the housing extending from the detector element towards the front of the detector housing permits the obtainment of a compact housing structure which as well as saving space and reducing weight, can provide several important advantages for fast cool-down and for the optical efficiency of the detector. Thus, the core forming the third part of the housing may be hollow and form part of an enclosure with the first part and with the infrared transmissive means, nay serve to direct incident infrared radiation towards the detector element, may carry electrical connections for the detector element (for example as part of a reflective pattern for directing the radiation), and may carry a lens and/or window and/or filter which may also be cooled during operation by the Joule-Thomson cooling element accommodated between the second and third parts of the housing. It should be noted that, as described in United Kingdom patent specification (GB-A) 1 061 130, it is known to use a light-pipe to direct infrared radiation onto a detector element for the 20 to 200 µm waveband and for the light-pipe to carry a cooled filter in front of the detector element. The detector element surrounded by a superconducting magnetic coil is cooled to liquid-helium temperature by being mounted in the bottom of a tapered tube inserted in a cryostat containing liquid helium which is itself inserted in a cryostat containing liquid nitrogen. Because it is not practical to provide infrared windows in the walls of this double cryostat construction, the radiation is incident along the tapered tube which is designed to act as a light pipe. The construction is very bulky, heavy and very slow to cool down to its operating temperature. It is not practical to use a Joule-Thomson cooler for cooling the detector element. The detector is not easily mobile, and if turned on its side or upside down a loss of the cryogenic liquid results. The field of use of this detector is quite different from that of detectors in accordance with the present invention. Various features in accordance with the invention will now be illustrated in particular embodiments of the invention, by way of example, with reference to the accompanying diagrammatic drawings, in which:- Figure 1 is a cross-sectional view of part of one infrared detector in accordance with the invention and mounted on a coolant bottle, and Figures 2 and 3 are cross-sectional views of parts of other infrared detectors in accordance with the invention. The drawings are diagrammatic and not drawn to scale. For the sake of convenience and clarity in the drawings, the dimensions and proportions of various features of these detectors have been shown exaggerated or diminished in size. The same reference signs used in one embodiment are generally used to refer to corresponding or similar parts of the other embodiments. The infrared detector of Figure 1 comprises a housing 1 within which an infrared detector element 2 and a Joule-Thomson cooling element 3 are accommodated. The infrared radiation 50 to be detected is incident on the detector element 2 via an infrared transmissive window 15, for example of germanium, at the front of the housing 1. This radiation 50 may be in the 3 to 5 or 8 to 14 µm wavelength bands, depending on the nature of the detector element 2. In accordance with the present invention the Joule-Thomson cooling element 3 is formed around a core which forms a third part 13 of the housing 1 and is accommodated between second and third parts 12 and 13 respectively of the housing 1 which extend towards the front (15,61) of the housing 1 from a first part 11 where the detector element 2 is mounted. The third part 13 of the housing 1 has a hollow shape in front of the detector element 2 to permit transmission of the infrared radiation 50 therethrough from the front window 15 to the detector element 2. The second part 12 in which the Joule-Thomson cooling element 2 is accommodated is located concentrically around the hollow third part 13. In the detector of Figure 1, at the wavelength or wavelengths of the radiation 50 detected by the detector element 2, at least a major portion 18 and 19 of the whole inner surface of the hollow third part 13 is highly reflective so acting as a light-pipe directing the incident radiation 50 towards the detector element 2. For this purpose it is advantageous for the reflective hollow in the third part 13 to have a width which increases from the first part 11 towards the front (15) of the housing 1. Thus, both the hollow third part 13 and the facing surface of the second part 12 of the housing 1 may have a generally conical shape as illustrated in Figure 1. The Joule-Thomson cooling element 3 may be of any known form appropriate to the detector design. In the form illustrated in Figure 1, the core forming the hollow third part 13 of the housing 1 is of a frusto-conical shape on which there is wound a helical coil of metal tubing 31 of the Joule-Thomson cooler. A bottle 30 of pressurised fluid (for example argon or dry air) is connected to one end of the coiled tubing 31 via an electrically-initiated gas-flow valve 34 (commonly termed a pyrotechnic gas motor). At the opposite end of the coiled tubing 31, an orifice 32 is present from which the pressurised fluid expands into a space 20 adjacent the first housing part 11 where the detector element 2 is mounted. This expansion of the coolant fluid from the orifice 32 effects a rapid cooling of the fluid (and hence a cooling of the detector element 2) in accordance with the Joule-Thomson effect. After expansion into the space 20, the cooled fluid flows back over the outside of the tubing 31 between the housing parts 12 and 13 so pre-cooling the pressurised fluid in the tubing upstream of the orifice 32. This regenerative cooling effect rapidly reduces the temperature of the coolant fluid so that, for example, a coolant such as argon or air is liquified in the coiled tubing 31 prior to its expansion from the orifice 32. To increase heat transfer between the tubing 31 and the venting coolant, the outer surface of the tubing 31 normally includes metal fins. A laminate foil 35 of superinsulation material (for example polyimide and oxidised aluminium) may be present between the outer housing part 12 and the finned metal tubing 31 to increase the efficiency of the regenerative cooling. In this way the detector element 2 is rapidly cooled to its operating temperature, for example below 100°K or at least below 120°K. The detector can be designed to have a cool-down time of less than 1 second and a maximum operational duration of, for example, 1 to 5 minutes. In the front of the housing, vent holes 36 are preferably provided adjacent the infrared window 15 for venting at least some of the expanded coolant over the front of the window 15 to prevent misting of the window by condensation. The holes 36 may be formed in a retention plate 61 bonded to the front of the housing part 12 and forming part of a clipping arrangement clamping the cooler 2 and housing part 13 in the housing 1. As illustrated in Figure 1, the third part 13 of the housing 1 not only provides the core for the coiled metal tubing 31 of the cooling element 3 but also forms an enclosure with the first housing part 11 and the front window 15. The detector element 2 of Figure 1 is mounted within this enclosure. This multi-functional role of the housing part 13 results in a compact space-saving arrangement which can significantly reduce the distance between the front (15,61) of the detector housing 1 and the bottle 30 and can also reduce the weight of the detector. Thus the detector housing 1 may be mounted directly on the bottle 30 as illustrated in Figure 1. Furthermore the thermal capacity needing to be cooled by the cooling element 2 can be reduced for achieving very rapid cool-down times. The Figure 1 housing for both the detector element 2 and the cooling element 3 simply consists of an outer part 12 and an inner part 11 and 13 with a front window 15. Both the outer and inner parts 12, 11 and 13 may be formed of moulded plastics material, although the part 12 preferably has a jacket of expanded polyurethane or other good thermally insulating material around it. The part 11 on which the detector element 2 is mounted is preferably formed as a circuit substrate carrying connnections for the detector element 2. The detector element 2 may be mounted on the substrate 11 in known manner, for example by using techniques similar to those described in United Kingdom patent (GB-A) 1 559 474. Thus the detector element 2 which may be of passivated cadmium mercury telluride having gold electrodes may be secured to the substrate 11 by a layer of insulating epoxy adhesive. Gold layer interconnections deposited over the edges of the detector element 2 may connect these electrodes to the conductors on the circuit substrate 11. This circuit substrate 11 may be secured by adhesive to an end of the hollow, inner plastics part 13. At least part of the reflective portion 18 and 19 of the inner surface of the hollow part 13 may be formed by an electrically conductive layer pattern (for example, of gold) on this surface which provides separate electrical connections 18 and 19 for the detector element 2. These parts 18 and 19 are insulated from each other by gaps 17 in the conductive layer pattern. The conductors on the circuit substrate 11 may be connected by solder or conductive epoxy to the conductive layer parts 18 and 19 on the hollow housing part 13. External connections 28 and 29 on the outside of the housing part 12 may be formed as a printed lead extension of the cnductors 18 and 19 on protrusions of the housing part 13, only one of which is illustrated in Figure 1. The hollow housing part 13 providing the core for the cooling element 3 may also be used to carry a cold filter 25. Such a filter may be used to selectively transmit infrared wavelengths to the detector element 2, for example wavelengths in the range 10.6 ± 0.3µm (micrometres). The filter 25 is mounted within the hollow housing part 13 (for example by epoxy) and is thermally coupled thereby to this part 13 to permit cooling of the filter 25 by the cooling element 3 during operation of the detector. As illustrated in Figure 1, the window 15 is mounted on the hollow part 13 in front of the filter 25. The expanded coolant may be vented over the filter 25 by providing a vent hole or even omitting the window 15. The filter 25 may even be omitted to vent the exhausted coolant over the front face of the detector element 2. In Figure 1 the helical coil of finned tubing 31 is connected to the bottle valve 34 by a length of unfinned tubing 38 on the outside of the outer housing part 12. However this tubing 38 may be accommodated in a channel in the housing part 12 and possibly even in the space between the housing parts 12 and 13. Many modifications are possible. In the detector of Figure 1 the detector element 2 may be mounted on the opposite side of the substrate 11 (i.e. on the cooling-element side) so as to be located in the space 20 into which the coolant expands from the orifice 32. Such an arrangement permits direct cooling of the detector element 2 so that an even faster cool-down is possible. A variation is illustrated in Figure 2, in which the circuit substrate forming the first housing part 11 on which the detector element 2 is mounted is secured in a recess of the outer second part 12 of the housing 1. The inventors have found that the passivation conventionally provided on detector elements 2 of cadmium mercury telluride (as described in, for example, GB-A 1 559 474 and GB-A 1 568 958) can provide adequate environmental protection for such a detector element 2 located in the coolant space 20 in detectors having short maximum operating durations (for example at most 3 minutes). However, a transparent, additional protective film of, for example, plastics material may be provided over the detector element 2 and possibly also over its conductors on the substrate 11. In the detector of Figure 2, the bulk 60 of the outer second housing part 12 may be of expanded polyurethane contained within walls 62, 63 and 64 of, for example, plastics material. In this case the external connections 28 and 29 may be formed as insulated leads or pins which extend through the side wall 64 and through the bulk 60 to be connected directly to the conductors on the circuit substrate 11 by solder or conductive epoxy. As illustrated in Figure 2, these conductors may be brought through the substrate so that the connections to the leads 28 and 29 is at the back face of the substrate 11. If desired, most of the back face of the substrate 11 may be coated to render it highly reflective at the wavelength(s) of the radiation 50, and the coated part of this back face may be convex so as to concentrate the reflected radiation onto the detector element 2. As well as a front infrared window 15, there is an additional infrared window 65 at the inside end face of the hollow third housing part 13 forming the core of the Joule-Thomson cooler 3. The whole inner surface of the cavity in the hollow part 13 may be made highly reflective or highly absorbent as desired for any particular application. The cooler 3 and housing part 13 are firmly secured in the outer housing part 12 by a spring-loaded retention plate 61 which is clamped to a flange at the front of the side-wall 64. These members are shown separated in Figure 2. Instead of using finned metal tubing 31, the Joule-Thomson cooler 3 may be formed by providing a grooved intermediate member 46 between the facing surfaces of the second and third parts 12 and 13 of the housing 1. Such an embodiment is illustrated in Figure 3. The member 46 may be connected to a coolant bottle by tubing 38 and a threaded mount 48, narrow channels 43 corresponding to the bore in the finned tubing 31, a space 20 into which the coolant expands from the orifice 32 at the end of the narrow channels 43, and wide channels 42 for venting the expanded coolant. The wide and narrow channels 42 and 43 are arranged to have good heat exchange therebetween for regenerative cooling. The narrow channels 43 are sealed by the hollow third housing part 13 which is bonded to the member 46. The wide channels are similarly sealed by a conical plate 41 which may form the inside of the outer housing part 12. Thus the walls 13 and 41 form part of the cooler 3. For clarity in the drawing, the various parts are shown separated in Figure 3. The bulk of the second housing part 12 may be formed by one or more masses 40 of thermal insulating material (for example expanded polyurethane) providing good thermal insulation between the exterior of the housing and both the detector element 2 and the cooling element 3. A laminate foil 35 of superinsulant material may be present between the plate 41 and the mass 40. This arrangement of a channelled member 46 sealed to the hollow third part 13 of the housing 1 permits very efficient cooling of the part 13. This is particularly advantageous for cooling the filter 25 and for reducing the emissivity of the inner surface of the hollow third part 13 if that surface is not very highly reflective or is even absorbent at the wavelength(s) detected by the detector element 2. However it is particularly advantageous for the inner surface of the hollow third part 13 in the detector of Figure 3 to be highly reflective so as to form a light-pipe for the radiation 50. Although conical shapes have been illustrated in the drawings, it will be evident that different shapes are possible, for example parabolic or other shapes curving in three dimensions or for example cylindrical. It is particularly convenient to form the infrared transmissive third part 13 of the housing as a hollow member. However for some detector applications it may be acceptable to have a solid member 13 of infrared transmissive material having a low thermal conductance, for example a particular plastics material or germanium. It has been stated above that the drawings illustrate examples of embodiments of the invention and, in order to avoid any misunderstanding, it is hereby further stated that, in the following claims, where technical features mentioned in any claim are followed by reference signs relating to features in the drawings and placed between parentheses, these reference signs have been included in accordance with Rule 29(7) EPC for the sole purpose of facilitating comprehension of the claim, by reference to an example.

Claims: An infrared detector comprising a housing (1) in which, at least one infrared detector element (2) is mounted at a first part (11) of the housing, a Joule-Thomson cooling element (3) formed around a core (13) and accommodated in a second part (12) of the housing, which cooling element (3) serves to effect cooling of the detector element (2) during operation of the detector by expansion of coolant into a space (20) adjacent the first part (11) of the housing, and infrared transmissive means (15) at a front (15, 61) of the housing to permit transmission of infrared radiation to the detector element (2), characterised in that the core (13) of the Joule-Thomson cooling element (3) forms a third part (13) of the housing (1), that the second (12) and third (13) parts of the housing (1) extend between the first part (11) of the housing (1) and said front (15, 61) of the housing, and that the third part (13) of the housing (1) is infrared transmissive in front of the detector element (2) to permit transmission of the infrared radiation (50) through the core (13) of the Joule-Thomson cooling element (3), from the front (15, 61) of the housing to the infrared detector element (2). A detector as claimed in Claim 1, further characterised in that the core (13) forming the third part of the housing (1) has a hollow shape, and that, at the infrared wavelength or wavelengths of the radiation detected by the detector element (2), at least a major portion (18, 19) of the whole inner surface of the hollow third part of the housing (1) is reflective so as to direct incident infrared radiation (50) towards the detector element (2). A detector as claimed in claim 2, further characterised in that the hollow space in the third part (13) of the housing (1) has a width which increases from the first part (11) towards said front (15, 61) of the housing. A detector as claimed in claim 3, further characterised in that both the hollow third part (13) of the housing and the facing surface of the second part (12) are generally conical in shape. A detector as claimed in anyone of the preceding claims, further characterised in that the core (13) forming the third part of the housing has a hollow shape forming an enclosure with the first part (11) of the housing (1) the infrared transmissive means (15), the detector element (2) being located within said enclosure. A detector as claimed in anyone of the preceding claims, further characterised in that electrical connections for the detector element (2) are carried by the core (13) forming the third part of the housing (1). A detector as claimed in claim 6, further characterised in that the core (13) forming the third part of the housing (1) has a hollow shape, and that the electrical connections for the detector element comprise an electrically conductive layer pattern at the inner surface of this hollow third part (13) of the housing (1). A detector as claimed in claim 7 when dependant on claim 2, further characterised in that the electrically conductive layer pattern forms at least part of the reflective portion (18,19) of said inner surface. A detector as claimed in anyone of the preceding claims, further characterised in that the core (13) forming the third part of the housing (1) has a hollow shape, and that a selectively infrared transmissive filter (25) is mounted within and thermally coupled to this hollow third part (13) of the housing (1) to permit cooling of the filter (25) by the Joule-Thomson cooling element (3) during operation of the detector. A detector as claimed in anyone of the preceding claims, further characterised in that an infrared transmissive window (15) of the housing (1) is mounted on the third part (13) at the front of the housing (1). A detector as claimed in claim 10, further characterised in that vent holes (36) are present in the front of the housing (1) adjacent the infrared transmissive means (15) for venting the coolant over the infrared transmissive means (15). A detector as claimed in anyone of the preceding claims, further characterised in that between the facing surfaces of the second (12) and third (13) parts of the housing (1) there is provided a grooved intermediate member (46) to form channels (42,43) of the Joule-Thomson cooling element (3). A detector as claimed in claim 12, further characterised in that at least the third part (13) of the housing (1) forms a part of the Joule-Thomson cooling element (3) by sealing a channel (43) in the grooved intermediate member (46). A detector as claimed in anyone of the preceding claims, further characterised in that the second part (12) of the housing (1) comprises one or more masses (40) of thermal insulating material providing thermal insulation between the outside of the housing (1) and both the detector element (2) and cooling element (3).

Inventors: DUNN, WILLIAM ANDREW ERNEST, GOWLETT, DAVID JOHN
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN, PHILIPS ELECTRONICS UK LIMITED
Application year: 1987
Main classification: H01L23
IPC codes: G01J5, F25D19, F25B9, G01J1