Cooling arrangements of this type are for example applied for cooling infrared detectors used in infrared sensors.

The sensor may consist of a camera housing which accommodates the infrared detector. A system of lenses mounted in the camera housing focuses infrared radiation onto the infrared detector. The infrared detector generally comprises a focal plane array of detector elements. For specific applications, diode elements of the type Mercury- Cadmium-Tellurium (Hg-Cd-Te) may serve as detector elements. This substance is eminently suitable for the long-wave infrared range (8-12 ßm) and the medium-wave infrared range (3-5 gm). To ensure its proper functioning, the detector array is in operation cooled down to a temperature of 80 Kelvin. To this end, use is made of cooling arrangements of the type set forth in claim 1.

Widely used in this context are cryogenic cooling arrangements, such as Stirling coolers. Other cooling arrangements are of the Peltier or Joule-Thomson type. In Peltier cooling arrangements, a Peltier element realizes a certain temperature drop. By connecting several Peltier elements in series, a far greater temperature drop can be realized than would be possible using individual elements.

Cooling arrangements of the Joule-Thomson type are based on the principle of cooling through an expanding gas.

With a view to wear and energy consumption, it is generally inadvisable to leave the cooling arrangement switched on if the sensor is not used. In that case, the cooling arrangement will be disconnected, which causes the temperature of the infrared detector to rise to an ambient

value. Under certain conditions, the ambient temperature may rise to a high level, caused for example by the action of solar heat on the camera housing the detector. This frequently occurs in military environments, where the ambient temperature may well exceed 50°C. However, detectors of the (Hg-Ce-Te) type appear to entail the problem that they are subject to disintegration at temperatures exceeding room temperature, due to diffusion effects in the detector diodes. The disintegration rate will increase in proportion to the temperature. This irrevocably reduces the effectiveness of the detector diodes. This particularly constitutes a problem because these types of detectors are extremely high-priced.

Additionally, this will shorten the useful life of the detector array to a considerable extent. The known cooling arrangements consequently have the drawback of being unsuitable for application in detector arrays whose useful life is seriously shortened when exposed to relatively high temperatures.

According to the invention, the cooling arrangement as described in claim 1 obviates this drawback.

The concomitant advantage is that the detector array can be maintained at an acceptable standby temperature without the cooling arrangement consuming too much energy.

In an special embodiment the cooling arrangement comprises one cooler which, in the standby mode, operates on reduced cooling power as compared to the operational mode.

A further advantageous embodiment is set forth in claim 3.

In this embodiment, the infrared detector temperature is measured and applied to a regulator which controls the cooling arrangement's power. The regulator maintains the

temperature substantially constant at the values required for use in the operational and standby modes.

A further advantageous embodiment is set forth in claim 6.

The innovative principle underlying this embodiment is implemented simply by powering fewer Peltier elements in the standby mode than in the operational mode.

Alternative effective embodiments are set forth in the claims 7,8 and 9. These embodiments realize the innovative principle through the application of a main cooler for the operational mode and an auxiliary cooler for the standby mode.

The invention will now be described in further detail with reference to the following figures, of which: Fig. 1 represents a split-Stirling cooler incorporating a regulator for controlling the cooling power in two operational modes; Fig. 2 represents a split-Stirling cooler incorporating an auxiliary cooler according to the Joule- Thomson principle; Fig. 3 represents a Peltier cooler incorporating a regulator.

Fig. 1 shows a cooling arrangement according to the invention, incorporating a split-Stirling cooler 1, suitable for use in both operational and standby mode. The split-Stirling cooler comprises a compressor 2 which is connected to a cold finger 4 via a split tube 3. The cold finger 4 includes a displacer and a regenerator, not shown in the figure. The compressor 2, split tube 3 and cold finger 4 constitute a closed system filled with a cryogenic gas, such as helium. By means of two pistons 5 and 6, compressor 2 generates a time-varying pressure in the system. Pistons 5 and 6 are actuated by linear

electromotors (not shown here), to which an alternating voltage is applied, which causes both pistons to perform an oscillating motion in opposite directions 7 at the frequency of the alternating voltage and with an amplitude dependent on the amplitude of the alternating voltage. This time-varying pressure will give rise to four consecutive Stirling cycles in the cold finger 4, i. e.: heat is drawn from the cold side 8 to the warm side 9. As a result, the cold side 8 of the cold finger 4 assumes a temperature of 70 to 80 Kelvin. The cold finger 4 may be positioned in a camera housing provided with a lens (not shown here). The lens focuses infrared radiation onto a staring array 10 of detection diodes of the Hg-Cd-Te (Mercury-Cadmium- Tellurium) type, attached to the cold side of the cold finger.

The cooling power for the cooling arrangement is controlled/governed by varying the amplitude of the compressor piston motion. To this end, a regulator 11 is provided to actuate an amplifier 12 which generates the alternating voltage for the compressor motor. Regulator 11 comprises a selector switch, not shown here, to allow the cooling arrangement to function in the operational mode or in the standby mode. In the operational mode, the cooling power is controlled such that the cold side of the cold finger 4 attains a temperature of approximately 70 or 80 Kelvin; the temperature in the standby mode may range from-20 to 40 degrees Celsius, but shall preferably not exceed 20 degrees Celsius. The cold side 8 of the cold finger may be provided with a temperature sensor 13, connected to the regulator 11. Regulator 11 will, based on temperature measurements, adjust the cooling power to the desired value.

Fig. 2 represents an alternative embodiment of the cooling arrangement according to the invention. The cooling

arrangement comprises a main cooler, implemented, in the example to the embodiment, as a split-Stirling cooler 2,3, 4 and an auxiliary cooler 14, in the example implemented as a cooler that operates according to the Joule-Thomson principle. Via a supply line 15, an expanding gas is blown along the staring array 10. In the operational mode, the main cooler cools staring array 10 until an operating temperature of 70 to 80 Kelvin is attained. In the standby mode, the main cooler is disconnected and the auxiliary cooler 14 activated. The cooling capacity of the auxiliary cooler is sufficient to prevent disintegration of staring array 10.

Fig. 3 shows an embodiment of the invention where the cooling arrangement comprises a stack of Peltier elements El,..., El. Each Peltier element E; realizes a certain temperature drop. A staring array 10 to be cooled is disposed on top of the stack. The Peltier elements are individually powered through connections Au,..., An by means of a power source 16. The power source has an operational mode and a standby mode. In the operational mode, all Peltier elements are powered; in the standby mode only a limited number, although this number of elements is sufficient to maintain the staring array at a temperature at which array disintegration is virtually impossible.

Preferably, only those Peltier elements that are in closest proximity to staring array 10 are connected, for example the first three elements. The side to be cooled may be provided with a temperature sensor (not shown in the figure) to be hooked up to a power source which either connects or disconnects Peltier elements on the basis of the temperature sensor measurements.