GB2259981A - A cryogenic electrical substitution radiometer - Google Patents

Abstract

A cryogenic electrical substitution radiometer 2 comprises a reference block (heat sink) 4, a radiation absorber eg. blackened cavity 6 and a mechanical cooler 8, 10 for cooling the reference block 4 and the absorber 6. The cooling means may comprise a refrigeration device operating on the Gifford-McMahon or Stirling cycles such that superconducting connecting wires may be used. In use, radiation passing through window 26 heats the absorber 6 measurably raising its temperature above that of the reference block. An electrical heater connected to absorber 6 may be used to effect the same temperature rise as that caused by the radiation, where the power delivered to the heater is known. The device has cold shields 16, 18 and an outer vacuum jacket 22. <IMAGE>

Description

A CRYOGENIC ELECTRICAL SUBSTITUTION RADIOMETER This invention relates to a cryogenic electrical substitution radiometer.

Electrical substitution radiometers were first developed about one hundred years ago. They have been used for many purposes such for example as measuring the radiant output of the sun, the radiance of black bodies, the intensity of lasers and the setting up of radiometric scales. These electrical substitution radiometers usually consist of an absorber, for example a black cavity or a disc, which is connected to a heat sink via a heat link. This arrangement allows the absorber to rise in temperature above the heat sink, setting up a temperature gradient between the absorber and the heat sink when a source of heat is applied to the absorber.

The heat sink needs to remain at a constant temperature during a measurement and so is usually relatively massive compared to the absorber, and can actively be controlled in temperature if necessary. The electrical substitution radiometers measure optical radiation by comparing the heating effect of optical radiation with the heating effect of electrical power at the absorber. The absorber absorbs a measurable proportion of the optical radiation incident upon it and this raises the temperature of the absorber above that of the heat sink. This temperature rise can then be measured using a thermometer. Similarly, electrical power can be supplied to a heater attached to the absorber by passing a measurable electrical current through the heater and by measuring the potential difference thus generated at each end of the heater.The electrical power generates heat at the absorber and, if totally absorbed, will cause the absorber to rise in temperature by an amount directly proportional to the applied power. Assuming that there is no difference between the way that the absorber responds to different heat sources, then the optical power can be determined by adjusting the electrical power to give the same temperature rise of the absorber as the optical power.

The above described known electrical substitution radiometers thus operate on a relatively simple concept but, as a practical matter, many problems arise and in order to obtain the highest accuracy, corrections have to be applied.

The development of a cryogenic electrical substitution radiometer in the early 1980's allowed operation of the electrical substitution radiometer at the temperature of liquid helium, i.e. 4.2K, and also at the temperature of pumped liquid helium, i.e. 2K. Operation at these temperatures allowed the use of superconducting leads, large absorbing cavities without loss of sensitivity, and near exact equivalence between electrical and optical heating. The cryogenic electrical substitution radiometers allowed the accuracy of electrical substitution radiometers to be improved to better than 0.01%, which is nearly fifty times better than the above mentioned conventional room temperature electrical substitution radiometers.

The presently used existing cryogenic electrical substitution radiometers operate at temperatures of around 7K or less, and use liquid helium as the main coolant to provide such temperatures. These temperatures are required to make use of conventional superconducting wire (which has an operating range of up to approximately 11K) and to gain from the specific heat capacity of copper. However, the use of liquid helium limits the flexibility and use of the cryogenic electrical substitution radiometers to locations which have a ready supply of the liquid helium (needing to be refilled approximately every 30 hours or less) or to the use of complicated automatic refilling systems/condensers which are expensive and not easily portable.Thus the presently known and used cryogenic electrical substitution radiometers are basically instruments which are confined to reasonably well equipped laboratories.

It is an aim of the present invention to reduce the above mentioned problems.

Accordingly, in one non-limiting embodiment of the present invention, there is provided a cryogenic electrical substitution radiometer comprising a reference block, absorber means, and mechanical cooler means for cooling the reference block and the absorber means.

The use of the mechanical cooler means avoids the above mentioned need for the periodic refilling with the liquid helium or the use of the complex recycling plants.

The absorber means may be an absorber cavity or an absorber disc.

The mechanical cooler means may be mechanical cooler cold finger.

The mechanical cooler means may be a Stirling cycle operated mechanical cooler means. Alternatively, the mechanical cooler means may be a Gifford-McMahon refrigeration cycle operated mechanical cooler means, or a similar mechanical cooler means.

The cryogenic electrical substitution radiometer may include at least one radiation shield. There may be an inner radiation shield and an outer radiation shield.

The cryogenic electrical substitution radiometer may include secondary temperature pick-off points on the mechanical cooler means for cooling other radiation shields. The use of the secondary temperature pick-off points may minimise the cooling power required by a main cooling tip, allowing the main cooling tip to maintain a stable operating temperature.

The cryogenic electrical substitution radiometer may include electrical connections which are formed by superconducting wire.

The cryogenic electrical substitution radiometer of the present invention provides a new approach to cooling electrical substitution radiometers insofar as it avoids the need for liquid helium or complex recycling plants. The cryogenic electrical substitution radiometer of the present invention effects the required cooling to required cryogenic temperatures of not more than 30K using the above mentioned mechanical cooler means. The mechanical cooler means, which may be one or more coolers, may allow the temperature of the reference block and the absorber means to be cooled to temperatures as low as approximately 1OK.

The mechanical cooler means may operate by direct connection to mains or battery-generated electricity. Thus the mechanical cooler means is extremely flexible and does not require liquid cryogens.

The cryogenic electrical substitution radiometer of the present invention may be designed to operate at different temperatures without loss of accuracy by using different superconducting leads. For example niobium superconducting leads may be used at lower temperatures and leads made of high temperature superconductors may be used for higher temperatures.

One of the advantages of the present invention is that the performance of the radiometer can be maintained at elevated temperatures of around 30K as compared to those radiometers currently available operating at around 4.2K. It should also be noted that whilst improved operation will be available by using superconducting wire, a good performance, equivalent to current instruments, can still be obtained without the wire being superconducting.

The thermal environment of the cryogenic electrical substitution radiometer of the present invention may be improved by using super insulation.

Since there may still be improved performance at temperatures not greater than 10K, due mainly to more sensitive thermometers, these temFeratures may be reached by adding a Joule Thompson stage to the cold tip of the mechanical cooler means.

If desired, the mechanical cooler means may be of generally known construction and operation, modified as necessary to operate in the cryogenic electrical substitution radiometer.

The cryogenic electrical substitution radiometer of the present application has more applications than the known cryogenic radiometers and it is able to fulfil the role of many other detectors, because it can be made sensitive to all optical radiation by the choice of a suitable black absorber means. Such a suitable black absorber means is preferably a small electro-formed cavity, although for some applications requiring high sensitivity, the absorber means may be a blackened disc.

The cryogenic electrical substitution radiometer can be produced to be small and portable, to have low running costs, and to be very easy to use.

The cryogenic electrical substitution radiometer of the present invention may be flown in space for up to ten years in order to measure the irradiance from the sun with unprecedented accuracy. The cryogenic electrical substitution radiometer of the present invention may also be used to view radiation emitted from or reflected back from the earth, and to perform high accuracy measurements for long periods.

The cryogenic electrical substitution radiometer of the present invention may have the ability to measure both radiant power of a collimated beam (all absorbed) or irradiance using an aperture.

An embodiment of the invention will now be described solely by way of example and with reference to the accompanying drawing which shows a cryogenic electrical substitution radiometer.

Referring to the drawing, there is shown a cryogenic electrical substitution radiometer 2 comprising a temperature controlled reference block 4, absorber means in the form of a blackened absorber cavity 6, and mechanical cooler means in the form of a mechanical cooler cold finger 8. The mechanical cooler cold finger 8 is for cooling the reference block 4 and the absorber cavity 6. The mechanical cooler cold finger 8 is connected to a mechanical cooler unit 10.

The radiometer 2 further comprises a thermal mass 12 for smoothing pulses. This mass 12 is connected by a poorly conducting heat link 14 to the reference block 4.

The radiometer 2 is provided with an outer cold shield 16 which may operate at approx. 110K. The radiometer 2 is also provided with an inner cold shield 18 which may operate at 30K or less. As shown, a heat link 20 is provided between the reference block 4 and the absorber cavity 6.

The radiometer 2 has an outer vacuum jacket 22 which is provided with a vacuum port 24. The outer vacuum jacket 22 is also provided with a window 26. The window 26 may be a flat window or a brewster angled window.

The radiometer 2 may be optionally provided with defining apertures 28, 30 for irradiance. The radiometer 2 may also optionally be provided with a photo diode 32 having a hole for collimated laser radiation.

The radiometer 2 may be approximately 750mm in length although larger or smaller radiometers 2 may be produced.

The temperature controlled reference block 4 has a thermometer. The absorber cavity 6 also has a thermometer.

The ratio between thermometer readings is used for measurement purposes.

The absorber cavity has an electrical heater. The electrical heater has electrical connections made using superconducting wire wrapped around the reference block 4 before emerging out of the system.

In the radiometer 2, all electrical leads undergo thermal anchoring at each shield 16, 18 before emerging.

The shields 16, 18 are wrapped with super insullation.

The photo diode 32 may be used depending upon the intended use of the radiometer 2 and it would be expected to exchange a complete cold shield for ease of change between configurations.

It is to be appreciated that the embodiment of the invention described above with reference to the accompanying drawing has been given by way of example only and that modifications may be effected. Thus, for example, the mechanical cooler means may be a Stirling cycle operated mechanical cooler means, a Gifford-McMahon refrigeration cycle operated mechanical cooler means, or a similar arrangement.

Claims (10)

1. A cryogenic electrical substitution radiometer comprising a reference block, absorber means, and mechanical cooler means for cooling the reference block and the absorber means.

2. A cryogenic electrical substitution radiometer according to claim 1 in which the absorber means is an absorber cavity or an absorber disc.

3. A cryogenic electrical substitution radiometer according to claim 1 or claim 2 in which the mechanical cooler means is a mechanical cooler cold finger.

4. A cryogenic electrical substitution radiometer according to claim 1 or claim 2 in which the mechanical cooler means is a Stirling cycle operated mechanical cooler means.

5. A cryogenic electrical substitution radiometer according to claim 1 or claim 2 in which the mechanical cooler means is a Gifford-McMahon refrigeration cycle operated mechanical cooler means.

6. A cryogenic electrical substitution radiometer according to any one of the preceding claims and including at least one radiation shield.

7. A cryogenic electrical substitution radiometer according to claim 6 and comprising an inner radiation shield and an outer radiation shield.

8. A cryogenic electrical substitution radiometer according to claim 6 or claim 7 and including secondary temperature pick-off points on the mechanical cooler means for cooling other radiation shields.

9. A cryogenic electrical substitution radiometer according to any one of the preceding claims and including electrical connections which are formed by super conducting wire.

10. A cryogenic electrical substitution radiometer substantially as herein described with reference to the accompanying drawing.