The invention, in particular further, though not exclusively, relates to using one or more standing-wave patterns in an antenna-cavity structure to obtain a desirable form of radiometric spatial weighting function for coupling a radiometer to a region of source material whereby a temperature of the source material inside the cavity may be measured.

BACKGROUND OF INVENTION Many industrial processes require non-invasive, non- destructive measurement of temperature within a mass of material, for example, processed food product inspection and also medical applications.

Travelling wave and induction field surface-contact type antennas have previously been used for medical applications. However, these types of antennas are not well suited to many envisaged applications. In particular, the following problems are evident: (a) The weighting functions for this class of antenna are strongly biased to a region in direct contact with the antenna, ie the induction or near-field zone. The radiometric temperature measurement is, therefore, strongly biased to an object's surface region adjacent to the antenna.

(b) If there is a region of low dielectric loss material between the antenna and the lossy material of an object, such as a packaging air gap, wave modes propagating perpendicular to the

antenna measurement axis may be set up, dramatically reducing the radiometric coupling to the product.

(c) If an object is electromagnetically thin in the direction of the antenna measurement axis, radiation from sources beyond the object will be coupled into the antenna together with the object material radiation giving a false temperature measurement (eg fluorescent lighting microwave radiation).

(d) If there is a region of low dielectric loss material between the antenna and the lossy material of an object (as in (b) above), radiation from sources around the antenna and the object will enter the antenna, again leading to a false temperature measurement.

Thermal/Infra-red imaging also only gives a surface reading and has problems in penetrating water and grease.

It is also not possible to see through packaging for food.

It is an object of at least one embodiment of at least one aspect of the present invention to obviate/mitigate one or more of the disadvantages of the prior art.

It is also an object of at least one embodiment of at least one aspect of the present invention to provide a method of using natural microwave thermal radiation to measure the temperature of an object.

SUMMARY OF INVENTION According to a first aspect of the present invention there is provided an apparatus for measuring a temperature of an object comprising: means for establishing at least one (and preferably one) standing wave pattern of radiation emanating from the object, and means for coupling the radiation to a measuring radiometer.

Preferably, the measuring radiometer is a radiation signal measurement radiometer receiver wherein the

radiation temperature signal detected/measured by the radiometer is representative of substantially the temperature of at least a portion of the object.

It is preferred if one or more ports, probes or loops electromagnetically couple the means for establishing the standing wave pattern and the measuring radiometer. The position, size, shape, orientation and construction of the ports, probes or loops may determine the coupling to the electric or magnetic fields, or a combination of these fields, of the standing wave pattern.

Preferably, the means for setting up the standing wave pattern is an enclosed wall structure. Advantageously, the enclosed wall structure is substantially rectangular or cylindrical, or at least part conical or pyramidal.

Preferably, the radiation is in the microwave range.

Preferably, the microwave radiation has a frequency in the range of O. lGHz to 30GHz.

Preferably, the means for setting up the standing wave pattern (s) are made from electrically conducting material.

Preferably, the means for setting up the standing wave pattern have low-loss, high-wave impedance areas within them that provide electro-magnetic isolation.

According to a second aspect of the present invention there is provided a method of measuring the temperature of an object comprising the steps of: establishing a standing wave pattern of radiation emanating from the object, and coupling the radiation to a measuring radiometer.

According to a third aspect of the present invention there is provided a transport container including an apparatus according to the first aspect.

According to a fourth aspect of the present invention there is provided a transportation means including an apparatus according to the first aspect.

According to a fifth aspect of the present invention there is provided an apparatus for measuring a temperature of an object comprising: a member having an open cavity and a plate which are

moveable relative to one another so as to enclose the object within the cavity, wherein, in use, a standing wave pattern of radiation emanating from the object is set up within the cavity, and means for coupling the radiation to a measuring radiometer.

Preferably, the member comprises a sleeve portion.

According to a sixth aspect of the present invention there is provided a method of measuring a temperature of an object comprising the steps of: providing a plate for receiving the object; providing a member having an open cavity wherein, in use, the member and the plate are moveable relative to one another so as to enclose the object within the cavity, establishing a standing wave pattern of radiation emanating from the object within the cavity, and coupling the radiation to a measuring radiometer.

According to a seventh aspect of the present invention there is provided a transport container including an apparatus according to the fifth aspect.

According to an eighth aspect of the present invention there is provided a transportation means including an apparatus according to the fifth aspect.

According to a ninth aspect of the present invention there is provided a objection line including an apparatus for measuring a temperature of an object according to the fifth aspect.

BRIEF DESCRIPTION OF DRAWINGS Embodiments of the present invention will now be described by way of example only, and with reference to the accompanying drawings, which are: Figure 1 a schematic view of an apparatus for measuring a temperature of an object according to an embodiment of the present invention; Figure 2 a series of graphs of electric field

and power for 1 = 1, m =1 and n = 0, (with 1, m and n being integral numbers defining the form of standing wave patterns in three dimensions of a cavity of the apparatus of Figure 1); Figure 3 a series of graphs of electric field and power for 1 = 1, m = 1, and n = 1; Figure 4 an experimental plot of cavity response versus position across a floor of a cavity of a further embodiment of an apparatus according to the present invention similar to that of Figure 1; Figures 5 (a)- (b) schematic perspective views of an apparatus for measuring a temperature of an object according to a yet further embodiment of the present invention.

DETAILED DESCRIPTION OF DRAWINGS Referring initially to Figure 1, there is shown an apparatus, generally designated 5 according to an embodiment of the present invention, for measuring a temperature of an object or product such as a processed food product 14, and comprising: means for establishing a standing wave pattern of radiation emanating from the object, and means for coupling the radiation to a detector such as a measuring radiometer 11.

The means for establishing a standing wave pattern comprises an enclosed wall structure 6, including a cavity 10 forming a measurement region. The means for coupling comprises a coupling port 12 provided on the wall structure 6.

The coupling port 12 electro-magnetically connects the cavity 10 to one or more radiation signal measuring radiometer receivers 11 (forming an"antenna-cavity" structure). The cavity 10 is made from a suitable conductive material such as copper, copper plated steel and/or silver plated brass. Such materials are chosen

because they have good microwave surface conductivity.

Enclosing of the measurement region provides isolation from external sources of electro-magnetic radiation allowing proper measurement of an effective radiation temperature of an object or product 14 within the cavity 10. As can be seen from Figure 1, the object 14 is substantially centrally placed within the cavity 10.

A suitable cavity 10 size is typically 0.25m x 0.25m x 0.15m. However, a wide range of sizes, eg from a few millimetres to a few metres can be used depending on the frequency and mode of the measurement. It is, for example, in embodiments of the present invention possible to monitor a temperature of an object within the apparatus 5 which can comprise a transport container 20 on a transportation means 25 such as a lorry, or a train, or ship, or the like.

In use, to measure the temperature of the object 14, a microwave standing wave pattern of naturally occurring microwaves from the product 14 is set up inside the cavity 10. The microwaves are typically in the wavelength region 0.1 GHz to 30GHz.

An apparent radiation temperature"seen"by a radiometer 11 connected to the cavity 10 will be substantially that of the material of the object 14 within the cavity 10 weighted according to a weighting function applying to the whole source (cavity and product). By weighting function is meant that although the temperatures across the whole cavity 10 contribute to the obtained measured temperature, the temperature reading is biased due to the standing wave arrangement in the cavity 10 towards a certain region of the cavity 10. Therefore, by altering the properties of the standing wave pattern, different regions of the object 14 within different parts of the cavity 10 can have their temperature measured. In general, if the radiation losses of the object 14 are much larger than the losses in the enclosing/coupling antenna-cavity structure 10, the measured temperature will be close to that of the object 14 and the antenna-cavity temperature has only a small effect.

It is characteristic of low-loss conducting wall enclosing structures such as that shown in cavity 10 that the electro-magnetic radiation within them tends to form standing-wave patterns. The form of the weighting function within an antenna-cavity-which determines the spatial form of the radiation coupling of the source material to the radiometric signal measured by the radiometer 11-is determined by the standing wave pattern of the antenna- cavity and enclosed material combination. Antenna-cavity structures can thus be designed to support standing-wave patterns which provide weighting functions that give stronger or weaker radiation coupling to source material at different positions in the structure. This can then allow the temperature measurement process to be biased towards regions of a product of particular importance. An example of this would be the case of a disc-shaped object or product (eg a food product such as a quiche), where the cooking process heats the periphery more than the centre, but it is the central temperature that is particularly important for product quality and food safety.

Referring to Figure 2, there are shown field and power density profiles across the cavity 10 for a given frequency of radiation. It can be clearly seen that the signal is strongest in the centre of the cavity 10, and therefore, the temperature reading will be predominantly biased by the temperature in that region. To measure the temperature of the object 14, the object 14 should therefore be placed in the centre of the measurement region. The arrangement in Figure 2, therefore, provides a centrally weighted microwave temperature measurement.

It should be noted that in Figure 2 a uniform Z-field is used and there is therefore no variation in the vertical direction. If the Z-field was varied, a 3-D topographic surface would be obtained for the electric field and power density.

Figure 3 shows electric field and power density profiles for a further given frequency of higher frequency than the given frequency of Figure 2. In Figure 3, the

obtained temperature reading therefore tends towards a "quasi-uniform"response across the whole of the cavity 10 tending to give an"average"temperature for the object 14 with the cavity 10.

Referring to Figure 4, there is illustrated an experimental plot of cavity response versus position across a floor of a cavity of an apparatus according to a further embodiment of the present invention. As can be seen from Figure 4, the cavity of this embodiment exploits standing wave patterns to produce a strongly peaked central response.

Referring now to Figures 5 (a) and 5 (b), there is shown an apparatus, generally designated 105 according to an embodiment of invention, for measuring a temperature of an object 114 and comprising: a member 106 having an open cavity 110 and a plate surface 118 which are moveable relative to one another so as to enclose the object 114 within the cavity 110, wherein, in use, a standing wave pattern of radiation (such as naturally occurring microwave radiation) emanating from the object is set up within the cavity 110, and means for coupling 112 the radiation to a measuring radiometer 111.

The member 106 comprises a sleeve portion which fits over and encloses an object 114 residing on plate surface 118 of an electrically conducting material. The plate surface 118 may comprise at least part of a conveyor belt or the like of a production line 130.

In use, the plate surface 118 is moved in direction "A"until the object 114 is below the member 106. The member 106 is then lowered in direction"B"onto the plate surface 118. After a measurement has occurred, the member 106 is lifted and plate surface 118 moved on to a next object 114, and so on.

Figure 4 (a) shows member 106 initially held above a product 114 whose temperature is to be measured and is then lowered, as shown in Figure 4 (b), to be positioned around product 114. Product 114 sits on conducting plate surface 118. A standing wave pattern is set up within the cavity

110 such that temperature of the product 114 can be measured by radiometer 111. This nature of measurement is useful on a high speed production line as a measurement operation may only take around 0.5 seconds.

The form of radiation standing-wave pattern in an antenna-cavity structure can be strongly influenced by the positioning, orientation and form of radiometer coupling port 112 (a probe or loop may also be used). If two or more appropriate coupling ports 112,112' are provided, each can couple to a different antenna-cavity weighting function, and each of these ports 112,112'will then allow the measurement of a radiometric temperature having a different spatial weighting across the product 114. The radiometric signals at the coupling ports 112,112', can be measured sequentially or the two coupling ports 112,112', can be measured at the same time using a synchronised 180° out of phase source reference switching configuration (not shown) in the radiometer 111.

Radiometric temperatures measured with different spatial weightings are used to estimate temperature variations across a source object 114.

It will be appreciated that the embodiments of the present invention hereinbefore described, are given by way of example only, and are not meant to limit the scope thereof in any way.

For example, although the member 106 illustrated in Figures 5 (a) and 5 (b) is part conical or pyramidal, it will be appreciated that the member may be of any suitable shape, and may indeed be merely rectangular.

It will be further appreciated that the form of the radiation standing-wave pattern in an antenna-cavity structure is dependent on the frequency of the radiation typically in the region 0. lGHz to 30GHz. Radiometric temperature measurements made at different frequencies, even through the same coupling port 112, will in general have different weighting functions. For example, referring to the resonant frequency expression for a rectangular cavity the low frequency resonance corresponding to

standing wave modes designated by l, m, n = 1,1,0 can have a weighting function for an appropriately polarised electric field which is strongly peaked in the centre of the cavity; at higher frequency resonances around 1, m, n = 3, 3,1 etc, the weighting function will become relatively uniform cross much of the cavity. The antenna-cavity dimensions must be chosen so as to provide the appropriate resonances at suitable radiometer measurement frequencies. Depending on the material of the source object, the frequency dependence of the attenuation factor may also be used to obtain change in weighting function within the material. For example, if the source object material has a high-water content, the antenna-cavity could be dimensioned to support relatively high-order resonances producing near uniform weighting functions external to the source object, for a range of frequencies over which the radiation attenuation in the source object varied appreciably. These different frequency radiometric temperatures could be efficiently and accurately measured with a multi-frequency-reference radiometer.