Background to the Invention Microwave ovens are commonly used in the catering industry and domestic kitchens for heating and cooking foods and liquids. A key drawback of microwave ovens is the unevenness of cooking, where some areas of the product to be heated become excessively hot and others remain unheated. This is due in part to variations of microwave absorbence of the foodstuff to be heated.

However, a significant part of the problem lies with microwaves themselves in the form of destructive and constructive interference.

Referring to Figure 1 herein, there is illustrated a commercially available prior art container for containing foodstuff during microwave cooking. The container 101 comprises a base 102, four walls 103,104, 105,106, a lid 107 attached to one of the walls 106, and closure means 108 attached to the lid 107.

The base 102, the walls 103,104, 105,106 and the lid 107 define a cavity in which foodstuffs can be placed for microwave oven cooking or heating. The container 101 is substantially transparent to microwave radiation, allowing incident microwaves to easily penetrate the container 101 and be absorbed by the foodstuffs contained within. It is known to locate a microwave-reflecting material 109 at the base 102 of the container 101. Any microwave radiation that is not absorbed by the foodstuff is reflected by the reflective material 109 back toward the food stuff, thereby improving the cooking or heating efficiency.

Prior art microwave reflector materials 109 typically comprise at least three layers. A substrate layer 110 comprises a dielectric material, on which a reflecting layer 111 is formed. The reflecting layer 111 typically comprises a

metal such as aluminum foil. A substantially microwave-transparent layer 112 is formed on the reflecting layer 111 to protect the reflecting layer 111 from atmospheric conditions or mechanical damage. Incident microwave radiation 113 is reflected from the reflecting layer 111. The resultant reflected microwave radiation 114 may have slightly lower energy than the incident microwave radiation 113 due to absorbence by the reflecting layer 111 or the protective layer 112, or partial transmission through the reflecting layer 111.

GB 2292509 discloses a cooking vessel for microwave cooking. The floor of the vessel has a many-sided body unit which refracts incident microwaves back into the cooking vessel, thereby creating a concentration of microwave energy in the interior of the vessel. The many-sided body unit is concave with respect to the cooking vessel to ensure that the incident microwave radiation, when refracted, is concentrated within the vessel. The concentration of microwave radiation leads to localized hot spots within the vessel to increase heating efficiency in a required region.

EP 0161739 discloses a microwave heating package and method. The microwave heating package comprises a cover which does not transmit reflected energy, thereby concentrating incident microwaves on the package inside the package, and improving the efficiency of heating of the food stuff contained within the package. However, the microwave radiation is not dispersed, and hot spots can still arise due to constructive interference of both incident and reflected microwave radiation.

EP 0000797 discloses a microwave energy moderator for partially attenuating microwave energy to achieve more uniform cooking of food stuffs in microwave ovens. The moderator is substantially comprised of a pre-determined array of alternately disposed portions of microwave reflective material and portions which are substantially transparent to microwave energy. However, the construction of the device is complex, requiring an aluminum foil sheet to have a predetermined set of perforations disposed throughout it prior to enclosing the aluminum sheet in thermoplastic layers.

WO 92/03358 discloses a container for use in microwave oven heating.

The container comprises microwave transparent wall portions, and at least one wall portion having a microwave field modifying coating which is disposed to form an array of discrete, electrically conductive pattern elements. The processing is complicated because discrete microwave reflecting areas must be applied to a wall of the container.

US 5,910, 268 discloses microwave packaging structures comprising active elements which modify the heating of foodstuffs. The active elements may be loop slots or strips and are constituted so as to become resonant or non-resonant during microwave heating. This disclosure requires cutting loops and annular slots into a package, the slots and loops being configured to provide constructive interference for intensification of microwave heating of the foodstuff in a predefined region, and destructive interference to reduce the microwave heating in the foodstuff in a separate predefined region.

US 5310976 discloses a microwave energy intensifier comprising an array of dots of electrically-conductive material of microwave reflecting thickness supported on a dielectric substrate. This disclosure is not designed to disperse microwave energy, but to intensify it. Furthermore, it requires the processing step of locating small dots of aluminum or other metal on a suitable dielectric substrate.

US 4,866, 234 discloses a microwave container which comprises one or more electrically conductive plates and/or microwave transparent apertures for generating a microwave field pattern having a higher order than that of the fundamental mode to the container. That is to say, the electrically conductive plates and apertures modify microwave radiation to form constructive interference of microwave radiation, thereby creating hot spots within the container for more rapid heating.

EP 0212936 discloses reflective apparatus for microwave cooking. A plurality of cells are installed within a microwave oven, to reflect microwaves and improve temperature uniformity of food heated in the oven. Each cell includes a reflector which moves in response to a temperature sensor and varies the concentration of reflected microwaves incident on the food. This dynamic system allows for more uniform heating within the microwave oven itself, and requires discrete moveable reflecting cells. This is designed to dynamically concentrate microwave radiation in cool spots.

JP 6196257 discloses a microwave oven with a concave base. The concave base reflects microwave radiation to increase the heating efficiency and increase the number of hot spots within the microwave oven due to focusing of the reflected microwave radiation.

Of the above disclosures, only EP 0000797 and WO 92/03358 seek to disperse incident microwave radiation thereby creating more uniform heating, whereas most of the disclosures seek to concentrate microwave energy in particular spots thereby increasing the heating in those spots.

The inventor has realised that the problems with prior art devices for dispersing microwave radiation include complicated production processes, involving layering discrete microwave reflecting regions on a dielectric substrate, or perforating a microwave reflector material prior to encapsulating it in protective materials.

Summary of the Invention According to the present invention there is provided a device for dispersing microwave radiation. Incident microwave beams are reflected from the device in the form of spherical wave fronts called planes. These planes will cover more localised areas and will do so more evenly. Secondly the planes will have less intensity at a given point from the initial contact than would the incident microwaves, thus when interference occurs and the wave fronts superimpose the

intensity will represent the sum of those two waves at that point, meaning the reflective planes all have less intensity than the initial waves.

According to a first aspect, there is provided a device for dispersing microwave radiation during microwave oven heating comprising: a reflective material configured to reflect incident microwave radiation; and at least one raised portion located on said device; wherein said at least one raised portion is configured to substantially disperse reflected microwave radiation.

Preferably, the device for dispersing microwave radiation during microwave oven heating comprises: a dielectric substrate; wherein said reflected material substantially covers a surface of said dielectric substrate.

Preferably, the device for dispersing microwave radiation comprises a convex bulge on a surface of said device.

Preferably, the convex bulge is substantially hemispherical.

Preferably, the raised portion has a maximum dimension in a range of 2 mm to 2000 mm.

Preferably, a plurality of raised portions are disposed on said device in a predetermined pattern.

Preferably said device is located in a container, said container being used to hold food or liquid during microwave oven heating.

Preferably said device is formed integrally with said container.

Preferably said device is located at a base of said container.

Alternatively, said device is located at a wall of said container.

Alternatively said device is located in a microwave oven.

Preferably said device is formed integrally with an interior wall of said microwave oven.

Preferably said reflective material comprises an electrically conducting material.

Preferably said electrically conducting material comprises aluminum.

Alternatively said electrically conducting material comprises steel.

Preferably said device further comprises a protective layer formed on said reflective material, wherein said protective layer is substantially transparent to microwave radiation.

According to a second aspect there is provided a method of manufacturing a device for dispersing microwave radiation during microwave oven heating comprising: forming a reflective material configured to reflect incident microwave radiation; and forming at least one raised portion on said device;

wherein said at least one raised portion is configured to substantially disperse reflected microwave radiation.

Preferably, said method comprises: forming said reflective material substantially over a surface of a dielectric substrate.

Preferably said raised portion comprises a convex bulge on a surface of said device.

Preferably said convex bulge is substantially hemispherical.

Preferably, said method comprises: forming each said raised portion of said plurality of raised portions with a maximum dimension in a range of 2mm to 2000mm.

Preferably, said method comprises: forming said plurality of raised portions on said device in a predetermined pattern.

Preferably, said method comprises: locating said device in a container, said container being used to hold food or liquid during microwave oven heating.

Preferably, said method comprises: forming said device integrally with said container.

Preferably, said method comprises: locating said device at a base of said container.

Alternatively, said method comprises: locating said device at a wall of said container.

Alternatively, said method comprises: locating said device in a microwave oven.

Preferably, said method comprises: forming said device integrally with an interior surface of said microwave oven.

Preferably said reflective material comprises an electrically conducting material.

Preferably said electrically conducting material comprises aluminum.

Alternatively said electrically conducting material comprises steel.

Preferably, said method comprises: forming a protective layer on said reflective material, wherein protective layer is substantially transparent to microwave radiation.

According to a third aspect there is provided a method of manufacturing a device for dispersing microwave radiation comprising:

forming a substantially deformable material, said material comprising a reflective material configured to reflect incident microwave radiation; and feeding said deformable material through a pair of rollers, said rollers located in opposition to each other, a first roller of said pair of rollers having a series of raised portions and a second roller of said pair of rollers having a series of depressions, said raised portions being configured to substantially interconnect with said depressions; whereby said substantially deformable material is deformed to comprise a plurality of convex bulges in a surface of said substantially deformable material.

Brief Description of the Drawings For a better understanding of the invention and to show how the same may be carried into effect, there will now be described by way of example only, specific embodiments, methods and processes according to the present invention with reference to the accompanying drawings in which: Figure 1 illustrates schematically a prior art container for microwave heating of foodstuffs.

Figure 2 illustrates schematically an elevation cross-section view of a single raised portion of a device for dispersing microwave radiation.

Figure 3 illustrates schematically an elevation cross-section view of the interaction of incident microwave radiation with a raised portion of a device for dispersing microwave radiation.

Figure 4 illustrates schematically an elevation cross-section view of the interaction of incident microwave radiation with two raised portions of a device for dispersing microwave radiation.

Figure 5 illustrates schematically a perspective view of a device comprising a plurality of raised portions for dispersing microwave radiation.

Figure 6 illustrates schematically plan views of predetermined patterns of raised portions on a device for dispersing microwave radiation.

Figure 7 illustrates schematically a perspective view of a container for microwave heating of foodstuffs configured to disperse microwave radiation.

Figure 8 illustrates schematically a perspective view of a container for liquids configured to disperse microwave radiation.

Figure 9 illustrates schematically a perspective view of a microwave oven comprising a device configured to disperse microwave radiation.

Figure 10 illustrates schematically an elevation view of apparatus for forming a device configured to disperse microwave radiation.

Detailed Description There will now be described by way of example a specific mode contemplated by the inventors. In the following description numerous specific details are set forth in order to provide a thorough understanding. It will be apparent however, to one skilled in the art, that the present invention may be practiced without limitation to these specific details. In other instances, well known methods and structures have not been described in detail so as not to unnecessarily obscure the description.

Referring to Figure 2 herein, there is illustrated schematically a single raised portion of a device for dispersing microwave radiation. The device 201 comprises a substrate layer 202, a microwave-reflecting layer 203 formed on the substrate 202, and protective layer 204 formed on the microwave-reflecting layer 203. A raised portion 205 is also shown.

The substrate layer 202 comprises any suitable dielectric material. Various materials may be used, including plastics and cardboard. The microwave- reflecting layer 203 comprises any electrically conductive material with a suitable thickness for substantially reflecting incident microwave radiation. Such materials include aluminum foil, aluminum flakes, or other metal foils, flakes or pressed steel. The microwave-reflecting layer 203 is formed substantially over an entire surface of the substrate layer 202, thereby eliminating the need for applying discrete regions of microwave reflecting material to the substrate, or using a perforated microwave-reflecting layer. The protective layer 204 comprises a material which is substantially transparent to microwave energy and has a relatively low dielectric loss factor, and which is substantially impervious to gases and liquids which may be released during microwave oven heating of a foodstuff.

Examples of such materials are polypropylene or polyethylene.

The device is manufactured by embossing a flat laminate sheet comprising a substrate layer 202, a microwave-reflecting layer 203 and a protective layer 204 to form the raised portion.

Referring to Figure 3 herein there is illustrated schematically an elevation cross-section view of the interaction of incident microwave radiation with a raised portion of a device for dispersing microwave radiation. The device 201 comprises a raised portion 301. When microwave radiation 302 is incident on a flat portion 303 of the device 201, it is substantially reflected to give reflected microwave radiation 304. An intensity of the reflected microwave radiation 304 is slightly lower than an intensity of the incidence microwave radiation 302. The difference in intensity is between the incident microwave radiation 302 and the substantially reflected microwave radiation 304 are accounted for by absorption by the device 201 and transmission through the device 201.

Interference patterns caused by both incident and reflected microwave radiation given rise to regions of high intensity microwaves, and regions of low intensity microwaves. In regions of high intensity microwaves, food will absorb more energy and heat up more quickly. These regions are known as hot-spots.

In regions of low intensity, food will absorb less energy, and therefore not heat up so quickly. These regions are known as cold-spots.

Where microwave radiation 305 is incident on a raised portion 301 of the device 201, incident microwave radiation 305 is reflected in many directions owing to the curvature of the raised portion 301. This results in a spherical wave 306. An intensity of the spherical wave 306 at any given point is lower than an intensity of the incident microwave radiation 305. The intensity of the reflected microwave radiation at any given point is therefore lower around a raised portion 301 than around a flat portion 303 of the device 201, but it is also more evenly dispersed and less likely to give rise to hot or cold spots.

Referring to Figure 4 herein, there is illustrated schematically an elevation cross-section view of the interaction of incident microwave radiation with two raised portions of a device for dispersing microwave radiation. The device 201 comprises of a first raised portion 401 and a second raised portion 402. When microwave radiation 403 is incident on the first raised portion 401, a spherical wave 404 is propagated away from the raised portion 401. Similarly, when microwave radiation 405 is incident on the second raised portion 402 a spherical wave 406 is prorogated away from the second raised portion 402. Some interference between the first spherical wave 404 and the second spherical wave 406 may occur where the two spherical wave fronts 404,406 overlap 407.

However, as the intensity of each spherical wave at any given point is lower than the intensity of reflected microwave radiation from the flat portion of the device 201, the degree of constructive or destructive interference is less pronounced, and any hot or cold spots that develop are minimised.

Referring to Figure 5 herein, there is illustrated schematically a perspective view of a device comprising a plurality of raised portions for dispersing microwave radiation. The device 501 comprises a plurality of raised portions 502. The raised portions 502 are disposed on the device 501 in a predetermined pattern.

The raised portions are formed by embossing the device 501 using a die 503 that has protrusions 504 corresponding to the desired raised portions 502. By

pressing a flat sheet comprising a substrate layer, a microwave-reflecting layer and a protective layer against the die, 503, the protrusions 504 deform the flat sheet to form raised portions 502.

Each raised portion 502 comprises a convex bulge on a surface of the device 501. It will be apparent to those skilled in the art that other shapes may also be used for the raised portion. Other shapes having a rounded profile, for example tetrahedral raised portions, will also reflect incident microwave radiation.

Each facet of the raised portion will substantially reflect incident microwave radiation in a particular direction, and therefore raised portions with a greater number of facets will lead to a greater dispersal of the reflected microwave radiation. It has been found that the shape most suitable for dispersal of microwave radiation is a convex bulge in the shape of a portion of a sphere, for example a hemisphere. It has been found that a maximum dimension of between 2 and 1000 mm is suitable for each raised portion 502.

Referring to Figure 6 herein, there are illustrated schematically plan views of predetermined patterns of raised portions on a device for dispensing microwave radiation. Figure 6a shows the raised portions 502 on the device 501 disposed in a regular manner, with each raised portion 502 at substantially 90° to each adjacent raised portion 601, and evenly spaced from each adjacent raised portion 601.

Figure 6b shows each raised portion 502 on the device 501 disposed in a more closely packed arrangement whereby each raised portion is at substantially 60 ° to the closest adjacent portion 602 with respect to an edge of the device 501.

It will be appreciated that other predetermined patterns may be used.

Furthermore, different patterns and variations in pattern can be used to alter the dispersal of reflected microwave radiation in different regions.

Referring to Figure 7 herein, there is illustrated an embodiment of the device used in a container for microwave oven heating of foodstuffs. The container comprises a base 701, walls 702,703, 704,705 attached to the base

701, a lid 706 attached to a wall portion 705, and fastening means 707 attached to the lid 706. The base 701, lid 706 and walls 702,703, 704,705 define a cavity in which foodstuffs to be heated in a microwave oven are located. A device 708 for dispersing microwave radiation is located substantially on the base 701 of the container prior to locating foodstuffs in the cavity of the container. When the container and foodstuffs are heated in a microwave oven, the device 708 disperses reflected microwave radiation more evenly, thereby cooking the foodstuff more evenly and minimizing hot and cold spots.

Alternatively, the device 708 can be formed integrally with the base 701 of the container. It has been found that the device 708 is ideally located at the base 701 of the container, although a device for dispersing microwave radiation could be located or formed integrally with any of the wall portions 702,703, 704,705 or the lid 706.

In an alternative embodiment, the device can be incorporated into a container for heating liquid, such as hot beverages or soup. Referring to Figure 8a herein, there is illustrated schematically a perspective view of a container for heating liquids in a microwave oven comprising a device for dispersing microwave radiation. The container 801 comprises a cylindrical wall 802 and a base (not shown). The cylindrical wall 802 and base define a cavity for containing liquid. The cylindrical wall 802 has an internal surface on which raised portions 803 are disposed in a predetermined pattern. The raised portions 803 comprise a microwave-reflective material. When the container 801 is filled with liquid and subjected to microwave radiation, the raised portions 803 disperse the incident microwave radiation to more evenly heat the fluid contained within the container 801. This prevents the formation of layered hot and cold spots that may otherwise lead to scalding from the liquid contained within the container 801.

Referring to Figure 8b herein, a raised portion 803is shown in a cross- section elevation view. It comprises a first support layer that is substantially transparent to microwave radiation 804 having a raised portion. A microwave reflective layer 805 is located on the support layer 804. A second support layer

806 is formed on the first support layer 804, thereby enclosing the microwave reflective layer 805.

An alternative arrangement is shown in Figure 8c, where the microwave reflective layer 805 is formed between the first support layer 804 and the second support layer 806, and extends throughout the wall 802 of the container 801.

Referring to Figure 9 herein, there is illustrated schematically a perspective view of a microwave oven 901 comprising a device configured to disperse microwave radiation. The device is located on an interior surface of the microwave oven, for example the base, walls or ceiling of the heating chamber.

The microwave oven 901 comprises a base 902 in the heating chamber.

The base 902 comprises a plurality of raised portions 903 configured to disperse reflected microwave radiation.

In an alternative arrangement, the microwave oven comprises at least one raised portion configured to disperse microwave radiation located in the walls of the microwave oven 901. A raised portion 904 is located in a side wall of the microwave oven. A raised portion 905 may additionally or alternatively be located in a rear wall of the microwave oven. These raised portions 904,905 may have a maximum dimension substantially the same as a maximum dimension of the internal walls of the microwave oven 901. However, the curvature of the raised portions 904,905 is suitable to disperse reflected microwave radiation.

Referring to figure 10 herein, there is illustrated schematically an elevation view of apparatus for forming a device configured to disperse microwave radiation. The device comprises a first roller 1001, a second roller 1002. The first roller 1001 comprises a series of raised portions 1003, and the second roller 1002 comprises a series of depressions 1004. The raised portions 1003 and the depressions 1004 are configured to substantially interconnect.

A sheet of substantially deformable material 1005 comprising a reflective material configured to reflect incident microwave radiation is fed through the rollers 1001,1002 in a direction shown by the arrow 1006. The rollers rotate in opposition to each other in directions shown by the arrows 1007,1008. As the sheet 1005 passes through an interconnecting raised portion 1003 and depression 1004, it is deformed so as to form a convex bulge 1009.

After passing through the rollers 1001,1002, the sheet 1005 comprises a series of convex bulges 1010, which are configured to disperse microwave radiation.