JP 609 4244-A2 discloses a microwave oven and a specifically designed cooking vessel, which has been provided with thermo-colour fields arranged around a periphery of the round vessel. A reflection type light sensor is provided for detecting a change of colour of said thermo-colour. By means of this feedback information a heating operation of a beverage or foodstuff in said container may be controlled.
GB 20 52 731-A discloses in general terms a set-up of a temperature responsive device in which light from a light source is directed towards a sensor of which the reflective characteristics changes with temperature. Reflected light from the sensor and received by a receiver provides information about such changes by temperature.
U.S. Pat. No. 5,285,041 discloses an automated food vending system comprising a microwave oven provided with a code reader for reading a bar code printed on a package.
Navy Technical Disclosure Bulletin, vol.10, no.1, Sep. 1, 1984, Arlington, Va. USA, pages 75-79,XP002006334 T. R. Odgen "Liquid Crystal Temperature Measurement Device" discloses a measurement apparatus for detecting the temperature of surfaces coated with thermographic liquid crystals, which can operate in water.
U.S. Pat. No. 4,933,525 discloses a microwaveable container provided with a plurality of liquid crystal film display devices which change colour in response to a preset temperature.
A general problem when using optical sensing/reading devices of the actual type is to establish a sustainable security in reading. Mainly this may be referred to the fact that the information carrying differences in the received reflected light may be small and therefore difficult to detect. Possibly this may be solved by the use of a more complicated light receiver including means for a deeper analysis of the spectrum of the received light, changing when the thermo sensitive surface changes colour at a temperature increase, for example. However, this further complication and increased costs therefore are highly undesirable in a consumer type equipment like a microwave oven. Another, related problem is that an accurate positioning of the cooking vessel or container, more specifically the sensor or sensors thereof, is required in relation to the light source and light receiver in order to obtain the symmetrical arrangements disclosed in said references, i.e. transmitting a light beam of a given angular off-set in relation to a perpendicular of the sensor surface and receiving a reflected beam of the same but opposite angular off-set.
In order to maintain the reflective characteristic of the sensor surface it is generally necessary to provide the sensor surface with a transparent cover layer or coating. In case the thermo-colour has been realized by means of a liquid crystal, such a transparent cover is a pure necessity. The use of this cover has the consequence that the amount of light in the surface reflected light beam will substantially not change with changes of the underlying thermo-colour. This problem is specifically focused in case of disposable type containers or packages regularly having a coating of this type. Moreover, use of packages or containers of this type further aggravates said sensor positioning problem, because the container wall and thereby the sensor carrying surface may bulge, thereby disturbing the reception of a symmetrically reflected reading beam.
Another problem in case of disposable containers is that non-approved packages may wrongly be interpreted as approved ones, which may lead to an unintended start of the heating progress with consequent risks for the user. Still another user risk may occur if a user inadvertently places an empty approved-type container in the oven. Within seconds this may lead to development of smoke and open fire.
These problems have neither been discussed in the referenced prior art, nor solved thereby.