Electric heating – Heating devices – Combined with container – enclosure – or support for material...
Patent
1991-09-18
1993-02-02
Walberg, Teresa J.
Electric heating
Heating devices
Combined with container, enclosure, or support for material...
219465, H05B 374
Patent
active
051839970
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a heating apparatus for cooking foods by means of at least one radiation source which is directed at a cook site for accommodating uncooked food or a container for food to be cooked, and which emits optical radiation with a visible content and an infrared content. Between the radiation source and the cook site a filter is disposed having a support body made of a material substantially transparent to all the radiation, which is provided with an interference layer system which reflects the visible content of the radiation at least predominantly back to the radiation source and passes the infrared content of the radiation at least predominantly to the cook site.
A heating apparatus of this kind, configured as a cook plate, is state of the art. In this apparatus what is involved is heating a cooker (cooking pot) placed on the cook plate with the infrared content of the optical radiation without letting the intensely visible part of the optical radiation penetrate into the kitchen space. The boundary here in question, between the visible and the infrared content of the optical radiation is at about 750 nm. The support body and interference layer system form a filter with a spectral transmission characteristic whose center lies at the above mentioned 750 nm.
Interference layer systems are characterized by alternating sequences of alternating layers of high refraction and low refraction. In the case of the system pertaining to the state of the art, titanium dioxide with an index of refraction of about 2.25 and silicon dioxide layers with an index of refraction of about 1.45 are used as the layer materials. In order then to obtain a sufficient division by the filter of a corresponding spectral transmission characteristic, it is necessary to apply a total of 43 alternating layers of the materials mentioned. This is a comparatively very difficult process which has to be performed in a vacuum, preferably by depositing the said layer materials by electron beam vapor deposition. Manufacture, therefore, is accordingly expensive.
The smaller the number of layer is, the better is their tension free build up and the firm bonding of these layers to the support body, the so called substrate, and to one another.
SUMMARY OF THE INVENTION
It is therefore the object of the invention to offer a heating apparatus of the kind described above, whose filter has a substantially simpler layered structure and gets by with a very much smaller number of individual layers, while the layer system at the same time is very largely free of internal tensions and has good adherence of the layers to one another and to the substrate (support body).
The solution of the stated problem is achieved in accordance with the invention in the heating apparatus described above by the fact that the interference layer system consists of an alternating sequence of layers of silicon and silicon dioxide, whose physical layer thicknesses in the case of silicon are between 20 and 50 nm in at least the predominant part of the Si layers, and between 80 and 110 nm in the case of the SiO.sub.2, at least in the predominant part of the SiO.sub.2 layers.
On the basis of the selection of the layer materials and the dimensions of the individual layers it is possible to make do with a very much smaller number of pairs of layers. With even only 5 layer pairs, i.e., a total of 10 individual layers, a quite outstanding filter characteristic is produced, as will be further explained with the aid of a transmission curve in the detailed description. The layer system according to the invention is very largely free of internal tensions.
The refractive index for silicon in the infrared range is approximately n=3.5; the refractive index for SiO.sub.2 is n=1.45. The small number of individual layers, among other things, is to be attributed to this very great difference of the refractive indices. To this is added, however, the relatively very great absorption capacity of silicon for optical radiation in the visible range. A high
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patent: 1954128 (1934-04-01), Heyroth
patent: 2601011 (1952-06-01), Wilcox
patent: 4892997 (1990-01-01), McWilliams
patent: 4902876 (1990-02-01), Mewissen
patent: 5026970 (1991-06-01), Buttery
Leybold Aktiengesellschaft
Walberg Teresa J.
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