US2920174A - Microwave ovens - Google Patents

Description

Jan. 5, 1960 D. B. HAAGENsEN MICROWAVE OVENS Filed June 28, 1957 F162 F763 F7614:

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tllli.. Illu Duane B. Haagensen, Wayland, Mass., assigner to Raytheon Company, a corporation of-:Delaware Application June 28, 1,957, ySerial No. ,668,748 19 Claims. (Cl. vH9-710.575)

This invention relates to microwave electronic heating apparatus, and more particularly, to a microwave oven having at least a portion Vcapable of transmitting optical or infrared radiation, or both, while at the same time being either partially or totally reectiveof microwave energy. g t

In microwave electronic ovens the microwave energy is contained within the cooking compartment by means of metal walls or screens. Such construction renders it dicult for the operator to observe properly the' manner in which the cooking process is proceeding. The use of wire or screen windows further presents a'problem of contamination when vapor or foodstuffs collect on the irregular window openings. An optically transparent substance, such as glass, obviously is unsuitable since it presents such low losses to microwave energy that leakage of microwave energy from the cooking compartment becomes prohibitive.v One of the objects of this invention is to provide a microwave oven with one or more optically transparent windows which also is substantially opaque to microwave energy. This object may be achieved by constructing the window of glass, plastic, ceramic, and the like, which is capable of passing readily the visible spectrum coated on one or both opposed surfaces thereof with an extremely thin layer or film of a low resistance metal or a coating comprising a combination of metals, each of which have diierent transmission response to portions of the visible spectrum. This thin metallic coating should be sufficiently thick to insure adequate reilcction of microwave energy therefrom without cutting down unduly the transmission therethrough of light energy. An extension of the above technique is to make one or more of the oven walls, or even the entire microwave oven enclosure, of coated glass.

Another object of this invention is to provide the microwave oven with an optical transparent window which is coated so that microwave energy is prevented from leaving the cooking compartment, while at the same time the coating partially absorbs microwave energy. When the oven is energized without any load, or with a very small load, this object is achieved by proper choice of material and thickness of the metal layer. For example, a metal of relatively high volume resistivity, such as Nichrome, could be used for this purpose so that the power absorbed in the metal coating is correspondingly high. Since the surface resistivity of the metal layer increases inversely as its thickness, `the amount of energy dissipated by the metallic coating may be increased as the coating is made thinner. The amount of energy absorbed by the metallic coating at low load or the degree of opacity to microwave energy, as the case may be, may be increased further by using more than a single layer of metal, each layer being separated from the adjacent metal layer by a layer of dielectric or low loss material, such as an oxide or a ceramic of the alumina type, since each layer will contribute to the total impedance totransmission ,of energy.

aram- Ffatented Jan. 5, 1960 .2 In addition, when the thickness of the separating layers between adjacentmetal layers is made any oddnumber of quarter wavelengths at the microwave frequency, the microwave energy at each of the `various metal layers effectively tends to cancel and the composite layer acts like a single layer of decreased thickness. 4r[-'his arrangement may also 'be used simultaneously to provide proper optical filtering action for best light transmission. i While the effective thickness of the composite layer `may be decreased `to effect a decrease in Vmicrowave transmittance, it is also possible to increase the microwave transmittance by making the thickness of the separating layers any odd number of half wavelengths, whereuponreinforcement of the yenergy at each of the metal layers tends to occur and the equivalent of a single layer of increased thicknessy may be obtained. The technique of combining more than a single metal layer with a space between layers of any odd number of quarter wavelengths is also useful to` obtain a coating of practical thickness when asingle layer having the desired transmission characteristic would be below the lower practical limit `for coating techniques, namely, about one-hundredth of a number of half wavelengths becomes useful. I y

Another object of this invention is to provide means in a microwave oven which also supplies infrared energy to the load contained withinithe heating compartment whereby the radiant energysource may be placed out-` side the heatingv compartment. `Infthis manner, interference with the microwave ields within the heating chamber and absorption of microwave energy by the infrared heating source may be'eliminated; 1 To achieve this result, the wall of the oven facingthe radiant energy source comprises a coated object similar to that already described, except that the coating is capable of transmitting energy in the infraredv portion of the electromagnetic spectrum while simultaneously presenting a substantial barrier to transmission of microwave energy. `The coating material and thickness will determine the relative transmission of both-microwave energy and infrared energy and will be chosen such that the best compromise is achieved between infrared transmission through the coated material and microwave reection from the coated material. The technique of' applying successive layers, previously referred to, maybe resorted to in achieving this result.

Another object of the invention is to provide a microwave oven with a resonant slot mode stirrer consisting of a metallic layer having resonant slots therein, said metallic layer being coated on a homogeneous surface of a low loss material, such as plastic or glass. Energy from the microwave source may pass through the energy-radiating slots vin the mode stirrer and the coating on the stirrer may be-constructed of the proper material and thickness to permit transmission therethrough of infrared energy should an infrared sourcebe employed. Y

The mode stirrer may be-designed to absorb a portion of the microwave energy at no-loadfconditions by making the metal coating of the proper material and thickness, in line with remarks previously made in connection with the no-load protective feature.

Another object of the=invention isto provide a-microwave oven door having a capacity seal composed of a portion of the metal oven body, themetal'layer coated on the optically transparent basegmemberI and a low loss film or. strip interposedfbetween the metal layer ,andthe oven body. The capacitive reactance representedfby this, capacitor insures a proper closure, R.F.wise, of the oven, even though there may be irregularities in the metalttl-metal contact between theedge ofpthetcyen; d QQr MAQ.

'wall20 of the oven. Y .1f f;

Microwave energy having a, frequency ofthe Qrderof the metallic oven wall. This capacityseal may supplement the usual metal-to-metal door seal or may be used even in the absence of such metal-to-metal Contact.

The foregoing, and 'other objects-of the inventionwill be best understood from the following description of some embodiments thereof, referencebeing had to the accom-l panying drawings, wherein: f A

Fig. 1 is a vertical Vsection View through a microwave oven embodying the invention; v j Figs. 2 to 4 are section views representing. possible construction of either the shelf or the window in the oven Y ofFig. 1; f Fig. 5 is a vertical section view of a microwave oven which does not contain an i n frared-heat ing unit; f

Fig. 6 is a vertical section view of an embodiment lof the oven showing a lslightly/different door construction and energy feed means than shown in Fig. 1;

Fig. 7 is similar to Fig. 6 exceptthat a different mode ing in the top wall 37 of the oven 10 and which extends through a bearing 38 fixed to the top surface or shelf 40 ofthe microwave heating compartment 15. The stirrer motor may be supported in any desired manner,'such as from the wall 37 .of the oven. The mode stirrer may be located within the microwave heating compartment, as shown in'Figs. 1 and 5, or external thereto, as indicated in Figs. 6 and 7.

As shown in Figs. 1, 6 and 7, the space 42 between the toptoven wall 37 and the shelf, viz., the top surface 40 of the heating compartment 15,V also Vmay contain an infrared heater unit 44 suitably attached to the portion of the back'wall 20 ofthe oven lying above the top surface 40" of heating compartment, 15. The heater stirrer isr shown and. that-, the infrared Vheating unit'is mountedabove'the mode V stirrer, rather kthan below the mode stirrer, as in Fig..6;v

Fig. 8 is a view of amode stirrer-of ,the type shown in Y.

Figs. 1, 5, and 7; and I Fig. 9 is a toprplanV- viewofl'an infrared VlieratingguiiitV which'is used in the ovens` shown in Figs. l, 6, and '7.,

Referring to the drawings in which like elements vare indicated by the same reference numerals, there Vis illustrated a microwavey ovennltl constructed of aY suitable 2 500V megacycles Vper second is derived, vfrom Va suitable source, such as a magnetron 21, 'and includes, forY example, an electrically-conductive .evacuated Aenvelope 22 provided with several radially-disposed anode vanesY 23, a cathode 24, and means indicated by the reference character B and the cross within, a circle, for-establishing a magnetic field directed transverse' to-the electron path Y between cathode 24 and anode vanes 23. The energy from the magnetron 21 is delivered* to a rectangular waveguide 25 through a coaxial transmission line 26 whichV is coupled inductively to the magnetronby'a loop 27. 'Ihe inner conductor 28 of transmissionfline 26 eX- tends into the interior'of waveguide 25 near one end thereof through an aperture in one wall, and serves as an exciting probe for the waveguide. The waveguide 25 is coupled to the interior of the microwave heating compartrnent 15 of oven 10 through an aperture'29 provided in the rear wall Y of the oven, one end of the waveguide -being secured to the rear Wall 30 by fastening meansl. Alternately, the microwave energy Vfrom the high frequency source may be fed into thejtop ofthe oven through a slot29, as explained later in connection with Figs. 6 and 7, and the output coupling means for element 44 preferably takes the form ofV a serpentine coil 45 Whose loops maybe supported further by a supf port rod 46 placed under the coil loops 45 and having the. ends thereof .attached tooppositek side 'walls 'of the oven.' It Vshould be understood :that the top wall of the oven'maybeprovidedfbythe shelf `40` and that the portion ofv oven 1`0"ly ing,above this shelfv may be'V omitted,

:in Awhich cas'exthe heater;unit144co11ld bel mountedin Vseveral ways, including by means-of a-bracket attached tojone of the walls of the microwave )heating compartment. Theppurposefof the heaterfunit'z44isto provide means for browning the surface .ofthe food mass in container 12 and acts :in conjunction withmicrowave heating to improvetheappearance yof thefood mass. [The heater unit'44,of course, may be eliminated, as shown in Fig. 5 in cases Where absence'of :external crusting or browning isV not considered objectionable.. i

.The lovendoor Y17 includesv a generally rectangular -frame' 48in which is fitted acomposite viewing window 50 comprisingjan optically transparent base member 51, Y

such Las glass, LuciteY or the like,'at least one surface of which'is Vcoated with a very thin metallicv layer or film 52.'V Thethickness of-,film 52V has been` greatly exaggerated in Fig. 1, as well asinother figures ofthe drawing, forV theV sake of yclarity. A typical cross section of the window A,50.,isshown in Fig. 2.V As .shown in Fig. 2, the metal' film 52 may' be icovered with a protecting layer 53, whichjprefe'rably` isa low loss. dielectric, such as an yoxide or. a Vceramic of the alumina type. For the Y sake of clarity,fthis protective film 53 has been elimi- Y at the same time-.preventing substantial loss of microthemicrowave energy source maybe a waveguide 32,

rather'than a coaxial line, also as indicated'inFigs. 6

and 7.

In order 'to provide more uniform heating, the field distribution of the microwave energy, within the oven may be continuously varied by a so-called mode stirrer 33, which may be a fan-type stirrer, as indicated in Fig. 6, or a stirrer consisting of a coated, slotted disk, such as shown in Fig. 8 and described in more detailV subsequently. The lstirrer- 33 is drivenby'a motor 34 either directly or through a speed reducer. The stirrer`33 is mounted on avv motor shaft 35 which protrudes through an open- Wave energy from the heating compartment 15 by way of transmission of microwave energyV through the window. 'The thin-metallic film 52 of vwindow 50 acts-to reflect microwave energy imp inging upon it in thesame mannerras from the metal walls ofthe heating compartment V15. YThe film 52,1`however, is Lsufiiciently thin to yallow a substantial portion of vthe'optical spectrum to be transmitted.v A compromise between .optical transmissivity andl microwave reflectivity of the metal film must be madeV in. proper design of the lm thickness and film material.

It has been foundA that a more uniform transmission over the optical frequency range may be Vobtained by using-more than onelayer of metal, each ofthelayers preferably being spaced apart as by a thin dielectric layerVY 54, each metallayer 52 being 'of a different material. in this-manner, a resultant of the-individual frequency transmission oharacteristic of `the various metals may be Vobtained so that the appearance-of the contents of container 12 is natural andnot distorted Yby virtue of the predominance of transmission through the window of any one color or colors. `The arrangementfor achieving this effect may be asfshown in Fig. 3, where the several layers 52 of.metal' 'are' deposited on `the glass base member'V 51. The metal layersV may be' separated by spacci' layers 54 offa dielectric material which may be ,ofvthesame material as? used vforY the protective coating 53. One example of a'composite layer struc-tureis a combination of successive metal ylayers of-chrinim, copper and chromium.

An alternative door arrangement `is shownr inA Fig. `t5 wherein a capacitive door seal isernploye'd. V'The door 17 of Fig. 6 is provided with van optically'transparent member 51, such as glass, coated with a thin'metallic lm 52, like the window "ofFig, 1. llaye'r `55"*of fa low-loss dielectric material, Vsuch as"analurriifia layer, may be disposed'incontact with metal rfilm 52 in vsuch a manner that'a portion l`of the dielectric material contacts the front oven wall'2l) :whenth'e dooriis closed. The dielectric material y55 may take the'fo'r'm Eof athin sheet which is secured to the windowby appropfiate fastening means, or it may be a thin'tilrn of plastic which is sprayed in liquid form over Vthe-metal iilm'SZ and allowed to harden. The successive `layers 52 and 55, together with the portion of oven'wall S2 contacting layer 55, constitute a capacitor fwhich tasextremely low impedance at microwave frequcnceathereby insuring an effective closurel of vthe oven access aperture 19 when the door 17 is in rthe closed position, even `in spite of possible irregularities in the metal-to-rnet'al contact between the edge ofthe door frame and theouter surface of wall 20 of the oven.

In applications using infrared heating for browning purposes, iti s `desirable to mount theinfrared'heating unit 44 external to the microwave heatingcompartment 15 to insure against interference of theheating unit'wi'th microwave eld inside the heating compartment and against absorption of the Vmicrovtave'v energyby the heat-A ing unit. Although thisV problemhas been alleviated by coating the heating unit, obviously it is simpler to make use of a conventional'uncoated heating element. This can be done by separating theheating element from the microwave heating compartment by means of the shelf 40 shown in Figs. l, 6, and 7'@ This shelfy may form the top wall of the open instead .of wall 37, as previously pointed out. This shelf 40, like the oven door 17 previously described, may consist of abase member 51, such as glass or any low-loss dielectric which is capable of transmitting infrared radiation. This base member 51 is coated with a thin film 52 of a metal capable of transmitting a substantial portion of infrared energy while simultaneously acting as la ybarrier to the passage of microwave energies. The shelf y40 "then serves as an effective top wall of the microwavey heating compartment to prevent appreciable egress of microwave energy from within the microwave heating compartment, while being transparent to infraredradiation from the externally-positioned heating unit-4.

The mode stirrer 33 of Fig. 8 comprises a resonant slot structure which may be made up somewhat in the same manner as the window 17 and shelf 46A described above', that is, it may include a base member 5?., such as glass, plastic or ceramic, upon one surface of which a thin metallic coating 52 is deposited.` Portions of the base member are left uncoated to form resonant'slots 60 through which microwave energy may be directed. When no infrared heating unit is employed, as in the case of Fig. 5, the metal lm 52 may be of thickness such that the surface resistivity is largeenough to allow for dissipation of a portion of the microwave energy under no-load conditions, whereby damage to the magnetron may be prevented. Microwave energy absorption in the mode stirrer may be enhanced if the metal used to coat the stirrer inherently has a high Volume resistivity. When an infrared heating unit 44 is used, as in Figs. l and 7, it is also essential that the thickness of the stirrer Vcoating 52 of Fig. 8 be of such surface resistivity as to permit appreciable transmission of 'infrared energy through the coating. The mode stirrer 33 may be mounted either above or below the I,infrared heating unitwhen positioned outside 'the microwave heatingv` The Imefauized" coaurigsz `on' the 'top "sh1f'4o=f""`om kpart'rn'ent 15 of 'Figsfand 7 maylb'e of 'Suchthicknes's as to permit both ,infrared energy'fro'm heating unit 42@ and microwave'energ'y yfro'i'nhigh frequency vsource r- 21 to pass therethrough, fbut "at 'the 'same time partially absorbing microwave "energy 'tnolload or i low-'load `cnditions. The metallic coating 52 may be on ,elithr'f'side of the glass m'einb'r 51, as'shown'by "a comparison of Figs. 6 and7. K Y

vThe use of several 'spaced meralllicrayers, 's'how'n in Fig. 3.,' for l"o"bta'ii'iiirgj'prop'e'r transmission "throughout the 'opticaVfre'qlieny band already has beenm'nft'ioned. AS these meranic Y1a'yersarje spaced apajrrbyrytn eed-number `of quarter wavelengtlisatythenidladfrequ/encyvf the infrared region,"in`the "case -of'lthlems'lielf `40fand vf'c stirrer 33, or at the 'midbard 'frequencyoffthe'opt al region, in the Vcase'of 'the'win'dow 1 7, "it'fis *possiblef't achieve substantial 'cancellationfof "energy fro'rrrtheE various la'yers and thetran'smis'sivity achieved, incomparison with that obtained froma single lmetal layer ora plurality of layers randomly spacedgcan he`redu'ced.I Moreover, by maintaining the spacing between metal Ylay'ers'at fan vodd number o'f Ahalf wavelengths; substantial reinforcement of energy from the various/layers'maybe obtained, whereupon the transmissivity is cor're''sp'o'ndiirgly''yV ,increased. Sincethe optical or infrared region'of the electromagnetic spectrumfas'the casern'a'y be,'is VVquite lfar removed from the Ymicrowave frequency region, no a'p- -preciable reinforcementor cancellation effects are likely posed surfaces of'base member 51,- as 'shown 'in`tFig."4,

because of the additional Vpath-length throgh'-whi'ch transmission must occur. Y If the thickness of memberS-l is so vdesigned that the'spacing'between the layers 52 of Fig. 4 is made equal to an odd number of` half wavelengths, howe'ver,1the*effect is the same as a single layer of increased thickness, while the effective thickness lmay be decreased provided the spacing between'layers '52 of Fig. 4 is `made an yodd number of quarter wavelengths.

What'is claimed is:

1. A high frequency heating' apparatus comprising a metallic enclosure forming a compartment lfor reception ofa body to be heated,`a1'1"d means forfsupplying microwavefenergy,y all portions ofA said element 'being capable enclosure having a portion thereof constructed ofa"c'o'm positeY elementwhich is substantially impervious'to microwave energyall porttions of said element being capable of transmitting a substantial amount of radiant energy lying outside the microwave region higher in the electromagnetic frequency spectrum.

2. A highv frequency heating apparatus comprising a metallic enclosure formingL a compartment for lreception of a bodyto be heated, and means for supplying microwave energy to the interior of said compartment, said enclosure having a. portion thereof constructed of a composite element which is substantially impervious to microwave energy, all portions of said element being caergy lying within the infrared spectrum and the visibleV 4. A high vfrequency heating apparatusy comprising a metallic enclosure forming a compartment for reception of a body to be heated, and means for supplying'micro- 'n wave energy tothe interior of lsaid compartmenaa heating unit positioned outside said metallic enclosure for radiating infrared energy, said metallicenclosure having :a portion thereof 'constructed of'a composite element "which is substantially impervious to microwave energy, all portions of said element being capable of permitting transfer. of infrared energy fromvsaidheating unit into l the interior of said heating compartment.V

Y 5, A high frequency heating apparatus comprising a metallic enclosure forming a compartment for Vreception of a body to be heated, means for supplying microwave energy to the interior of saidicoimpartment, and'aV heating unit positioned outside said metallic enclosure for radiating infrared energy, said metallic enclosure .having `a portion thereof constructed of a composite element, said element including a ybase member transparent to .in-

Vfrai'ed radiation having yat least'one of its major surfaces coated with at leastone thin metallicV film which is substantially Yimpervious v to microwave energy, all portions of said element Ybeing capable of permitting transfer of infrared energy from said heating unit into the interior of said heatingY compartment.

6. A high'frequencyheating'apparatus comprising as' metallic enclosure forming a compartment .for receptionk of a body'to be heated, Vmeans for supplying microwave energy to the interior-of said compartment,"and a heating unit positioned outside said metallic enclosure. Vfor radiating infrared energy, said metallic enclosure having a portion thereof .constructed of a composite element, said element including a base member transparent to infrared radiation having atleastone of its major surfaces coated with at least one thin metallic film capable" of absorbing a portion of said microwave energy and of transmitting infrared `energy from 'said heating unit intoV .wave energy..YV

8. VA highv frequency heating apparatus comprising a metallic,enclosureforming a compartment for receptionV of a body Yto be hearted, means fof supplying microwave energy to the interior ofy said compartmennand a'heating unitvpositioned outside said metallic enclosure for radiating infrared energy, said metallic enclosure hav-i ing a portion thereof constructed of a composite element, said'element including aV basemember transparent to infrared radiationhaving Va surface thereof. coated with a 'plurality of metallic lmsspaced .apart by dielectric spacing layers, saidV metallic/*films being transparentftoV infrared, radiation and substantially impervious to micro- `wave energy.` f ,Y "j

9. A high frequency heating apparatus comprisinga metallic enclosure forming a compartment Aforfreception of a body to be heated, means for supplying microwave energy to the interior o f saidcompartmenL-and a heating unit positioned outside said metallic enclosure forV radiating infrared energy, said metallicrenclosure having a portion thereof constructed of` a composite element, said element including'a base member transparentY to infrared radiation having a surface thereof coated with a plurality of metallic films'spaced apart byrdielectric spacing layers, said metallic lms being transparent to infrared'radiation andsubstantially impervious to microwave energy, said metallic films being spaced by an odd number of quarter wavelengths ata frequency lying` within the infrared region of the electromagnetic spectrum.

10. A high frequency heatinglapparatus comprisingn a metallic enclosure forming a compartment for reception of a body torbe heated, means for supplying microwave energy to the interior of said compartment, Vand a heating unit positioned outside said metallic enclosure for radiating infrared energy,1said metallic enclosure having a portion thereof constructed of a compositeelement,

said element including a base member transparent to infrared radiation having a surface thereof coated with a plurality of metallic films'spaced apart by dielectric spacing layers, said metallic lilms being transparent to infrared radiation and substantially'impervious to microwave energy,y said metallic lilmsbeing spaced by an odd number of half wavelengths at a frequencylying within the infrared region Aof the electromagnetic spectrum. Y

1l. A high frequency heating apparatus comprisinga metallic enclosure forming a compartment for reception of a body to be heated, and means for supplying micro-v wave energy, ally portions of said element being capableV of transmitting energy lying within the visible portion Y of the electromagnetic spectrum.

12. A- high frequency heating` apparatusV comprising a metallic enclosure forming a compartment for reception Vof a body to be heated, and means for supplying microwave energy'to Vthe interior of said compartment, said enclosure having a portion thereof constructed-of a composite element, said element including a base member transparent to visiblevradiation having Vat least one of its major surfaces coatedwith atV least one thin metallic film which is substantially impervious to microwave energy and capable Vof transmission of visible radiation whereby the interior of said heating compartment Vmay be inspectedvisually. I K Y 13. A high frequency heatingrapparatus comprising a metallic enclosure forming a compartment for reception of a body to be heated, and means for supplying microwave energy to the linterior of said compartment, said Vmetallic-enclosure having a portion thereof constructed of a composite element, said element includinga base .member transparent to visual radiation having a surface Vthereof Vcoated with a plurality of metallic lms, said n metallic films beingV transparent to visual radiation and metalliccenclosureV havingV aportion thereof constructed` of a composite element, said element including a base member transparent to visnal radiation having a Vsurface e thereof coated-withV a pluralityVV of metallic films spaced Yapart by dielectric spacingV layers, said metallic filmsV being transparent to VvisualV radiation and` substantially impervious to microwave energy, said metallic films being Y spaced 'by an odd number of quarter wavelengths at a frequency lyingY within .the Visualre'gion of the electroe magnetic spectrum.V Y Y 15; A highrfrequency heating apparatus comprising a i metallic enclosure forming a compartment for reception of a body to be heated, and means for supplying microwave energy to the interior of said compartment, said metallicenclosure having a portion thereof constructed of a composite element, said element including a base member transparent to visual radiation having a surface thereof coated with a plurality of metallic films spaced apart by dielectric spacing layers, said metallic films being transparent to visual radiation and substantially impervious to microwave energy, said metallic films being spaced by an odd number of half wavelengths at a frequency lying within the visual region of the electromagnetic spectrum.

16. A high frequency heating apparatus comprising a metallic enclosure forming a compartment for reception of a body to be heated, means for supplying microwave energy to the interior of said compartment, a heating unit positioned outside said metallic enclosure for radiating infrared energy, and a movable structure adjacent said means for supplying for effecting a variation in the electric field distribution pattern within said compartment, said structure including a base member transparent to infrared radiation and a thin metallic film coated on at least one of the major surfaces thereof for dissipating microwave energy, said base member containing uncoated portions to provide resonant slots through which microwave energy can be directed.

17. A high frequency heating apparatus comprising a metallic enclosure forming a compartment for reception of a body to be heated, means for supplying microwave energy to the interior of said compartment, a heating unit positioned outside said metallic enclosure for radiating infrared energy, and a movable structure positioned adjacent said means for supplying for eecting a variation in the electric field distribution pattern within said compartment, said structurev including a base member transparent to infrared radiation and a thin metallic film coated on at least one of the major surfaces of said base member which is capable of transmitting infrared radiation, said base member containing uncoated portions to provide resonant slots through which microwave energy may be directed.

18. A high frequency heating apparatus comprising a metallic enclosureforming a compartment for reception of a body to be heated, means for supplying microwave energy to the interior of said compartment, a heating unit positioned outside said metallic enclosure for radiating infrared energy, and a movable structure positioned outside said heating compartment for effecting a variation in the electric field distribution pat-tern within said compartment, said structure including a base member transparent to infrared radiation and a thin metallic film coated on at least one of the major surfaces of said base member which is capable of transmitting infrared radiation, said base member containing uncoated portions to provide resonant slots through which microwave energy may be directed.

19. A high frequency heating apparatus comprising a metallic enclosure forming a compartment for reception of a body to be heated, and means for supplying microwave energy to the interior of said compartment, said enclosure having access means forming a portion thereof, said access means including a composite element which in conjunction with said metallic enclosure constitutes a capacitor, said composite element having a base member transparent to visible radiation and a surface of said base member coated with at least one thin metallic film which is substantially impervious to microwave energy and capable of transmitting visible radiation, said composite element further including a dielectric layer superimposed upon said metallic film and adapted to contact a portion of the metallic enclosure when said access means is in the closed position.

References Cited in the file of this patent UNITED STATES PATENTS 2,594,971 Moullin Apr. 29, 1952 2,599,944 Salisbury June 10, 1952 2,656,535 Neher Oct. 20, 1953 2,707,235 Townes Apr. 26, 1955 2,748,239 Long et a1. r. May 29, 1956 2,761,942 Hall Sept. 4, 1956 FOREIGN PATENTS 643,605 Great Britain Sept. 20. 1950 UNITED STATES PATENT oTTTCn CETINCATE GF CEUHN Patent No. 2,920,174 January 5, 1960 l Duane B I-aagens en It is hereby certified that error appears in the printed specification of' the above numbered patent requiring correct-ion and that the said Letters Patent should readas corrected below.

Column 5, line 27, for "iti s" read e it is line 38Y for "open" read oven column lin 58Y for all portions of said element being capable" read to the interior of said compalfmentv said 3 line 6l, for "porttions" read portions Signed and sealed this 14th day of June 1960 (SEAL) Attest:

KARL H. AXLINE. ROBERT C. WATSON Attesting Oicer Commissionei` of Patents UNITED STATES PATENT oTTTCn CETINCATE GF CEUHN Patent No. 2,920,174 January 5, 1960 l Duane B I-aagens en It is hereby certified that error appears in the printed specification of' the above numbered patent requiring correct-ion and that the said Letters Patent should readas corrected below.

Column 5, line 27, for "iti s" read e it is line 38Y for "open" read oven column lin 58Y for all portions of said element being capable" read to the interior of said compalfmentv said 3 line 6l, for "porttions" read portions Signed and sealed this 14th day of June 1960 (SEAL) Attest:

KARL H. AXLINE. ROBERT C. WATSON Attesting Oicer Commissionei` of Patents UNITED STATES PATENT OFETCE CERTIFICATE OE CORRECTION Patent No. 2,920,174 january 5, 196e Duane B.s I'iaagensen 'It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should readas corrected below.

Column 5, line 27, for "iti s" read -jit is line 38, for Open" read oven column lin@v 58Y for all portions of said element being capable read to the interior of said compartmentq said --3 line 6l, for "p0rttions read portions Signed and sealed this 14th day of June 1960.,

(SEAL) Attest:

KARL H. AXLINE ROBERT C. WATSON Attesting Officer Commissioner of Patents

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