US3604783A

US3604783A - Microwave cavity viewers

Info

Publication number: US3604783A
Application number: US837446A
Authority: US
Inventors: Howard Roth
Original assignee: DCA Food Industries Inc
Current assignee: DCA Food Industries Inc
Priority date: 1969-06-30
Filing date: 1969-06-30
Publication date: 1971-09-14
Anticipated expiration: 1988-09-14

Abstract

Viewing ports for microwave ovens. In a first embodiment, a transparent mirror is placed over a relatively large aperture in the wall of the oven with the metallic coating of the mirror bearing against the metallic wall of the oven. The interior of the oven is illuminated. The metallic coating of the mirror prevents microwave energy from being transmitted through the mirror, while the one-way viewing characteristic of the mirror allows visible light to be transmitted through it. In a second embodiment, a waveguide is connected between the aperture in the oven and a viewing port. Two prisms are provided, one at the oven aperture and the other at the viewing port. Visible light from inside the oven is reflected by the aperture prism down the waveguide and by the viewing port prism out of the viewing port. Any microwave energy which is transmitted down the waveguide is attanuated both as a result of the waveguide dimensions and special attenuators provided for this purpose. The prism mounted at the oven aperture can be rotated so that the interior of the oven can be scanned.

Description

14 71 United States Patent [111 ,6 4,7

[72] Inventor Howard Roth 3,457,138 7/1969 Miller 126/200 2] A I N ggg l g- OTHER REFERENCES 1 1 P Beers, o. L. Minimizing the Effects of Ambient Light on [22] Filed June 30, 1969 I mage Reproduction, Journal of the SMPTE, Volume 66, [45] Pammcd M1971 J 1957 347 354 c r d' 350 276 [73] Assignee DCA Food Industries Inc. une opy m New York, N.Y. Primary Examiner-David Schonberg Assistant Examiner-Toby H. Kusmer AttorneyAmster and Rothstein [54] MICROWAVE CAVITY VIEWERS 6 Claims, 3 Drawing Figs.

ABSTRACT: V|ew|ng ports for microwave ovens. In a first I52] U.S.(.l 350/163, embodiment, a transparent mirror is placed Over a rclativdy 126/200, 219/1055, 350/22, 350/276, 350/2 large aperture in the wall of the oven with the metallic coating 350/319 of the mirror bearing against the metallic wall ofthe oven. The [51] Int. Cl F23m 7/00 interimf the oven is i||uminated ,The metamc coating f h [50] Field ofSearch 350/163, min-or prevents microwave energy f being transmitted 22v 291; 26/200; 219/1055 through the mirror, while the one-way viewing characteristic of the mirror allows visible light to be transmitted through it. [56] References cued 1n a second embodiment, a waveguide is connected between UNITED STATES PATENTS the aperture in the oven and a viewingport. Two prisms are 723,303 3/1903 Powell [26/200 UX provided, one at the oven aperture and the other at the view- 728,063 5/1903 Wilson 350/291 ing port. Visible light from inside the oven is reflected by the 1,520,245 12/1924 Humbrecht 350/22 UX aperture prism down the waveguide and by the viewing port 3,088,453 5/1963 Grahn et al. i 126/200 X prism out of the viewing port. Any microwave energy which is 3,260,832 7/1966 Johnson 219/1055 transmitted down the waveguide is attanuated both as a result 3,329,795 7/1967 Long 219/1055 of the waveguide dimensions and special attenuators provided 3,390,447 7/1968 Mears 350/276 UX for this purpose. The prism mounted at the ovenaperture can 3,430,023 2/1969 Tingley 126/200 X be rotated so that the interior of the oven can be scanned.

I 111111,!!! G 20 t 1 q a f6 /5 4 Z! v .42 2 .564 5 ;-0 44 265.4 f/

c/ I j a4 7! 5a A Ill ll] MICROWAVE CAVITY VIEWERS This invention relates to microwave cavity viewers, and more particularly to windows which provide an unobstructed view of the interior of a microwave oven without permitting microwave energy to pass out of the oven.

A typical home-use microwave oven is operated at a power level below 1.6 kilowatts, generally at a frequency of either 915 MHz. or 2,450 MI-Iz. Electromagnetic energy of a frequency in the microwave part of the spectrum moves freely through air and through most substances with little of it being absorbed or reflected. On the other hand, there are other substances which interact with electromagnetic waves in the microwave part of the spectrum and are heated by them. This is especially true of water and any substance containing water.

A microwave oven typically consists of an aluminum or stainless steel enclosure since these metals reflect most of the microwave energy which impinges upon them inside the oven. The energy is contained within the oven and is used to heat a food or any other item which contains water and must be heated rapidly throughout.

In order to look into a microwave oven, or any microwave cavity for that matter, it is necessary to provide a window" or other viewing device which permits electromagnetic energy in the visible part of the spectrum to be transmitted from the interior of the cavity to the exterior of the cavity while at the same time blocking the microwave energy. If the microwave energy escapes from the cavity, the eyes of the observer may be injured due to the heat generated as a result of the water content of the human eye. It is apparent that a metallic window cannot be used because it is opaque to the visible spectrum, and a glass window cannot be used because it would permit the microwave energy in the cavity to escape.

At the present time, a viewing port is generally constructed by providing holes in one of the walls of the cavity. Microwave energy is attenuated when it passes through an opening smaller than one-half of its wavelength. The degree of attenuation varies inversely with the size of the aperture and directly with the distance the energy travels through the aperture or a tube of that dimension. Generally, the holes which are made to view the interior of a microwave oven are in the order of V4 inch in diameter; with a wall thickness of only l/l6 inch, almost no microwave energy gets through to the exterior of the oven. If larger diameter holes are used, each one is generally provided with a tube in order that the aperture length be v greater than the thickness of the oven wall so that the microwave energy can be attenuated before it reaches the eye of the viewer. In almost all cases, the resulting viewing port, while safe as far as the observer is concerned, hinders unobstructed viewing into the cavity. At best, a typical configuration simply allows a small area of the cavity to be seen without obstruction.

It is a general object of my invention to provide a viewing port for the interior of a microwave cavity which allows for safe, wide area unobstructed viewing.

In a first illustrative embodiment of my invention, a relatively large metallic-coated piece of glass is placed over a large aperture (e.g., 6 X 6 inches) on the front of the microwave oven, with the metallic coating facing the interior of the oven. The metallic-coated glass is simply a transparent mirror of conventional design widely used heretofore for one-way viewing. The metallic coating is quite thin and serves, in the conventional use of a transparent mirror, to block visible light from being transmitted through the mirror from the glass side while allowing visible light to be transmitted through the mirror from the dark or metallic side. In my invention, the transparent mirror is not utilized as a one-way window. I have found that such a transparent mirror, provided the metallic coating is held in intimate contact with the wall of the oven surrounding the aperture over which it is placed, effectively shields the viewer from the microwave energy inside the cavity while at the same time allowing visible light to be transmitted through it.

With very high energy ovens, a conventional transparent mirror will not shield against the microwave energy. However, a conventional microwave oven for home use is operated below L6 kilowatts and a conventional transparent mirror is adequate to completely protect a viewer from the microwave energy inside the cavity. Moreover, in the event a conventional transparent mirror is utilized with a higher power oven, the mirror does serve to attenuate the microwave energy transmitted along with the visible light to the viewer. Thus if an additional shield is provided, for example, with the conventional apertures for completely attenuating the microwave energy, it is apparent that the attenuation requirements of the conventional shield are not as stringent and a less obstructed view of the interior of the oven can be had.

In a second illustrative embodiment of my invention, a first prism is included in front of an aperture provided in the microwave cavity. Visible light from inside the cavity is reflected from this prism to a second prism which directs the light toward the viewer through a viewing port. The first prism can be rotated so that the interior of the cavity can be scanned. Between the two prisms, light travels along a tunnel which has a width small enough to attenuate any microwave energy which escapes through the aperture in the oven. Moreover, the tunnel is provided with various tuning posts as well as a microwave energy absorbing material for further attenuating the microwave energy. Thus no microwave energy exits the viewing port and the entire cavity can be scanned with minimal attenuation of any light emanating inside the cavity. I

It is a feature of my invention, in one illustrative embodiment thereof, to utilize a conventional transparent mirror for containing microwave energy within a cavity while at the same time allowing the transmission through it of visible light.

It is another feature of my invention, in a second illustrative embodiment thereof, to provide two prisms for allowing the interior of a microwave oven to be viewed, while at the same time coupling the viewing port to the interior of the oven through a tunnel which attenuates any microwave energy which would otherwise be coupled to the viewing port.

Further objects, features and advantages of my invention will become apparent upon a consideration of the following detailed description in conjunction with the drawing, in which:

FIG. 1 depicts schematically a first illustrative embodiment of my invention;

FIG. 2 depicts schematically a second illustrative embodiment of my invention; and

FIG. 3 is a partial cross-sectional view taken through the line 3-3 of FIG. 2.

In FIG. I, microwave oven 10 includes conventional walls 12. On one side of the oven are two lamps 14 for illuminating its interior. Light is transmitted to the interior of the oven through apertures I6, as shown by arrow 18. The apertures serve to permit light to enter the cavity without allowing microwave energy to leave the cavity to damage light sources 14. The function of the apertures is similar to the function of the conventional apertures in blocking microwave energy from leaving the viewing port of a prior art microwave oven.

Microwave energy source 20 is coupled via waveguide 22 to the interior of the oven, and as shown by arrow 24 is the source of the microwave energy for cooking whatever is placed in the oven.

The front of the oven has a rectangular opening 74 which, in the first illustrative embodiment of the invention, is approximately 6 X 6 inches. Over this opening is placed transparent mirror 28 which includes a metallic coating 30 on the side facing the oven cavity. In order not to disturb the normal cooking operation which takes place in a closed cavity, it is important that the metallic coating make intimate contact with the oven wall all along the edge of the aperture, and for this reason frame 34 is provided. The frame is attached by bolts 38 and nuts 40 to the oven wall around the edge of aperture 74. The frame includes a diaphragm spring 36 which bears against the transparent mirror to force its metallic coating into intimate contact with the oven wall. Intimate contact is insured by wetting the contacting surfaces with mercury.

Surrounding frame 34 is another frame 42, hinged to the oven wall by bolts 80 and nuts 82. This frame serves to reduce the amount of light impinging on the viewer's side of mirror 28 to contribute to better viewing. The metallic coating of the mirror prevents microwave energy from leaving the oven cavity through aperture 74. However, visible light is transmitted from the relative bright side of the mirror (inside the oven) to the relatively dark side of the mirror (outside the oven) so that the interior of the cavity can be viewed.

Shown in dotted lines, and represented by the numeral 44, are the conventional holes used to block microwave energy and to allow viewing of the cavity. These holes are contained in frame 42 which due to its contact with the oven wall actually forms part of the microwave cavity. In the case of high energy ovens, the transparent mirror may not be sufficient to block all of the microwave energy which would otherwise exit the viewing port. For this reason, the conventional viewing port 44 can be utilized. However, the transparent mirror still serves to attenuate the microwave energy which would otherwise be transmitted and this allows the construction of a viewing port 44 which obstructs the view to a far lesser extent.

FIG. 2 depicts a second illustrative embodiment of the invention. Microwave oven 50 is similar to microwave oven of FIG. 1 insofar as the light and microwave energy sources are concerned. However, the arrangement for viewing inside the oven is different.

A first prism 60 is mounted for pivotal movement around the axis designated 64, as shown by the double-ended arrow 62. Light from the oven is reflected by the prism along tunnel 58 toward prism 56. Prism 56 is fixed in position, and light reflected along the tunnel is reflected by prism 56 to viewing port 52. As prism 60 is turned about its axis, different regions of the oven can be viewed.

The tunnel 58 is rectangular; the distance between the two prisms is approximately 4 inches, the vertical dimension (shown in FIG. 3) is approximately 6 inches and the width (dimension D in FIG. 2) is approximately 1.75 inches for an oven operated at 2,450 MHz. Dimension D is small enough so that microwave energy is attenuated as it travels down the tunnel. To further insure that microwave energy from the oven is not transmitted along the waveguide and out through viewing port 52, two mechanisms for attenuating the energy are provided. The first entails the use of a series of posts 66; by adjusting them, microwave energy can be greatly attenuated as it is transmitted down the waveguide from the region enclosing prism 60 to the region enclosing prism 56. The posts are mounted in front wall 72 and, although only three are shown in the drawing, there are preferably nine posts arranged in three rows, one on top of the other. The other mechanism for attenuating microwave energy consists of a microwave absorbing material 54, typically a plastic filled with iron filings which absorb any microwave energy which impinges on them. Any microwave energy which travels down the waveguide is thus absorbed and cannot be reflected for possible exit from the waveguide at port 52.

FIG. 3 shows the mounting of prism within waveguide 58. The prism is provided with two

posts

66, 68, the latter of which simply fits into a hole in the bottom wall of the waveguide to enable the prism to rotate. Post 66 extends up through the upper wall of the waveguide to knob 70. As the knob is turned the interior of the cavity is scanned. Although scanning prism 60 is shown toward one end of the oven, for a maximum field of view it is preferable that the prism be mounted in the middle of the front wall of the oven. Although not shown in FIG. 2 due to space limitations, the lowermost wall in the drawing can be thought of as being further down such that its distance from prism 60 is the same as the distance of the uppermost wall from the same prism.

Although the invention has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the application of the principles of the invention. Numerous modi ications may be made therein and other arrangements may be devised without departing from the spirit and scope of the invention.

What I claim is:

I. In combination with means including a plurality of walls forming a microwave cavity, a viewing arrangement for said cavity comprising means for illuminating said cavity, an clongated tunnel mounted on one of said walls of said cavity and having one end opening into said cavity for allowing said tunnel to communicate with said cavity, first light reflecting means mounted in said tunnel adjacent said one end for reflecting visible light from the interior of said cavity along said tunnel, a tunnel opening providing a viewing port in said tunnel spaced from said one end and facing away from said cavity, second light reflecting means mounted in said tunnel adjacent said viewing port for reflecting light transmitted along said tunnel from said first light reflecting means through said viewing port, said tunnel having a width small enough and a length between said two light reflecting means long enough such that microwave energy transmitted from said one opening to said viewing port is attenuated to a safe level.

2. In combination with means including a plurality of walls forming a microwave cavity, a viewing arrangement for said cavity comprising means for illuminating said cavity, an elongated tunnel mounted on one of said walls of said cavity and having one end opening into said cavity, first light reflecting means mounted in said tunnel adjacent said one end for reflecting visible light from the the interior of said cavity along the tunnel, a tunnel opening providing a viewing port in said tunnel spaced from said one end and facing away from said cavity, second light reflecting means mounted in said tunnel adjacent said viewing port for reflecting light transmitted along said tunnel from said first light reflecting means through said viewing port and means for attenuating microwave energy transmitted along said tunnel such that the microwave energy which exists at said viewing port is at a safe level.

3. An arrangement for viewing the interior of a microwave cavity in accordance with claim 2 wherein said microwave energy attenuating means includes a plurality of posts mounted along said tunnel.

4. An arrangement for viewing the interior of a microwave cavity in accordance with claim 2 further including means in said tunnel for absorbing microwave energy transmitted therealong.

5. An arrangement for viewing the interior of a microwave cavity in accordance with claim 2 wherein said first and second light reflecting means are prisms.

6. An arrangement for viewing the interior of a microwave cavity in accordance with claim 5 wherein the first prism is mounted for rotatable movement within said tunnel such that the interior of said microwave cavity can be scanned.

Claims

1. In combination with means including a plurality of walls forming a microwave cavity, a viewing arrangement for said cavity comprising means for illuminating said cavity, an elongated tunnel mounted on one of said walls of said cavity and having one end opening into said cavity for allowing said tunnel to communicate with said cavity, first light reflecting means mounted in said tunnel adjacent said one end for reflecting visible light from the interior of said cavity along said tunnel, a tunnel opening providing a viewing port in said tunnel spaced from said one end and facing away from said cavity, second light reflecting means mounted in said tunnel adjacent said viewing port for reflecting light transmitted along said tunnel from said first light reflecting means through said viewing port, said tunnel having a width small enough and a length between said two light reflecting means long enough such that microwave energy transmitted from said one opening to said viewing porT is attenuated to a safe level.