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(11) | EP 0 790 755 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Lighting device for microwave oven |
| (57) A microwave oven has a cooking chamber (20) for cooking a food using microwave energy,
and a magnetron oscillator for producing microwave radiation to heat the food. The
oven also includes a hole (21b) formed on an upper surface (21) of the cooking chamber
and a lamp (41) for illuminating the cooking chamber through the hole. The lamp includes
a filament (41a) and a bulb (41b), and the filament is located about 2mm above the
upper surface (21). The hole, which is less than 19mm in diameter, is formed 2/14th of the upper surface's depth from the front of the upper surface. The upper surface
includes a beveled portion that slopes downward towards the front of the microwave
oven in which the hole is formed. The lamp is located at the hole so as to face the
rear of the cooking chamber and is covered by a protective glass cover (42) protruding
into the cooking chamber. |
[0001] The present invention generally relates to a microwave oven comprising a cooking chamber and a filament lamp for illuminating the cooking chamber through an aperture in a wall thereof.
[0002] A microwave oven heats food by using microwaves at approximately 2 450 MHz, usually generated by a magnetron. The microwaves are radiated into a cooking chamber in which the food food has been placed. Polar molecules, e.g. water molecules, in the food are drawn into alignment with the electric field of the microwave radiation. The rapid oscillation of the electric field of the microwaves causes the molecules to collide with each other, generating heat which warms the food. Recently, radiant electric heaters have been incorporated into microwave ovens to supplement the cooking process.
[0003] Figure 7 is a cutaway-perspective view of a conventional microwave oven as disclosed in JP-A-06-59707. The oven includes a cooking chamber 1 in which food is cooked, a magnetron 3 which produces microwaves to cook the food, a plurality of holes formed in the upper section of the cooking chamber 1, and a lamp 13 installed over the holes 12 which illuminates the cooking chamber 1 therethrough.
[0004] The degree of illumination of the cooking chamber 1 by the lamp 13 is proportional to the size of the holes 12. However, their size is limited by the possibility of microwave leakage through the holes 12. Also, the height of the lamp 13 above the cooking chamber 1 further decreases the illumination therein. Thus, food in the cooking chamber 1 is insufficiently illuminated the user cannot easily monitor the progression of the cooking process.
[0005] The present invention is a lighting device for a microwave oven that can obviate the above problems and disadvantages of the conventional technique.
[0006] A microwave oven according to the present invention is characterised in that the lamp illuminates the cooking chamber through a single aperture and the filament of the lamp is positioned relative to the aperture such that less than 2.0mW cm-2 of microwave power leaks through the hole when the oven is in use.
[0007] An oven according to the present invention may include a further filament lamp for illuminating the cooking chamber through an aperture in a wall thereof. The lamps preferably generate light having different colours.
[0008] Preferably, the or each aperture is provided in an upper wall of the cooking chamber. More preferably, the or each aperture is formed substantially on the front-to-rear centre line of the upper wall.
[0009] One lamp may be advantageously positioned approximately 2/14th of the distance, from the front of the cooking chamber, along the front-to-rear centre line of an upper wall of the chamber. In another form, the front margin of an upper wall of the cooking chamber slopes downwardly and the aperture is formed in said front margin so that light from the lamp is directed to a back wall of the cooking chamber.
[0011] Advantageously, a glass cover is provided for protecting the or each lamp and the cover protrudes through the associated aperture or apertures into the cooking chamber.
[0012] Preferably, the filament of the or each lamp is spaced approximately 2mm perpendicularly away from the opening of the aperture in the outer surface of said wall.
[0013] Reflector means may be provided to maximize the amount of light passing through the or each aperture.
[0014] Embodiments of the present invention will now be described, by way of example, with reference to Figures 1 to 6 of the accompanying drawings, in which:
Figure 1 is a cutaway-perspective view of a microwave oven with a lighting device in accordance with the present invention;
Figure 2 is an enlarged-sectional view taken along line II-II in Figure 1;
Figure 3 is an exploded-perspective view of a lighting device for a microwave oven in accordance with the present invention;
Figure 4 is a sectional view of another lighting device for a microwave oven in accordance with the present invention;
Figure 5 is a sectional view of another lighting device for a microwave oven in accordance with the present invention;
Figure 6 shows various preferred positions of a lamp hole in accordance the present invention; and
Figure 7 is a cutaway-perspective view of a prior art microwave oven with a lighting device.
[0015] Referring first to Figure 1, the construction of a microwave oven to which the present invention is applied is now described.
[0016] The microwave oven includes a cooking chamber 20 and a door 50 that opens and closes the entrance to the cooking chamber 20. The cooking chamber 20 is formed by an inner case 11, and this inner case 11 is joined to an outer case 12 to constitute the main body of the microwave oven. A rotatable tray is is located at the bottom of the cooking chamber 20 and is turned by an electric motor (not shown).
[0017] An electrical component compartment 30 is formed between the inner case 11 and the outer case 12. The electrical component compartment 30 houses a magnetron 31 which is powered by a high voltage transformer 32 and which radiates microwaves into the cooking chamber 20 so as to cook foods therein. Behind the electrical component compartment 30 is a fan 33 which cools the high-voltage transformer 32.
[0018] A lighting device 40 is provided on the upper surface 21 of the cooking chamber 20 to illuminate the cooking chamber's 20 interior.
[0019] The upper surface 21 has a beveled portion 21a at its front onto which the lighting device 40 is mounted. This beveled portion 21a has the slopes most at the rearmost part when viewed from the top. The lighting device 40 is installed on the beveled portion 21a perpendicular to its surface, facing towards the cooking chamber 20.
[0020] The construction of the lighting device will now be described in detail with reference to Figures 2 and 3.
[0021] The beveled portion 21a has a hole 21b formed in its centre through which light rays from the lighting device 40 pass. The lighting device 40 is installed outside the hole 21b and is turned on and off by means of an electrical connection with a circuit board (not not shown) in the electrical component compartment 30. The lighting device 40 includes a lamp 41 consisting of a filament 41a that emits light when power is applied to it, a terminal 41b through which electricity is supplied to the filament 41a, and a bulb 41c. Lamp brackets 44 on which the lamp 41 is installed are fixed on the beveled portion 21a and, being fixed by rivets 45, are designed to lie flush with the beveled portion 21a and its peripheral portion. In addition, each lamp bracket 44 has a guide piece 44a that protrudes upwards, securing a lamp cover 43 which is put on the lamp 41 in place. The lamp cover 43 is in the shape of a box whose bottom is open, and has apertures 43a formed on each of its sides. First bosses 44b, formed on each of the guide pieces 44a of the lamp bracket 44, are respectively inserted into the apertures 43a in such a manner that the lamp cover 43 is connected to the lamp brackets 44. The lamp 41 is fixed to the lamp cover 43, and the filament's 41a lower end extends outside the beveled portion 21a.
[0022] A protective glass 42 is mounted on the hole 21b to provide protection for the lamp 41 by protruding into the cooking chamber 20 and making the lamp 41 watertight, thus precluding disconnection of the lamp 41 due to moisture. The protective glass 42 also stops dirt and dust from directly adhering to the lamp 41 to thereby avoid damage to the lamp 41. Additionally, the protective glass 42, which is made of heat-resistant glass, serves to diffuse light rays from the lamp 41.
[0023] The protecting glass 42 includes a flange 42a lying on the outer surface of the beveled portion 21a, and a convex portion 42b protruding into the cooking chamber 20 from the flange 42a, passing through the hole 21b. A cone-shaped reflecting member 46, which reflects light lays from the lamp 41 toward the cooking chamber 20, is provided to the interior of the lamp cover 43 so as to encircle the lamp 41. The reflecting member's 46 lower end comes in contact with the protective glass 42, and its upper end contacts the lamp's 41 inner surface. Second bosses 43b are provided on the inner side of the lamp cover 43 within which the reflecting member 46's upper end is secured.
[0024] Figures 4 and 5 depict lighting devices for a microwave oven in accordance the present invention.
[0025] A plurality of holes 21b' and 21b" are formed in the beveled portion 21a, and at least one lamp 41 is provided to each hole 21b' and 21b". In the lamp 41, the filament's 41a lower end is positioned outside the beveled portion 21a, and a bulb's 41b lower end protrudes into the cooking chamber 20. A single protective glass 42' with a plurality of convex portions 42b' and 42b", corresponding to each hole 21b' and 21b", protects the bulbs and a lamp cover 43 covers the entire lamp 41.
[0026] Figure 4 shows a lighting device in which a lamp 41 is provided at each of the holes 21b' and 21b", and Figure 5 depicts a lighting device in which two lamps 41 are provided at one hole 21b' and one lamp 41 is provided at the other hole 21b". In Figure 5, the two lamps 41 at the hole 21b' provide white light, and one lamp 41 at the hole 21b" produces red light. A partition 47 is formed between these lamps 41, thus offering two-color lighting.
[0027] The following description relates to the operation of a microwave oven according to the present invention.
[0028] Once a user presses a start button (not shown) in order to start cooking, the magnetron 31 radiates microwaves into the cooking chamber 20 which cook food therein. A fan 33 is provided to cool the high-voltage transformer 32 and the magnetron 31 with air from outside the oven.
[0029] The lamp 41 is turned on during the cooking operation and light rays from the lamp 41 are diffused through the protective glass 42 into the cooking chamber 20 so that the intensity of the light provided to the cooking chamber 20 is increased. The lamp's 41 placement on the front of the cooking chamber's 20 upper portion so as to face the rear additionally enhances illumination of the cooking chamber 20. Most light rays generated by the lamp 41 are directly provided to the cooking chamber 20, and the rest is reflected by the reflecting member 46 into the cooking chamber 20, thus providing increased illumination in the cooking chamber 20.
[0030] There is a limit to the hole 21b's size because it is necessary to prevent a leakage of the high-frequency microwaves, which is more fully described referring to Table 1 - 1. Table 1 - 1 represents the relationship between the level of illumination of the cooking chamber 20 and the diameter (⌀) of the hole 21b, the amount of microwave leakage and the lifetime of the lamp (numerical values of Table 1 - 1 reflect experimental results).
[0031] Referring to Table 1 - 1, the larger the diameter of the hole 21b becomes, the more the level of illumination of the cooking chamber 20 increases, and the more the microwave leakage increases. When the amount of leakage exceeds a predetermined value (6mW cm-2), the lifetime of the lamp 41 is reduced. Thus, it is preferable that the diameter ⌀ of the hole 21b be about 14mm - 19mm.
[0033] Seven holes 21b, each 14mm in diameter, are formed in the middle of the upper surface 21. More specifically, according to experimental data, the holes 21b are respectively formed at 1/14, 2/14,..., 6/14 and 7/14 of the depth of the upper surface 21 from the front of the upper surface 21. In the experiment, the filament 41a is positioned level with, 3mm below, 1mm above, or 2mm above the upper surface 21.
[0034] As shown in Table 2 - 1, when the filament 41a is level with the upper surface 21, aproximately 0.8mW to 1.2mW of microwave power leaks. When the filament 41a is positioned 2mm below the upper surface 21 approximately 2.8mW to 3.5mW leaks out according to the hole 21b's position. When the filament 41a is positioned 1mm above the upper surface 21, approximately 0.5mW leak out regardless of the position of the hole 21b. When the filament 41a is positioned 2mm above the upper surface 21, approximately 0.1mW to 0.2mW leak out according to the position of the hole 21b. The standard amount of leakage of microwave power is 2.0mW cm-2 maximum so it is preferable that the position of the filament 41a is positioned 2mm above the hole 21b.
Table 2 -2 (unit: Lux)
| Position of hole | Centre of Door | Cooking chamber |
| 1 | 95.0 | 64.9 |
| 2 | 116.4 | 160.0 |
| 3 | 55.1 | 171.0 |
| 4 | 51.8 | 207.0 |
| 5 | 44.4 | 223.0 |
| 6 | 41.4 | 371.0 |
| 7 | 35.2 | 424.0 |
[0035] Table 2 - 2 represents the illumination levels in the cooking chamber 20 under different conditions. When an illuminometer is installed to the cooking chamber 20 to measure its illumination, the lighting efficiency of the lamp 41 provided at the hole 21b, which is in the centre of the upper surface 21, reaches its highest level. When the illuminometer is installed on the centre of the door 50, the lighting efficiency of the lamp 41 provided at the hole 21b at the second position 2/14 reaches the highest level. Therefore, the lamp 41 is preferably installed at the second position 2/14.
[0036] As described above, according to the present invention, a lamp is installed on the upper surface of the cooking chamber in close vicinity to the cooking chamber and most light rays from the lamp are provided directly to the cooking chamber, thus offering enhanced lighting efficiency. The light from the lamp is diffused by the protective glass that protrudes into the cooking chamber to thereby illuminate the cooking chamber more efficiently. Moreover, the light directed initially away from the cooking chamber is reflected by the reflecting member into the cooking chamber which enhances the lighting efficiency of the lamp. The lamp is installed in a position which ensures enhanced lighting efficiency and the diameter of the hole is as large as possible within the range in which the microwave leakage is acceptable. In addition, it is simple to install a lighting device according to the present invention on the top of the inner case of the microwave oven.
1. A microwave oven comprising a cooking chamber (20) and a filament lamp (41) for illuminating
the cooking chamber through an aperture (21b; 21b') in a wall (21) thereof, characterised in that the lamp illuminates the cooking chamber through a single aperture and the filament
(41a) of the lamp is positioned relative to the aperture such that less than 2.0mW
cm-2 of microwave power leaks through the hole when the oven is in use.
2. An oven according to claim 1, including a further filament lamp (41) for illuminating
the cooking chamber through an aperture (21b") in a wall thereof.
3. A oven according to claim 1 or 2, wherein the or each aperture is provided in an upper
wall (21) of the cooking chamber.
4. An oven according to claim 3, wherein the or each aperture is formed substantially
on the front-to-rear centre line of the upper wall.
5. An oven according to claim 1, wherein the lamp is positioned approximately 2/14th of the distance, from the front of the cooking chamber, along the front-to-rear
centre line of an upper wall of the chamber.
6. An oven according to claim 1, wherein the front margin (21a) of an upper wall of the
cooking chamber slopes downwardly and the aperture is formed in said front margin
so that light from the lamp is directed to a back wall of the cooking chamber.
7. An oven according to according to any preceding claim, wherein the diameter of the
or each aperture is in the range 14mm to 19mm.
8. An oven according to any preceding claim, wherein a glass cover (42) for protecting
the or each lamp protrudes through the aperture into the cooking chamber.
9. An oven according to any preceding claim, wherein the filament (41) of the or each
lamp is spaced approximately 2mm perpendicularly away from the opening of the aperture
in the outer surface of said wall.
10. An oven according to claim 2, wherein the lamps produce different coloured light.
11. A lighting device for a microwave oven having a cooking chamber for cooking food by
using microwave energy, and a magnetron oscillator producing microwave frequencies
to heat the food, comprising:
a hole formed on an upper surface of the cooking chamber; and
a lamp for brightening the cooking chamber through the hole.
12. A lighting device as set forth in claim 11, wherein the lamp includes a filament and
a bulb, and the filament is located above the upper surface.
13. A lighting device as set forth in claim 12, wherein the filament is positioned 2mm
above the upper surface.
14. A lighting device as set forth in claim 11, wherein the hole's diameter is between
14mm and 19mm.
15. A lighting device as set forth in claim 11, wherein the hole is formed in the center
of the upper surface.
16. A lighting device as set forth in claim 15, wherein the hole is formed on a spot that
corresponds to 2/14 of the upper surface's depth from the front of the upper surface.
17. A lighting device as set forth in claim 15, wherein the upper surface includes a downward-sloping
beveled portion with its lowest portion at the front of the cooking chamber, the hole
is formed on the beveled portion, and the lamp is provided to the hole so as to face
the rear of the cooking chamber.
18. A lighting device as set forth in claim 11, wherein the hole has a protective glass
protruding into the cooking chamber.
19. A lighting device as set forth in claim 11, wherein a reflecting member is provided
behind the lamp to reflect light rays from the lamp into the cooking chamber.
20. A lighting device as set forth in claim 11, wherein a lamp cover is fixedly connected
to the upper surface of the cooking chamber by means of lamp brackets so as to cover
and fix the lamp.
21. A lighting device as set forth in claim 20, wherein each of said lamp brackets has
guide pieces bent upward to be inserted into the lamp cover.
22. A lighting device for a microwave oven having a cooking chamber for cooking food by
using microwave energy, and a magnetron oscillator producing microwave frequencies
to heat the food, comprising:
at least two holes formed on the upper surface of the cooking chamber; and
lamps provided to each of the holes for brightening the cooking chamber through the holes.
23. A lighting device as set forth in claim 22, wherein the lamps produce light rays of
at least two different colors.
24. A lighting device as set forth in claim 22, wherein each of the lamps includes a filament
that is positioned on the upper surface, and a bulb that protrudes into the cooking
chamber.