WO2001022823A9

WO2001022823A9 - Lightwave conveyor oven and method of operating the same

Info

Publication number: WO2001022823A9
Application number: PCT/US2000/026867
Authority: WO
Inventors: Eugene R Westerberg; William H Schestedt; William P Minnear; Jay G Romiti; John W O'neal; Gay Winterringer
Original assignee: Quadlux Inc; Eugene R Westerberg; William H Schestedt; William P Minnear; Jay G Romiti; John W O'neal; Gay Winterringer
Priority date: 1999-09-29
Filing date: 2000-09-29
Publication date: 2002-09-26
Also published as: WO2001022823A1; AU7737500A

Abstract

A lightwave conveyor oven includes a housing, a moveable conveyor belt at least partially within the housing and extending from the input and output openings of the housing, a lightwave cooking lamp positioned within the housing and oriented to direct radiant energy onto the conveyor belt; and a hearth beneath the conveyor belt, the heart including a heating element, the hearth positioned to radiate heat onto the conveyor belt. Preferably, the lightwave cooking lamps are disposed in elliptical reflector housings.

Description

LIGHTWAVE CONVEYOR OVEN AND METHOD OF OPERATING THE SAME

Inventors: Eugene R. Westerberg, Chris Tatarian, William H. Sehestedt, John O'Neal, Jay G. Romiti, Gay Winterringer, William P. Minnear

Field of the Invention

This invention relates to the field of cooking ovens. More particularly, this invention relates to a conveyorized lightwave oven configuration.

Background of the Invention

Ovens for cooking and baking food have been known and used for thousands of years. Basically, these well-known oven types can be categorized in four cooking forms; conduction cooking, convection cooking, infra-red radiation cooking and microwave radiation cooking.

There are subtle differences between cooking and baking. Cooking requires only the heating of the food. Baking of a product from a dough, such as bread, cake, crust or pastry, requires not only heating of the product throughout, but also chemical reactions coupled with driving the water from the dough in a predeter ined fashion to achieve the correct consistency of the final product and finally browning the outside of the product. Following a recipe is very important for proper results during the baking operation. An attempt to substantially decrease the baking time in a conventional oven by increasing the temperature would result in a damaged or destroyed product.

In general, there are problems when one wants to cook or bake foodstuffs with high-quality results in short times. Conduction and convection provide the necessary quality, but both are inherently slow energy transfer methods. Long-wave infra-red radiation can provide faster heating rates, but it only heats the surface area of most foodstuffs, leaving the internal heat energy to be transferred by much slower conduction. Furthermore, the shallow heating depth of long-wave infra-red limits the rate at which heat energy can be introduced to a product, because high radiant powers at the food surface produce a burned food interface. Microwave radiation heats the foodstuff very quickly in depth, but during baking the loss of water near the surface stops the heating process before any satisfactory browning occurs. Consequently, microwave ovens cannot produce quality baked foodstuffs, such as bread.

In general, radiant cooking methods can be classified by the manner in which the radiation interacts with the foodstuff molecules. For example, starting with the longest wavelengths for cooking, the microwave region, most of the heating occurs because of the coupling of radiant energy into the bipolar water molecules causing them to rotate and thereby absorb energy to produce heat. If the wavelength is decreased to the long- wave infra-red regime, the molecules and their component atoms resonantly absorb the energy in well-defined excitation bands. This is mainly a vibrational energy absoφtion process. For the short-wave infra-red just above the visible part of the spectrum (the near-visible region), the main part of the absoφtion is due to higher frequency coupling to the vibrational modes. This absoφtion is generally weaker than the absoφtion for the long-wave infra-red. In the visible region, the principal absorption mechanism is excitation of the electrons that couple the atoms to form the molecules. These interactions are easily discerned in the visible band of the spectrum, where they are identified as "color" absorptions. Finally, in the ultraviolet, the wavelength is short enough, and the energy of the radiation is sufficient to actually remove the electrons from their component atoms, thereby creating ionized states and breaking chemical bonds. This short wavelength, while it finds uses in sterilization techniques, probably has little use in foodstuff heating, because it promotes chemical reactions and destroys food molecules.

Lightwave ovens are capable of cooking and baking food products in times much shorter than conventional ovens. This cooking speed is attributable to the range of wavelengths and power levels that are used.

Typically, wavelengths in the visible range (.39 to .77 μm) and the near-visible range (.77 to 1.4 μm) have a fairly deep penetration in most foodstuffs. This range of penetration is mainly governed by the absoφtion properties of water which is the principal constituent of most foodstuffs. The characteristic penetration distance for water varies from 30 meters in the visible to about 1 cm at 1.4 μm. Several other factors modify this basic absoφtion penetration. In the visible region electronic absorption (color absoφtion) reduces the penetration substantially, while scattering in the food product can be a strong factor throughout the region of deep penetration. Measurements show that the typical average penetration distance for light in the visible and near-visible region of the spectrum varies from a minimum of 2-4 mm for meats to as deep as 10 mm for some baked goods and liquids like non-fat milk.

It is this region of deep penetration that produces that fast cooking times seen in lightwave ovens. Because the energy is deposited in a fairly thick region near the surface of the food, the radiant power density that impinges on the food can be increased in lightwave ovens without overheating the surface temperature of the foodstuff. Consequently the radiation in the visible and near-visible regions does not contribute greatly to the exterior surface browning.

In the spectral region above 1.4 μm (infra-red region), the penetration distance decreases dramatically to fractions of a millimeter, and for certain peaks down to 100 μm (the thickness of a human hair). The power in this region is absorbed in such a small depth of penetration that the temperature at the surface rises rapidly, driving the water out and forming a water-depleted crust. With no water to evaporate and cool the surface, the temperature can climb very fast to 300°F. This is the approximate temperature where the set of browning reactions (Maillard reactions) are initiated. As the temperature is pushed even higher to above 400°F, the point is reached where the surface begins to burn.

It is the balance between the deep penetration wavelengths (.39 to 1.4 μm) and the shallow penetration wavelengths (1.4μm and greater) that allows the power density at the surface of the food to be increased in the lightwave oven, to cook the food rapidly with the shorter wavelengths and to brown the food with the longer infra-red so that a high-quality product is produced. Conventional ovens do not have the shorter wavelength components of radiant energy. The shallower penetration of the longer wavelength radiation means that if the radiant power in such an oven is increased, only the food surface is heated, with the result that the food surface is prematurely browned before its interior gets hot.

Conventional ovens operate with radiant power densities as high as about .3 W/cm² (i.e., at 400°F). The cooking speeds of conventional ovens cannot be appreciably increased simply by increasing the cooking temperature, because increased cooking temperatures drive water off the food surface and cause browning and searing of the food surface before the food's interior has been brought up to the proper temperature. In contrast, lightwave ovens have been operated from approximately 0.8 to 5 W/cm² of visible, near-visible and infra-red radiation. The greater useful power density results in substantially enhanced cooking speeds for lightwave ovens..

For high-quality lightwave cooking and baking, the applicant has found that a good balance ratio between the deeply penetrating and the surface heating portions of the impinging radiant energy is about 50:50, i.e., Power(.39μm to 1.4 μm/Power(l .4 μm and greater) * 1. Ratios higher than this value can be used, and are useful in cooking especially thick food items, but radiation sources with these high ratios are difficult and expensive to obtain. Fast cooking can be accomplished with a ratio substantially below 1, and the applicant has shown that enhanced cooking and baking can be achieved with ratios down to at least .6 for most foods, and lower for thin foods and foods with a large portion of water such as meats. If the power ratio is reduced below about .3, the power densities that can be used in cooking are comparable with conventional cooking and no speed advantage results.

If blackbody sources are used to supply the radiant power, the power ratio can be translated into effective color temperatures, peak intensities, and visible component percentages. For example, to obtain a power ratio of 1 , it can be calculated that the corresponding blackbody would have a temperature of 3000°K, with a peak intensity at .966 μm and with 12% of the radiation in the visible ranges of .39 to .77 μm. Tungsten halogen quartz lamps have spectral characteristics that follow the blackbody radiation curves fairly closely. Commercially available tungsten halogen bulbs have been successfully used as light sources for cooking with color temperatures as high as 3400°K. Unfortunately, the lifetime of such sources falls dramatically at high color temperatures (at temperatures above 3200°K it is generally less than 100 hours). It has been determined that a good compromise in bulb lifetime and cooking speed can be obtained using tungsten halogen bulbs operated at about 2900 to 3000°K as lightwave cooking lamps. As the color temperature of the bulb is further reduced and more of the shallow-penetrating infra-red is produced, the cooking and baking speeds are diminished for quality results. For most foods there is a discernible speed advantage using lamps having color temperatures down to about 2500°K (blackbody peak at about 1.2 μm and visible component of 5.5%) as the lightwave cooking lamps. In the region of 2100°K the speed advantage over convention thermal ovens vanishes for virtually all foods that have been tried. There is a need for a commercial oven that would display the characteristics of enhanced cooking speed and high quality cooking results of lightwave cooking combined with the convenience and capacity of a conveyor oven. Conventional conveyor ovens are gas-fired or electrically operated and use long-wave infra-red radiation, convection, or hot-air impingement to cook the food.

While there is a strong need for a faster and more energy efficient conveyor there has been a significant problem in providing a lightwave oven conveyor configuration. This problem is a result of the open architecture of the conveyor oven. The conveyor oven requires a cooking cavity having open input and output ends where the belt traverses the cavity. In a lightwave conveyor this means that a large portion of the high-intensity light could escape the cooking cavity through the open ends. If the escaped light is strong enough it could present a heating or eye hazard outside of the cavity. Furthermore, the light lost out of the cavity means that the overall efficiency of the oven would decrease, negating many of the advantages pointed out above. This invention seeks to overcome these problems with a novel apparatus configuration and to provide several new methods of conveyor operation.

Summary of the Invention

The present invention is a lightwave conveyor oven which includes a housing and a moveable conveyor belt at least partially within the housing and extending from the input and output openings of the housing. At least one lightwave cooking lamp is positioned within the housing and oriented to direct radiant energy onto the conveyor belt; and a hearth beneath the conveyor belt, the heart including a heating element, the hearth positioned to radiate heat onto the conveyor belt. Preferably, the lightwave cooking lamps are disposed in elliptical reflector housings.

Brief Description of the Drawings

Fig. 1 is a perspective view of the outside of the lightwave conveyor oven utilizing principles of the present invention.

Fig. 2 is a front elevation view of the lightwave conveyor oven of Fig. 1, with a portion of the housing removed to permit the upper lamp assembly to be viewed.

Fig. 3 is an end elevation view of the lightwave conveyor oven of Fig. 1.

Fig. 4A is a top plan view of the lightwave conveyor oven of Fig. 1 with a portion of the housing removed to permit the upper lamp assembly to be viewed.

Fig. 4B is a bottom plan view of a reflector, end cap, and lamp assembly of the oven of Fig. 1.

Fig. 4C is a perspective view of the reflector of Fig. 4B.

Fig. 4D is a front plan view of a reflector end cap of Fig. 4B.

Fig. 5 is a cross-sectional side view of the unfocused upper lamp- reflector assembly of the present invention.

Fig. 6 is a cross-sectional side view of the focused upper lamp- reflector assembly of the present invention.

Detailed Description of the Preferred Embodiment

The lightwave conveyor oven of the present invention is illustrated in Figs. 1 through 4C. The lightwave conveyor oven 10 includes a housing 12, a conveyor belt 14, a hearth plate 16, an upper lamp assembly 18, a lower lamp assembly 20, an electronic controller 22, and control panel 24. An input assembly 26 and an output assembly 28 are positioned at opposite ends of the housing 12. The housing 12 includes outer sidewalls 30, highly reflective inner sidewalls 32, top wall 34, and bottom wall 36. The oven cavity 38 is delimited on the sides by the inner sidewalls 32, below by the hearth plate 16 and above by a transparent splash shield 40. In one embodiment the oven cavity 38 is approximately 7" high, 28" wide (along the belt) and 19" deep (across the belt).

The conveyor belt 14 spans the oven cavity 38 and extends out through the input assembly 26 on the food entry side, and out through the output assembly 28 on the food exit side of the housing 12. In one embodiment the belt is 18" wide. It is driven by a drive assembly 42, which includes a chain drive and a drive motor. The rate of travel of the conveyor belt 14 is controlled using the control panel 24, which electronically communicates with the electronic controller 22. The control panel 24 is mounted on the front of the input assembly 26 so that it is easily viewable by an operator. The control panel 24 contains several adjustable controls 44 and switches 46 for controlling the lightwave conveyor, and a display 48 indicating the oven's mode of operation and operational parameters. The electronic controller 22 is mounted within the input assembly 26, below the conveyor belt 14. The electronic controller 22 contains a programmable microprocessor and the associated electronics so that it can control drive motor 41, the intensity of the upper lamp assembly 18, and the intensity of the lower lamp assembly 20.

A front door 50 (Fig. 1) is fitted into the housing 12so that it may be fully detached for cleaning. The front door 50 has a window 52 fitted near its center so that the food can be observed in the oven cavity 38. The window 52 is made of two pieces of glass with a low transmission film of aluminum sandwiched between so that the emerging visible and near-visible light intensity has been greatly reduced (for example by a factor of about 1000) from the lamp intensity within the oven cavity 38. This reduction is necessary to allow comfortable and non-hazardous viewing of the cooking process. A rear door 54 is similar to the front door 50 in that it is detachable, but it has no window.

Light shields 56 (Fig. 1) are positioned on both the input assembly 26 and the output assembly 28 to reduce the amount of escaping light so that the cooking cavity 38 cannot be viewed directly from either end of the conveyor belt 14.

The housing is preferably supported by four legs 58, each of which has four casters 60 for easy mobility. Each caster 60 has a braking mechanism to allow the oven to be locked against undesired rolling.

Lamp Assemblies

It has been found that improved heating and browning characteristics could be obtained using elliptical reflector configurations. If one focus of the ellipse is positioned at approximately the height of the top of the food, and if a linear lightwave cooking lamp (e.g. a tungsten-halogen bulb) is positioned at the other focus of the ellipse, a good browning source is produced. By focusing the light onto the surface of the food, the high power density drives off the surface water quickly and the incident radiation can heat the food surface above the boiling point of water (212°F) to temperatures of 300°F to 400°F. This easily browns the food. On the other hand if the lamp is positioned away from the second focus a broad, unfocused line of radiant energy results and this dispersed radiation cannot easily remove all of the surface water before more water is diffused up from the food interior. Thus this second (unfocused) configuration is useful when deep penetration without browning is desired.

It may be advantageous to use both focused and unfocused lamp- reflector unit configurations in an embodiment of a lightwave conveyor oven as is described below. In such an embodiment, the two units are put in line along the conveyor belt with the unfocused unit closest to the point of food entry. Thus when the food comes into the lightwave cavity it is first heated with the unfocused unit to obtain the optimal heating in-depth and then it passes to the focused unit to obtain its final brown finish.

Upper lamp assembly 18 includes lamps and reflectors positioned to direct radiant energy towards the conveyor belt 14. During use, the lamps illuminate the food carried through the oven cavity 38 by the conveyor belt, causing the food to be cooked with lightwave energy using an optimized mixture of visible, near-visible and infrared radiation. Upper lamp assembly 18 is disposed just above the transparent splash shield 40. Splash shield 40 is made from a strong, glass-ceramic material (pyroceram) with a low temperature coefficient. It is easily removed for cleaning and can be made in two pieces for easier handling.

The upper lamp assembly 18 is depicted in Fig. 4A. In one embodiment, upper lamp assembly 18 includes two focused lamp-reflector units 62 and two unfocused lamp-reflector units 64. The spaces between the lamp-reflector units are filled with flat reflector sheets 66 to improve the overall internal reflectivity of the oven cavity 38, and thereby improve the oven efficiency. It should be appreciated that other lamp arrangements may alternatively be used. For example, the lamp assembly may include all focused lamp-reflector units, all un-focused lamp-reflector units, or other combinations/arrangements of the units.

Each focused lamp-reflector unit 62 is comprised of an elongate elliptical reflector (Figs. 4B and 4C), a tubular 2000W 220V tungsten- halogen quartz lamp 70 (or other suitable lightwave cooking lamp), and a pair of reflector end caps (Figs. 4B and 4D). Each unfocused lamp-reflector is similarly comprised of an elliptical reflector 74, a tubular 2000 W 220V tungsten halogen lamp 76 (or other suitable lightwave cooking lamp), and two reflector end caps 78. See Fig. 3. The quartz lamps 70 and 76 have a nominal operating color temperature of 3000°K which provides a bulb lifetime in excess of 2000 hours, and a large portion of visible and near-visible radiation for deep penetration heating. The aluminum end caps 72 and 78 have highly reflecting inner surfaces facing towards the lamps and provide a template and support for the elliptically-shaped reflectors 68 and 74. The reflectors 68 and 74 are made of highly reflective (about 90%) sheet aluminum fastened to the elliptical profile of the end caps 72 and 78 respectively.

The unfocused lamp-reflector units 64 are located near the entry end of the oven cavity 38, and are used to deeply heat the incoming food with the visible and near-visible components of the lightwave radiation. These units are staggered across the center of the conveyor belt 14 to even out the illumination on the midline of the conveyor belt 14. The focused lamp- reflector units 62 are placed near the exit end of the oven cavity 38, and they are also arranged in a staggered fashion so that both units overlap the center line of the converyor belt 14, thus filling in across the width of the conveyor belt 14 with essentially unbroken, uniform illumination.

Fig. 5 depicts a ray tracing of one of the unfocused lamp-reflector units 64. The rays start from a linear lamp filament 80 positioned at about 3/8" lower than one of the foci 82 of the elliptical reflectors 74, reflect from the reflector surface and weakly reconverge to provide a fairly broad, low power density stripe just above the surface of the conveyor belt 14. This low power density is ideal for deep heating the food as it enters the oven cavity 38 with little unwanted early browning.

Fig. 6 shows a ray tracing of one of the focused lamp-reflector units 62. The rays start from a linear lamp filament 84 positioned at one of the foci 86 of the elliptical reflectors 68, bounce off of the reflector surface, and reconverge on the second ellipse foci 87. Each lamp provides a line focus 88 at approximately 5 cm above the top level of the conveyor belt 14. This focus height provides a maximum power density at approximately the height of most food products that would be cooked or baked in a conveyor oven, e.g., pizza, muffins, bagels, appetizers, and cookies. The high power density of the focus of the focused lamp-reflector units removes the water rapidly from the surface of any food item that passes the focus.

If the surface water removal rate is higher than the water replenishment rate from the food interior, the surface water is removed. The temperature of the surface is elevated above the boiling point of water before interior water from the food can diffuse to the surface and cool it by evaporation. When the temperature gets to about 300°F the browning reactions start to take place. It has been found that the browning rate for a focused reflector unit is several times faster than for unfocused illumination, even though the lamps are run at the same overall power levels, with the same color temperatures and the same outputs of visible, near-visible and infra-red power. The browning response to power density is suφrisingly non-linear, and the browning is triggered at a definite power density level with little browning below the level and considerable browning above the level. It is interesting to watch a brownable foodstuff, such as pizza crust, move beneath one of the focused reflector-units. As the conveyor belt moves the crust under the lamp focus, the lamp appears to "paint" the crust surface brown.

The lower lamp assembly 20 is illustrated in Figs. 2 and 3. It consists of three 1000W 2500° tungsten-halogen quartz lamps 90. The lower lamps 90 are 22" long and are arranged as a parallel group of three lamps with the center lamp on the midline of the conveyor belt and the outer lamps spaced 4.5" on either side of the center lamp. The lower lamps 90 are surrounded on five sides by highly reflective polished aluminum to uniformly illuminate the sixth side, the black hearth plate 16, with the minimum power loss. The bottom and side reflectors are combined in a lower reflector 92. It consists of a simple flat sheet with the sides bent upward at an angle of about 45° to improve hearth heating uniformity. The two end reflectors 94 are essentially flat reflecting plates. The center lower lamp 90 is connected to the electronic controller 22 and using the signal from the thermocouple 96 as a reference its intensity is controlled to maintain the hearth plate 16 at the desired temperature.

The hearth plate 16 is positioned above the lower lamp assembly 20. Hearth plate 16 is a thin blackened steel plate that is heated by direct radiation from the lower lamp assembly 20. In this manner, the visible radiation from the lower lamps is fully converted to long- wave infrared radiation. The long- wave radiation is appropriate for heating the food containers/supports on the conveyor belt without creating an eye hazard. It should be noted that although it is preferred to use a lightwave cooking lamp to heat the heart, other forms of heating elements known in the art may alternatively be used to heat the hearth.

When no food is on the belt, the black hearth absorbs most of the visible light from the top lamps and only a minimal amount is scattered and escapes.

Hearth Plate

Hearth plate 16 is supported in several small areas to insure low conductive heat losses. A thermocouple 96 is attached to the bottom of the hearth plate 16, and the electronic signal derived from the temperature of the hearth plate 16 is sent to the electronic controller 22. The electronic controller 22 uses that signal to electronically adjust the power going to the lower lamp assembly 20 so that the temperature of the hearth plate 16 can be set and maintained at a desired temperature.

It has been found that a thin blackened steel hearth plate positioned over the bottom lamps, so that the radiant energy of the lamps is used primarily (or even exclusively) to heat the plate, will not adversely effect to the speed of cooking to any great degree. This unanticipated result suggested that when the hearth plate was heated hot enough (750°F to 850°F) the radiation from the plate (long-wave infrared) heated the bottom of the food containers efficiently and uniformly. The bottom heating of the pans did not depend on the deeply penetrating parts of the lightwave illumination, and hence the speed was largely uninfluenced. However, heating the hearth plate with concealed lamps below it provides an added advantage in that there is negligible emission of visible light from the bottom lamp unit into the line of vision of a person using the oven. Eliminating escape of radiant energy from the lower lamps is highly desirable. If used in a typical conveyor oven, the lower lamps would be below eye level and their bright radiant energy would thus be upwardly directed, potentially towards the eye of an operator standing near one of the oven's open ends. By using the hearth, the light from below is confined and converted to heat directly with no visible exterior emission that might cause retinal damage to an operator..

Moreover, since the upper lightwave lamps heat the food in depth with deeply penetrating visible and near-visible radiation, directing or even focusing the radiation onto the black hearth surface causes most of the radiation to be absorbed into the hearth surface. Again, allows for highly efficient heating without scattering potentially harmful amounts of visible light outside of the oven.

The upper lamp assembly 18 is cooled with three small (5") axial flow fans 98. The cool air is brought in from the outside through two chevron louvers 100 positioned just above the output assembly 28, one at the front of the oven and the other in the rear. The chevron louvers are angled so that they pull cool air in from the side of the oven instead of the hot air from the bottom that has been pre-heated by the hearth plate 16. This greatly aids the cooling of the top lamps. The air from the fans 98 cools the upper lamp assembly 18 and is exhausted as hot air that is directed down on the food placed on the conveyor belt 14 above the input assembly 26. This is a energy efficient use of the hot exhaust air that serves to preheat the entering food. Operation

The operation of the lightwave conveyor oven of the present invention can be described as follows. If the oven has been turned off, the hearth plate 16 needs to be preheated to its starting temperature. Generally, the hearth plate 16 is operated at a temperature of 750°F to 850°F. This preheating operation takes about 7 to 10 minutes depending on the desired hearth temperature. When the hearth plate 16 has attained the prescribed temperature the electronic controller 22 will cycle the power to the lower lamp assembly 20 to maintain the hearth temperature. The hearth plate 16 acts like a heat capacitor, storing heat energy during its preheat phase. This helps it maintain a constant temperature and radiance when power is absorbed by the food load passing above it. Its heating time constant of about 5 minutes is short enough that it can quickly be brought back to the desired temperature.

In the simplest mode of operation the conveyor belt speed, the hearth temperature, and the upper lamp intensity are set manually by controls 44 on the control panel 24. The food to be cooked is placed in a suitable container such as a black pan or screen. This is put onto the moving conveyor belt 14 at the input assembly 26. The food is then preheated by the air that is used to cool the upper lamp assembly 18. The belt 14 moves the food into the cooking cavity 38 where the unfocused lamp assembly 64 deeply heats the food from above and the hearth plate 16 heats the food or food container from below. The food proceeds to move under the focused lamp assembly 62 where it is further heated and browned. It then exits to the output assembly 28 where it can be taken from the conveyor belt 14, and served.

The oven may optionally include a "stand-by" mode of operation, during which most of the unit is powered-down to save energy costs. Because of the fast response of the tungsten-halogen lamps the oven can be brought on-line quickly when triggered to do so by the operator or by feedback from a sensor that detects that food has been placed into the oven. In an embodiment having "stand-by" mode, another thermocouple 102 is added to the entrance end of the hearth plate. As the food passes over the thermocouple 102, its temperature drops, and this signal informs the electronic controller 22 to boost the power to the hearth plate 16 so that the hearth plate temperature is restored by the time the food has reached the center of the oven cavity. This feature is especially useful if the oven is operated in a stand-by mode during low usage periods. In the stand-by mode the power to the oven is reduced and the hearth temperature allowed to drop somewhat. When new food is detected by the thermocouple 102, the oven comes up to full power rapidly to cook the item. Other food detection devices such as mechanical switches and photoswitches could be used in place of the thermocouple, but the thermocouple has the added advantage that it detects the size of the temperature drop and this information allows the electronic circuitry to make a better correction to the hearth temperature.

In an alternative form of operation, the food may be placed in a container that identifies the food type. For example, the container plates may be colored, or they may have different bar codes. Each container is sensed by a suitable detector before it passes into the oven cavity 38. Each food has a recipe encoded in the electronic controller 22 and when the food is recognized the electronic controller 22 changes the upper lightwave intensity, the belt speed, and the hearth temperature, if necessary, to optimize the cooking for the particular food as it passes through the oven cavity 38. With the fast time response of the lamps and the advantage of microprocessor control, it is possible to cook many different food products in a row, each of which has a different recipe for light control, and to produce high-quality results in all of the food products cooked in sequence. This facility greatly expands the usefulness of the conveyor to cook a larger spectrum of foods than can now be cooked in traditional conveyors, and furthermore the quality of the finished food product would be superior from such an oven.

The lightwave conveyor oven described herein provides a number of advantages over conventional conveyors. For example, the conveyorized lightwave oven uses lightwave cooking lamps to provide a power density inside the oven cavity that cooks food faster than standard conveyor ovens for a given input power.

Moreover, lightwave conveyor ovens can be powered "on" when the oven is needed and left in a low-power standby mode for the rest of the time, whereas standard conveyors need to be kept at cooking temperature most of the time. This makes the lightwave conveyor oven more energy efficient than its conventional counteφart. The higher efficiency means that the heat load into the oven's surroundings is less, and this implies lower air- conditioning costs. Similarly, with higher efficiencies the outside surfaces of the oven would stay at lower temperatures than the conventional conveyors.

Another advantages include quieter operation, in that the oven would operate at lower sound levels than conventional impinger ovens, since there is no need for large, noise-producing blowers

It is to be understood that the present invention is not limited to the embodiments described above and illustrated herein. For example, it is within the scope of the invention to use a different number of lamps, lamps of different powers or different color temperatures. Similarly, it is within the scope of the invention to change the size of the cooking cavity, the materials used to obtain high reflectivities, the width and length of the belt, the number of belts per oven, and the total oven power.

Claims

CLAIMSWe Claim:

1. A lightwave conveyor oven comprising: a housing having an input opening and an output opening; a moveable conveyor belt at least partially within the housing and extending from the input opening and the output opening; a lightwave cooking lamp positioned within the housing and oriented to direct radiant energy onto the conveyor belt; and a hearth beneath the conveyor belt, the heart including a heating element, the hearth positioned to radiate heat onto the conveyor belt.

2. The lightwave conveyor oven of claim 1 wherein the heating element includes a second lightwave cooking lamp positioned to direct radiant energy onto the hearth.

3. The lightwave cooking oven of claim 2 wherein the second lightwave cooking lamp is positioned below the hearth.

4. The lightwave oven of claim 3 wherein the hearth is positioned to prevent visible radiant energy from the second lightwave cooking lamp from passing through the input and output openings.

5. The lightwave oven of claim 1, wherein the housing includes a light shield partially covering the input opening, the light shield positioned to minimize passage of visible radiant energy from the lightwave cooking lamp through the input opening.

6. The lightwave oven of claim 1, wherein the housing includes a light shield partially covering the output opening, the light shield positioned to minimize passage of visible radiant energy from the lightwave cooking lamp through the output opening.

7. The lightwave oven of claim 1, wherein the lightwave cooking lamp is mounted in a semi-elliptical reflector.

8. The lightwave oven of claim 7 wherein the semi-elliptical reflector includes two foci, and wherein the lightwave cooking lamp is positioned at one of the foci of the reflector.

9. The lightwave oven of claim 7 wherein the semi-elliptical reflector includes two foci, and wherein the lightwave cooking lamp is spaced apart from the foci of the reflector.

10. The lightwave oven of claim 1, wherein the lightwave cooking lamp is one of a plurality of lightwave cooking lamps positioned within the oven.

11. The lightwave oven of claim 1, wherein the lightwave cooling lamp is at least one of a plurality of lightwave cooking lamps positioned above the conveyor within the housing, and wherein at least one of the lightwave cooking lamps is mounted in a semi-elliptical reflector.

12. The lightwave oven of claim 1 , further including a sensor positioned to detect movement of a food item into the housing on the conveyor.

13. The lightwave oven of claim 12, wherein the sensor is a thermocouple positioned to detect a drop in temperature upon movement of a food item into the housing.

14. A method of cooking using a lightwave oven, including the steps of: providing a lightwave oven comprised of a housing, a conveyor belt extending through the housing, at least one lightwave cooking lamp positioned within the housing and above the conveyor belt, and a hearth positioned beneath the conveyor belt; placing a food item on the conveyor belt, and causing the conveyor belt to carry the food item beneath the cooking lamp within the oven housing; heating the hearth to an elevated temperature; and causing the heated heart to radiate cooking energy onto the conveyor.

15. The method of claim 14, further including the steps of: detecting movement of the food item into the housing; and illuminating the lightwave cooking lamp in response to detection of movement of the food item into the housing.

16. The method of claim 15 wherein the detecting step includes using a thermocouple to detect a temperature drop in response to passage of the food item into the housing.

17. The method of claim 14, wherein the providing step further provides a heating element, wherein the step of heating the hearth includes heating the hearth with the heating element, and wherein the method further includes the steps of: detecting movement of the food item into the housing; and causing the heating element to increase the temperature of the hearth in response to detection of movement of the food item into the housing.

18. The method of claim 17 wherein the detecting step includes using a thermocouple to detect a temperature drop in response to passage of the food item into the housing.

19. The method of claim 14, further including the steps of: determining the type of food item being placed in the oven; and controlling the speed of the conveyor belt to achieve the appropriate cooking time for the determined type of food item.

20. The method of claim 14, further including the steps of: determining the type of food item being placed in the oven; selecting a lamp intensity for the determined type of food item; controlling the intensity of the lightwave cooking lamp to achieve the selected lamp intensity.

21. The method of claim 14, further including the steps of: determining the type of food item being placed in the oven; selecting a hearth temperature for the determined type of food item; and controlling the hearth temperature to achieve the selected hearth temperature.

22. The method of claim 14, further including the steps of: determining the type of food item being placed in the oven; determining a recipe for the determined type of food item; and controlling the speed of the conveyor, the intensity of the lightwave cooking lamp, and the temperature of the hearth in accordance with the determined recipe,

23. The method of claim 22 wherein the food item is provided to be contained in a container bearing indicia representing the food type, and wherein the step of determining the type of food item is carried out using a sensor to detect the indicia from the food container.

24. The method of claim 23 wherein the indicia is a bar code and wherein the sensor is a bar code detector.

25. The method of claim 23 wherein the indicia is a color indicia and wherein the sensor is an optical detector.

26. The method of claim 14, further including the steps of: drawing air into the housing to cool the at least one lightwave cooking lamp, the drawn air becoming warm air; and directing the warm air onto food entering the housing on the conveyor.