Abstract:
The present invention relates to an oven that includes both radiant cooking elements and a microwave cooking element. The cooking elements are controlled to provide reduced cooking time as compared to known radiant ovens, yet a wide variety of foods can be cooked in the oven. The oven is operable in a speed cooking mode wherein both radiant and microwave cooking elements are utilized, a microwave only cooking mode wherein only the magnetron is utilized, and a radiant only cooking mode wherein only the lamps are utilized.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional application No. 60/115,744, filed Jan. 13, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to ovens and, more particularly, to a combination oven using both radiant and microwave energy. 
     Known ovens are either, for example, microwave or radiant cooking type ovens. For example, a microwave oven includes a magnetron for generating RF energy used to cook food in the oven cooking cavity. Although microwave ovens cook food more quickly than radiant ovens, microwave ovens do not brown the food. Microwave ovens therefore typically are not used to cook as wide a variety of foods as radiant ovens. 
     Radiant cooking ovens include an energy source such as lamps which generate light energy used to cook the food. Radiant ovens brown the food and generally can be used to cook a wide variety of foods. Radiant ovens, however, cook foods slower than microwave ovens. It would be desirable to provide an oven which provides the speed advantages of microwave ovens yet also can be used to cook a wide variety of foods such as with radiant ovens. 
     BRIEF SUMMARY OF THE INVENTION 
     In an exemplary embodiment of the invention, an oven includes both radiant cooking elements and a magnetron, or microwave, cooking element. The cooking elements are controlled to provide reduced cooking time as compared to known radiant ovens, yet a wide variety of foods can be cooked in the oven. The oven is operable in a speed cooking mode wherein both radiant and microwave cooking elements are utilized, in a microwave only cooking mode wherein only the magnetron is utilized for cooking, and a radiant only cooking mode wherein only the lamps are utilized for cooking. 
     In one aspect, the present invention relates controlling the power levels, and adjusting the power levels, in the speed cooking mode. More particularly, by controlling the power levels of the radiant cooking elements and the microwave cooking element, as well as the length of cooking time, desired cooking operations can be achieved. 
     In another aspect the present invention relates to adjusting the cooking time during cooking operations. Such adjustment is sometimes referred to herein as “active time adjustment.” By enabling an operator to easily and quickly adjust the selected cooking time during cooking operation, the desired cooking can be achieved. 
     In yet another aspect the present invention relates to illuminating the cooking cavity during cooking operations, such as during microwave only mode cooking operations and cooking operations in which the lamps are not sufficiently energized to enable visualization of food in the cooking cavity. Since halogen lamps are used for radiant cooking, the oven door has a very dark glass window which does not enable visualization of the cooking cavity with typical microwave oven lighting. The present invention, however, provides for visualization of food in the cooking cavity during cooking operations by energizing a selected halogen lamp for a short period of time (i.e., 4 seconds) so that the cooking cavity is illuminated but negligible cooking is performed by the light energy output by the lamp. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front view of an oven in accordance with one embodiment of the present invention; 
     FIG. 2 is a perspective schematic view of a portion of the oven shown in FIG. 1; 
     FIG. 3 is a schematic illustration of the radiant cooking unit and the microwave cooking unit relative to the cooking cavity; 
     FIG. 4 is a schematic illustration of the lower lamp of the oven shown in FIG. 1; 
     FIG. 5 is a schematic illustration of the reflector for the upper lamps of the oven shown in FIG. 1; 
     FIG. 6 is an illustration of a portion of the turntable of the oven shown in FIG. 1; 
     FIG. 7 is a schematic illustration of the cooking cavity of the oven shown in FIG. 1, including a damper to control air flow; 
     FIG. 8 is a functional block diagram of the oven shown in FIG. 1; 
     FIG. 9 is a circuit schematic diagram of the oven shown in FIG. 1; 
     FIG. 10 is a timing diagram illustrating target and command times for energizing the cooking elements; 
     FIGS. 11-14 illustrate messages displayed when adjusting/entering the power level and cooking time; 
     FIG. 15 is a flow chart illustrating process steps executed when adjusting the cook time; 
     FIG. 16 is a flow chart illustrating process steps for lamp power level control; and 
     FIG. 17 is a flow chart illustrating process steps for the soft start of the Halogen lamps. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed, in one aspect, to operation of an oven that includes at least two types of cooking elements, such as radiant and microwave cooking elements. Although one specific embodiment of radiant/microwave cooking oven is described below, it should be understood that the present invention can be utilized in combination with many other such ovens and is not limited to practice with the oven described herein. For example, the oven described below is an over the range type oven. The present invention, however, is not limited to practice with just over the range type ovens and can be used with many other types of ovens. 
     FIG. 1 is a front view of an over the range type oven  100  in accordance with one embodiment of the present invention. Oven  100  includes a frameless glass door  102  having an injection molded handle  104 . A window  106  is provided for visualizing food in the oven cooking cavity. Door  102  has an inner metal frame that extends around the door periphery and comprises an RF door choke. The glass of door  102  has, for example, a thickness of about ⅛″ and can withstand high temperatures, as is known in the art, and is secured to the inner metal frame by an adhesive. Handle  104  also is secured to the metal frame by bolts that extend through openings in the glass. Oven  100  also includes an injection molded plastic vent grille  108  and a frameless glass control panel  110 . 
     Rubber tactile switch covers  112  are located over each key pad of panel  110 , and an injection molded knob or dial  114  is provided for making multiple selections. Selections are made using dial  114  by rotating dial  114  clockwise or counter-clockwise and when the desired selection is displayed, pressing dial  114 . The various selections available, in an exemplary embodiment, from dial  114  are set forth in Appendix A. Instructions and selections are displayed on a vacuum fluorescent display  116 . 
     Referring now to FIG.  1  and Appendix A, wherein in one exemplary illustration of the operation of dial  114  and display  116 , the steps necessary to program oven  100  to cook a 0.5 inch (1.3 cm) beef steak until well done are presented. The SELECT FOOD TYPE menu is first displayed on display  116 . Next, dial  114  is rotated util the MEATS food type is displayed and dial  114  is then pressed to selected the MEATS food type. Next, the SELECT MEAT: menu is displayed. Next, dial  114  is rotated until the STEAKS (BEEF) meat is displayed then the dial is pressed to select STEAKS (BEEF) meat. Next, the SELECT THICKNESS: menu is displayed and 0.5 INCH (1.3 cm) beef thickness is selected. Next, a SELECT HOW DONE: menu is displayed and dial  114  is rotated until the MEDIUM menu is displayed, then the MEDIUM menu is selected. Next, an instruction is displayed indicating “Use ROUND METAL TRAY”, and the Upper Power Level (UPL) and Lower Power Level (LPL) settings are displayed. It is noted that UPL and LPL may be changed, as further described below. It is understood that the above illustration is but one example of many cooking selections which may be made for programming oven  100  to cook a plurality of foods. 
     The following functions can be selected from respective key pads of panel  110 . 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 CLEAR/OFF 
                 Selecting this pad stops all cooking and erases 
               
               
                   
                 the current program. 
               
               
                 DELAYED 
                 Selecting this pad results in a delay in the start 
               
               
                 START 
                 of cooking. 
               
               
                 HELP 
                 Selecting this pad enables an operator to find out 
               
               
                   
                 more about the oven and its features. 
               
               
                 MICROWAVE 
                 Selecting this pad enables defrosting, heating 
               
               
                   
                 beverages, reheating leftovers, popcorn, 
               
               
                   
                 vegetables, and all types of microwave cooking. 
               
               
                 MICROWAVE 
                 Selecting this pad enables quick and easy 
               
               
                 EXPRESS 
                 warming of a sandwich, or reheat of coffee. 
               
               
                 OPTIONS 
                 Selecting this pad enables access to the auto 
               
               
                 ON/OFF 
                 night light, beeper volume control, clock, clock 
               
               
                   
                 display, and display scroll speed features. 
               
               
                 OVEN LIGHT 
                 Selecting this pad during microwave cooking 
               
               
                   
                 illuminates the cavity. 
               
               
                 POWER LEVEL 
                 Selecting this pad enables adjusting the power 
               
               
                   
                 levels for speed cooking and microwave 
               
               
                   
                 cooking. 
               
               
                 REMINDER 
                 Selecting this pad enables an operator to select a 
               
               
                   
                 time at which an alarm is to sound. 
               
               
                 REPEAT LAST 
                 Selecting this pad facilitates cooking repetitive 
               
               
                   
                 items such as cookies and appetizers. 
               
               
                 SPEED COOK 
                 Selecting this pad enables an operator to 
               
               
                 MANUAL 
                 manually enter speed cooking time and power 
               
               
                   
                 levels. 
               
               
                 START/PAUSE 
                 Selecting this pad enables an operator to start or 
               
               
                   
                 pause cooking. 
               
               
                 SURFACE LIGHT 
                 Selecting this pad turns ON/OFF the surface 
               
               
                   
                 light for the cooktop. 
               
               
                 TIMER ON/OFF 
                 Selecting this pad controls a general purpose 
               
               
                   
                 timer (e.g., minutes and seconds) 
               
               
                 VENT FAN 
                 Selecting this pad enables an operator to clear 
               
               
                   
                 the cooktop area of smoke or steam. 
               
               
                   
               
             
          
         
       
     
     FIG. 2 is a perspective schematic view of a portion of oven  100 . Oven  100  includes a shell  120 , and a cooking cavity  122  is located within shell  120 . Cooking cavity  122  is constructed using high reflectivity (e.g., 72% reflectivity) stainless steel. Halogen lamps  124  and  126 , and a reflective plate  128  are mounted to an upper panel  130  of shell  120 . As described below in more detail, a halogen lamp also is located at a lower section of shell  120 . An exhaust system  132  also is mounted to shell  120 . Air flows through cavity  122  in a direction indicated by arrow  134 . A cooling system  137  is mounted to shell  120  for cooling oven components. Exemplary dimensions of oven  100  are set forth below. 
     
       
         
               
               
             
               
               
               
             
               
               
             
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Shell 
               
               
                   
                   
               
             
          
           
               
                   
                 Exterior Height (front) 
                 15 11/16″ 
               
               
                   
                 Exterior Height (rear) 
                 16 ½ 
               
               
                   
                 Exterior Width 
                 29 4/5″ 
               
               
                   
                 Exterior Depth 
                 14 4/5″ 
               
               
                   
                   
               
             
          
           
               
                   
                 Cooking Cavity 
               
               
                   
                   
               
             
          
           
               
                   
                 Cavity Height 
                  8 2/5″ 
               
               
                   
                 Cavity Width 
                 19 2/7″ 
               
               
                   
                 Cavity Depth 
                 13 3/5″ 
               
               
                   
                   
               
             
          
         
       
     
     FIG. 3 is a schematic illustration of oven  100 , and particularly of halogen lamp cooking units  150  and  152  and microwave cooking unit  154  relative to cooking cavity  122 . As shown in FIG. 3, upper cooking unit  150  includes two halogen lamps  124  and  126  and cooking unit  152  includes one halogen lamp  156 . Lamps  124 ,  126 , and  156 , in an exemplary embodiment, are 1500 W halogen lamps having a color temperature of approximately 2300 K, each with an output power of about 1.5 kW (4.5 kW total for all three lamps). Lamp  124  is referred to as the upper center lamp, and lamp  126  is referred to as the upper exterior lamp. Lamp  156  is referred to as the lower lamp. Glass plates  158  and  160  extend over cooking units  150  and  152  between lamps  124 ,  126 , and  156  and cavity  122 . Also, twist mesh screens  162  and  164  having an opening ratio of about 80% are provided for additional protection. Additional details are provided below with respect to reflector  128 . A magnetron  166  of microwave cooking unit  154  is located on a side of cavity  122 . Magnetron  166 , in an exemplary embodiment, delivers a nominal 950 W into cavity  122  according to standard IEC (International Electrotechnical Commission) procedure. 
     With respect to lower lamp  156 , and referring to FIG. 4, lamp  156  is located off center and at an angle relative to a bottom surface  172  of cavity  122 . Such location of lower lamp  156  results, for example, in lowering the temperature of the rollers on turntable  136 . 
     FIG. 5 is a schematic side illustration of reflector  128 . Reflector  128  includes angular side sections  180  and  182  and angular center sections  184  and  186 . The dimensions (in millimeters) indicated in FIG. 5 are exemplary and have been found suitable for at least one oven. By selecting the reflector dimensions as indicated in FIG. 5, upper lamps  124  and  126  are believed to provide more even cooking of items located on turntable  136 . 
     FIG. 6 illustrates a portion of turntable  136 . Turntable  136  has an open grille construction with about a 70% energy transmission. Turntable  136  rotates at about 6 r.p.m. and has a diameter of about 11⅛″. Turntable  136  includes metal segments  190  with ceramic rollers  192 , one of which is illustrated within circle  194 . 
     FIG. 7 illustrates a damper  194  located below microwave cooking unit  154 . Damper  194  is open when in the microwave only mode to enable air to flow through cavity  122 . In the speed cooking and radiant only mode, damper  194  closes to prevent air from flowing in a reverse direction and back towards microwave cooking unit  154 . 
     FIG. 8 is a functional block diagram of oven  100 . As shown in FIG. 8, oven  100  includes a mounting system  200 , a structural system  202 , a control system  204 , an electrical system  206 , RF generation  208 , a component cooling system  210 , halogen lamps  212 , and a food containment system  214 . Various features of each system are indicated in FIG.  8 . Mounting system  200  is provided to enable mounting oven over the range. Mounting system  200  also provides connection with an exhaust to enable removal of fumes from over the cooktop into the exhaust. Structural system  202  generally refers to shell  120 , which provides an enclosure. Control system  204  includes an interface, i.e., keypads  112  and dial  114 , and also distributes power to the other oven systems. Electrical system  206  powers the control and safety devices. RF generation  208  is performed by magnetron  166 , and RF energy output by magnetron  166  is selectively used to cook food in food containment system. Component cooling system  210  is provided to cool the other system and to remove moisture from cavity  122 . Halogen lamps  212  generate light energy used for cooking food in food containment system  214 . 
     FIG. 9 is a schematic diagram of oven  100 . Power is provided to oven  100  via lines L 1 , L 2 , and N. Relays R 1 -R 13  are connected to a microcomputer which is programmed to control the opening and closing thereof. Lower lamp  156  is electrically connected to line L 1  via a thermal cut off  300 . Energization of lower lamp  156  is controlled by relays R 1  and R 2 . An electronic switching device is in series with relay R 1  to provide a soft start, as described below in more detail. Upper lamps  126  and  124  are connected to line L 2  via thermal cut offs  304  and  306 . Electronic switching devices  308  and  310  are in series with relay R 4 . In one exemplary embodiment electronic switching devices  302 ,  308 ,  310  and  326  are TRIACS™. 
     Relays R 1  and R 4  are air gap type relays, and are in series with electronic switching devices  302  and  308 , respectively. Relays R 1  and R 4  are closed in the soft start operation of respective lamps  124 ,  126 , and  156  to enable energization of electronic switching devices  302  and  308 . After completion of the soft start, relays R 1  and R 4  are open. Relays R 2 , R 3 , and R 6  are controlled by the microcomputer to close after the soft start is completed to hold lamps  124 ,  126 , and  156  on based on the particular power setting. 
     Oven  100  also includes an upper blower motor  312  and a lower blower motor  314  for cooling. A small synchronous motor  317  when energized, closes damper  194 . Thermal cut outs  318  and  320  and a fuse  322  also are provided to protect oven components, e.g., from overheating or an overcurrent condition. Cooktop lamps  324  are electrically connected in series with an electronic switching device  326  and are provided for illuminating the cooktop. 
     A vent motor  328  having low, slow, and high speeds selectable via relays R 7 , R 8 , and R 9  is provided for removing fumes from over the cooktop. An oven lamp  330 , fan motor  332 , and a turn table motor  334  are controlled by separate relays R 10 , R 11 , and R 12 . A primary interlock switch  336  is located in door  102  and prevents energization of cooking elements unless door  102  is closed. A relay R 13  controls energization of microwave cooking unit  154 . Microwave cooking unit  154  includes a high voltage transformer  338  which steps up the supply voltage from 120V to 2000V. A high voltage capacitor  340  and a high voltage diode  342  circuit steps up the voltage from transformer  338  from about 2000V to about 4000V. This high voltage is supplied to magnetron  166  and the output of magnetron  166  is supplied to a waveguide  344  which directs RF energy into cooking cavity  122 . As also shown in FIG. 9, oven  100  includes a door sensing switch  346  for sensing whether door  102  is opened, a humidity sensor  350  for sensing the humidity in cooking cavity  122 , a thermistor  352 , and a base thermostat  354 . 
     With respect to speed cooking operation of oven  100 , the microcomputer controls relays R 1 -R 6  and R 13  based on the power level either associated with the preprogrammed cooking program or manually entered. In the speed cooking mode, for example, if a power level  9  is selected, the upper exterior lamp  126  has a target on-time of 29 seconds of a 32 second duty cycle, upper center lamp  124  has a target on-time of 25 seconds of a 32 second duty cycle, lower lamp  156  has a target on-time of 29 seconds of a 32 second duty cycle, and magnetron  16  has a target on-time of 29 seconds of a 32 second duty cycle. A duty cycle of 32 seconds is selected for one particular implementation. However, other duty cycles could be utilized. Set forth below is a chart which sets forth the target on-times based on power level. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
               
               
                   
                 Upper 
                 Upper 
                   
                   
               
               
                   
                 Exterior 
                 Center 
                 Lower 
                   
               
               
                 Power Level 
                 Lamp 
                 Lamp 
                 Lamp 
                 Magnetron 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                 1 
                 3 
                 3 
                 3 
                 3 
               
               
                 2 
                 6 
                 5 
                 6 
                 6 
               
               
                 3 
                 10 
                 8 
                 10 
                 10 
               
               
                 4 
                 13 
                 11 
                 13 
                 13 
               
               
                 5 
                 16 
                 14 
                 16 
                 16 
               
               
                 6 
                 19 
                 16 
                 19 
                 19 
               
               
                 7 
                 22 
                 19 
                 22 
                 22 
               
               
                 8 
                 26 
                 22 
                 26 
                 26 
               
               
                 9 
                 29 
                 25 
                 29 
                 29 
               
               
                 10 
                 32 
                 27 
                 32 
                 32 
               
               
                   
               
             
          
         
       
     
     To increase lamp reliability, a soft start operation is used when energizing lamps  124 ,  126 , and  156 . Particularly, in accordance with the soft start operation, triacs  302 ,  308 , and  310  are utilized to delay lamp turn-on. For example, upper exterior lamp  126  and lower lamp  156  are delayed for one second from commanded turn-on to actual turn-on. Upper center lamp  124  is delayed for two seconds from commanded turn-on to actual turn-on. Therefore, the target turn-on times are different from the commanded on-times. Set forth below is a table containing the commanded on-times based on power level selected. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
               
               
                   
                 Upper 
                 Upper 
                   
                   
               
               
                   
                 Exterior 
                 Center 
                 Lower 
                   
               
               
                 Power Level 
                 Lamp 
                 Lamp 
                 Lamp 
                 Magnetron 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                 1 
                 4 
                 5 
                 4 
                 3 
               
               
                 2 
                 7 
                 7 
                 7 
                 6 
               
               
                 3 
                 11 
                 10 
                 11 
                 10 
               
               
                 4 
                 14 
                 13 
                 14 
                 13 
               
               
                 5 
                 17 
                 16 
                 17 
                 16 
               
               
                 6 
                 20 
                 18 
                 20 
                 19 
               
               
                 7 
                 23 
                 21 
                 23 
                 22 
               
               
                 8 
                 27 
                 24 
                 27 
                 26 
               
               
                 9 
                 30 
                 27 
                 30 
                 29 
               
               
                 10 
                 32 
                 29 
                 32 
                 32 
               
               
                   
               
             
          
         
       
     
     For example, if upper lamps  124  and  126  are to operate at power level  7 , then upper lamp  124  would be commanded to operate for 21 seconds and upper exterior lamp  126  would be commanded to operate for 23 seconds. Lamps  124  and  126  would be commanded to turn-on for 21 and 23 seconds, respectively, at the beginning of each 32 second duty cycle. Due to the soft-start delays, lamps  124  and  126  would actually be on for 19 seconds (lamp  124 ) and 22 seconds (lamp  126 ) of each 32 second duty cycle. 
     FIG. 10 is a timing diagram illustrating the state of lamps  124 ,  126 , and  156 , and magnetron  166 . In the example, refrigerated crescent rolls are to be cooked in accordance with the following: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Total Time: 
                 4:30 
               
               
                   
                 Upper Power Level: 
                 10 
               
               
                   
                 Lower Power Level: 
                  3 
               
               
                   
                 Microwave Power Level: 
                  3 
               
               
                   
                   
               
             
          
         
       
     
     As shown in FIG. 10, upper center lamp  124  is commanded on (dashed line) two seconds before it actually turns on (solid line). Lamp  124  is on for 27 seconds of each 32 second period. Upper exterior lamp  126  is always on during this period. Lower Lamp  156  is on one second after it is commanded to turn on, and in on for 10 seconds out of each 32 second period. Magnetron  166  has no delay between command and execution of on time, and is on for 10 seconds of each 32 second period. 
     An operator may adjust the power level of the upper lamps, the lower lamp, and the microwave during operation. To change the power level, the operator selects the POWER LEVEL pad and a select icon flashes on display  116 . A message “Select UPPER POWER” then is displayed as shown in FIG.  11 . Rotation of dial  114  then enables an operator to select the upper power level (clockwise rotation increases the power level and counter clockwise rotation decreases the power level). When dial  114  is pressed to enter the selection, a short beep sounds and “Select LOWER POWER” is displayed as shown in FIG.  12 . Dial rotation then alters the current lower power level, and when dial  114  is pressed, a short beep is sounded. Then, “Select MICRO POWER” is displayed as shown in FIG.  13 . Dial rotation now alters the microwave power level. When dial  114  is pressed to enter the selection, a short beep is sounded and the OVEN icon flashes and the SELECT icon is turned off. “ADJUST TIME or START” is then displayed as shown in FIG.  14 . The time may be adjusted or the START pad pressed. 
     When the power level pad is pressed at an acceptable time during lightwave cooking, i.e., one or more of the lamps are energized, the cooking countdown continues and the UPL (FIG.  11 ), LPL (FIG. 12) and MPL (FIG. 13) displays appear. The same operation as described above is utilized except that after entering the new microwave power level, 2 short beeps are sounded and the countdown and UPL, LPL and MPL display continue for 2.0 seconds. After 2.0 seconds, the UPL, LPL and MPL displays are removed and only the cooking countdown continues. If the power level pad is pressed when it is not allowed to change/enter or recall the power level, a beep signal (0.5 seconds at 1000 hz) sounds and the message “POWER LEVEL MAY NOT BE CHANGED AT THIS TIME” scrolls on display  114 . After the scroll has completed, the previous foreground features return. If the power level pad is pressed at a time when a change/entry is allowed, but no dial rotation or entry occurs within about 15 seconds, the UPL, LPL and MPL display are removed and the display returns to the cooking countdown. 
     FIG. 15 is a flow chart  400  illustrating process steps executed when adjusting the cook time during cooking operations. During cooking operations, a main cooking routine COOK is executed. If dial  114  is not moved  404 , the main cooking routine continues to be executed  406 . If dial  114  is moved, then the microcomputer determines whether a time change can be made, e.g., is the time remaining within the change limits  408 . For example, if only 15 seconds remain in a cooking operation, no time change may be allowed to prevent an operator from shutting down a cooking operation by rotating dial  114  until zero is displayed, sometimes referred to as a “hard shutdown”, which may not be desirable. If the remaining time is not within the change limits, then the main cooking routine continues to be executed  406 . If the remaining time is within the change limits, then the microcomputer determines whether dial  114  was moved clockwise  410 . It is understood that the change limit may also be zero seconds. If no (i.e., dial  114  was moved counterclockwise), then for each increment that dial  114  is moved, the cook time is decremented by one second  412 . If yes, then for each increment that dial  114  is moved, the cook time is incremented by one second  414 . 
     FIG. 16 is a flow chart illustrating process steps  450  for lamp power level control. Such control is used to control energization of lamps  124 ,  126 , and  156  (FIG.  9 ). More particularly, a main cooking routine  452  is executed during normal cooking operations. A power counter is incremented  454  for each one second interval, and the microcomputer then checks whether a power cycle is complete  456 . For example, and as explained above, each duty cycle has a duration of about 32 seconds. If the duty cycle is complete, then the power counter is reset  458 . If the duty cycle is not complete, or after resetting the counter, then the microcomputer checks whether the power count is greater than the “on time”  460 . The “on time” is equal to the time corresponding to the selected power level for each lamp, as explained above. If the power count is greater than the “on time”, then the particular lamp is de-energized  462  and cooking continues with the main cooking routine  464 . If the power count is less than or equal to the “on time”, then the microcomputer checks whether the lamp is already on  466 . If yes, then cooking operations continue  464 . If no, then the microcomputer checks whether the soft start has been completed  468 . If the soft start has been completed, then operations continue with the cooking routine  464 . If soft start operations are not complete, then the soft start routine is called  470 . 
     FIG. 17 is a flow chart illustrating process steps for the soft start routine  500 . As explained above, the soft start for the halogen lamps is utilized to increase the lamp reliability. When routine  500  is called from the power level control routine  502 , the microcomputer then increments a soft start counter  504 . The microcomputer then determines whether the soft start is complete (e.g., depending on the lamp, the soft start has a duration of 1 or 2 seconds, as explained above). If soft start is complete, then the microcomputer resets the soft start counter  508 , turns on the lamp control relay  510 , and turns off the lamp control triac  512 . Operations then proceed to the cooking routine  514 . If soft start is not complete, then the microcomputer turns on the lamp control triac for a soft start count×10% of the line cycle  516 . Operations then proceed to the cooking routine. 
     The glass of the oven door is very dark and does not enable visualization of food within cavity  122  unless at least one of the Halogen lamps is on and sufficiently energized to illuminate cavity  122 . Therefore, in some cooking operations such as the microwave only mode of cooking or when radiant cooking at low power levels, and in order to visualize food in cooking cavity  122 , an operator may select the microwave button on keypad  112 . When this pad is selected during cooking, the microcomputer energizes upper center lamp  124  for four seconds at full power (i.e., power level  10 ), with a soft start, i.e., two seconds of soft start and two seconds of power level  10  energization for a total of four seconds, as described above. Lamp  124  illuminates the cooking cavity sufficiently so that an operator can visualize the food through window  106 . 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.