Abstract:
A method and apparatus for curing dental material with radiation from a halogen lamp provides for a shifting of a spectrum of the radiation to a shorter wavelength for increased rate of curing by adjusting current flow within a heating element of the lamp. The current is applied over an interval of time for accomplishing the curing. Feedback and feedforward circuitry, operative with a control signal of a regulated power source to reduce voltage applied by the power source to the heating element during an initial portion of the interval of time, prior to obtaining an operating temperature of the heating element, to maintain an upper bound on the magnitude of the current applied to the heating element, thereby to the vivid unnecessary and operations of the heating element for increased operating lifetime of the lamp.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to operation of a halogen lamp in a dental curing process and, more particularly, to the obtaining of more power in a usefull spectral region without diminishing useful lifetime of the lamp. 
     A halogen lamp has a tungsten filament which is heated by electric current during operation of the lamp. The lamp&#39;s lifetime is based on the integrity of the filament. Once the filament burns out, the lamp no longer operates. While the lamp is operating, heat produced by electric current in the filament induces evaporation of the filament. Thus, the filament is continuously evaporating during operation of the lamp and, when the diameter of the filament reaches the critical point, the lamp burns out. A higher current flow in the filament results in a more rapid evaporation. Thus, during the start-up interval of the lamp, before the filament has attained its operating temperature, a much larger current flows with a resultant higher rate of evaporation. 
     In order to preserve the lifetime of a lamp having a tungsten filament during operation wherein the lamp may be turned on and off frequently, it is known to use a current limiter such as an NTC (negative temperature coefficient) thermistor to suppress the initial inrush current while providing negligible electrical resistance once the filament has heated to operating temperature. Unfortunately, such a current limiter develops significant heat after suppressing the initial inrush current, and requires a cool-town (recovery) time after power is removed from the lamp and prior to reactivation of the lamp. Due to the temperature dependent operating characteristic of the thermistor, the thermistor must be allowed to cool down in order to restore its resistance to the appropriate value for reactivation of the lamp. The cool-town time is approximately one minute. This presents an inconvenience in the situation wherein it is desired to turn the lamp off momentarily before reactivating the lamp. Also, the current limiter suffers from the disadvantage of dissipating some of the electric power which would otherwise be employed usefully in operation of the lamp. 
     In the use of a halogen lamp for curing dental material, it is advantageous to operate the lamp in a fashion which accelerates the rate of the curing. Thereby, the curing can be accomplished advantageously in a lesser amount of time. One way to accomplish this is to raise the operating voltage of the lamp, resulting in increased power dissipation in the lamp with increased light output. However, this introduces the disadvantage of reduced lifetime of the lamp. Another factor to consider in accelerating the curing process is the spectral distribution of light produced by the lamp. The curing process is accomplished best with a light wavelength of approximately 450 nm (nanometers). However, more than 95 percent of halogen lamp light, in the case of a typical halogen lamp heated by a tungsten filament, has a wavelength over 600 nm. Therefore, normal operation of a halogen lamp suffers from a lack of optimization of the spectral distribution of the light for use in the curing of dental material. 
     SUMMARY OF THE INVENTION 
     In view of the aforementioned disadvantages and problems, it is an object of the present invention to operate a halogen lamp in a manner which accomplishes a more rapid curing of dental material while substantially retaining the expected lifetime of the lamp. In accordance with a feature of the practice of the invention, it is recognized that the spectrum of light radiated by the lamp is dependent on the electrical voltage employed for exciting the lamp. Thus, an increase of the operating voltage by 10 percent shifts the color temperature to give a 6 percent color temperature rise, in the case of a typical halogen lamp heated by a tungsten filament. The increase of the operating voltage by 10 percent in this lamp also results in an increase of radiated light energy by 34 percent. The increase in color temperature is manifested by a shifting of the maximum spectral radiation output from longer wavelength toward shorter wavelength. In the case of the foregoing example, the spectrum shifting results in an additional 5 percent light output. While the increase in applied voltage provides the foregoing advantageous features in the operation of the lamp, it is noted that the increased voltage is accompanied by a disadvantage in that the foregoing 10 percent increase in the operating voltage results in a shortening of the lamp&#39;s life by 40 percent. 
     In accordance with the methodology and the apparatus of the invention, enhanced usage of the halogen lamp for the curing of dental material is obtained by increasing the voltage applied to the lamp to shift the spectrum and obtain better utilization of the frequency spectrum of the output light, and to increase intensity of the output light, while regulating the applied voltage for a gradual increase of the voltage at the time of initial turn on of the lamp so as to avoid the initial surge current with its associated rapid evaporation of the filament. The design lifetime of the lamp is retained because the loss in use of the lamp associated with the increased voltage is balanced by the improved utilization of the spectrum, and the avoidance of the initial surge current so as to maintain an upper bound on the magnitude of the current. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The aforementioned aspects and other features of the invention are explained in the following description, taken in connection with the accompanying drawing figures wherein: 
     FIG. 1 shows a stylized view of a halogen lamp heated by an internal electrical filament, and radiating light useful in the curing of dental material; 
     FIG. 2 provides two graphs representative of spectral emission characteristics of the halogen lamp as a function of voltage applied to the lamp filament, the upper graph being unfiltered light and the lower graph being filtered light; 
     FIG. 3 is a diagram of an electric circuit for controlling voltage and current applied to the filament of the lamp in the practice of the invention; and 
     FIG. 4 is a set of graphs showing voltage as a function of time employed for heating the lamp filament. 
    
    
     Identically labeled elements appearing in different ones of the figures refer to the same element but may not be referenced in the description for all figures. 
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to FIG. 1, a halogen lamp  10  comprises an envelope  12  containing a halogen gas and enclosing a heating element in the form of an electrical filament  14  for heating the lamp  10 . A source of electric power for the lamp  10  is represented by a variable voltage source, depicted as a variable battery  16 , for applying voltage and current to the filament  14 . Upon providing sufficient current by the battery  16  to heat the filament  14  to operating temperature of the lamp  10 , the lamp  10  radiates light  18  through a filter  19  to illuminate dental material  20  held within a tray  22 . The rays of the light  18  incident upon the filter  19  are further identified by the legend  1   8 A, and the rays of the light  18  outputted by the filter  19  are further identified by the legend  18 B. The filter  19  has a spectral passband of 300 nm to 600 nm. The filtered light  18 B aids in the curing of the dental material  20 . A higher intensity of the light  18  results in a decreased time to accomplish the curing. Also, an adjustment of the spectral distribution of the light  18  to provide increased intensity in the vicinity of 450 nm light wavelength results in a decreased time to accomplish the curing. Increased lifetime of the lamp  10  is accomplished by a gradual application of the excitation voltage to limit the magnitude of the heating current to the filament  14  as the electrical resistance of the filament  14  rises with the temperature of the filament  14 . 
     The heating element has a resistance characterized by a positive temperature coefficient such that, at a relatively low temperature, the resistance is relatively low, and that at relatively high temperature, the resistance is relatively high, and wherein the resistance rises immediately after application of voltage to the heating element due to the rise in temperature of the heating element. By applying voltage of reduced magnitude during the initial stages of heating the filament, and allowing the voltage to rise in a manner substantially proportional to the resistance of the filament, the magnitude of the current is maintained substantially constant during the initial heating stage of the filament, thereby to place an upper bound on the magnitude of the current and to avoid the sudden inrush of current to the filament. This avoids unwanted evaporation of the filament to promote longevity in usage of the lamp. By use of the term “substantially proportional”, it is to be understood that the applied voltage may not follow the changes in resistance exactly and that the relationship may not be perfectly linear. However, the voltage follows the resistance sufficiently closely to maintain an upper bound on the current. 
     In FIG. 2, the graph shows the intensity of light emitted by the lamp  10  as a function of frequency of the light. The intensity is shown along the vertical axis, and increasing wavelength is shown toward the right along the horizontal axis. For ease of reference, the light  18 A incident upon the filter  19  is identified in FIG. 2 as white light, and the filtered light  18 B outputted by the filter  19  is identified in FIG. 2 as blue light. In both the upper graph for the unfiltered light and the lower graph for the filtered light, there are two traces of which the lower trace represents the light output for normal energization of the lamp, and the upper trace represents the light output for excitation at increased current and voltage to the filament. The value of 450 nm is shown in the graph, and represents a preferred value of light wavelength for the curing of the dental material. It is noted that while the invention is being demonstrated for the case of the curing of dental material, the invention may be employed also for the processing of other material wherein the intensity and the spectral distribution of the radiation may be chosen to optimize the processing. 
     FIG. 3 shows connection of a power source  24  to the halogen lamp  10 . Input line voltage, such as that of the power provided to homes and offices, is applied by a terminal pair  26 . Electric power at the terminal pair  26  is coupled by a step-down transformer  28  and a diode bridge rectifier  30  to the power source  24 . The transformer  28  reduces the AC (alternating current) line voltage to a lower value of AC voltage across terminals  32  and  34  for driving the bridge rectifier  30 . The AC voltage is converted by the bridge rectifier  30  to a DC (direct current) voltage appearing between line  36  and ground  38 , the DC voltage being applied via line  36  and ground  38  to the power source  24 . Voltage is outputted from the power source  24  to the halogen lamp  10  via line  40  and ground. 
     The power source  24  comprises a power regulator  42 , three capacitors C 1 , C 2  and C 3 , four resistors R 1 , R 2 , R 3  and R 4 , two diodes D 1  and D 2 , and a switch  44 . The capacitor C 1  connects between the line  36  and ground for filtering the input voltage on line  36 . The capacitor C 2  connects between line  40  and ground for filtering the output voltage on line  40 . The input voltage on line  36  is applied to the first input voltage terminal  46  of the power regulator  42 . The output voltage on line  40  is provided by an output voltage terminal  48  of the power regulator  42 . A second input terminal  50  of the regulator  42  is responsive to a DC voltage, such as the voltage on line  36 , to activate the regulator  42  to output the desired voltage on line  40 ; removal of the voltage from terminal  50  terminates the presence of the output voltage on line  40 . Also included in the regulator  42  is a negative feedback terminal  52  by which operation of the regulator  42  is responsive to a feedback signal for maintaining the output voltage on line  40  at a desired value. 
     The resistors R 1  and R 2  are connected in series between line  40  and ground for providing a sample of the output voltage of line  40  at the junction between the resistors R 1  and R 2 . The sample of the output voltage has a magnitude equal to only a fraction of the output voltage, and is applied, as feedback signal, to the feedback terminal  52  of the regulator  42 . This constitutes a feedback circuit of the power source  24  for control of the magnitude of the output voltage on line  40 . The magnitude of the feedback signal at terminal  52  establishes the magnitude of the voltage appearing on line  40 . By way of example in the construction of the circuit, the resistors R 1  and R 2  are shown as fixed resistors wherein their values are selected to provide the desired output voltage. However, if desired, the voltage divider circuit of the resistors R 1  and R 2  may include a variable resistor (not shown) allowing for manual adjustment of the output voltage. 
     The switch  44  has two positions, a first position, as shown in FIG. 3, for grounding the second input terminal  50  of the regulator  42 , and a second position in which the voltage of the line  36  is applied by the switch  44  to the second input terminal  50  of the regulator  42 . In the first position of the switch  44 , the regulator  42  is turned off so as to output no voltage on line  40  to the lamp  10 . In the second position of the switch  44 , the regulator  42  is turned on to output the voltage on line  40  for energizing the lamp  10 . 
     A feedforward circuit  54  connects between the second input terminal  50  and the feedback terminal  52  of the regulator  42 . The feedforward circuit  54  comprises the capacitor C 3 , the diodes D 1  and D 2 , and the resistors R 3  and R 4 . The feedforward circuit  54  is active immediately after the throwing of the switch  44  from the first position to the second position. The feedforward circuit  54  receives a step voltage upon the throwing of the switch  44  to its second position, and converts the step voltage to a ramp voltage which is applied to the feedback terminal  52 . The ramp voltage initially is sufficiently great to overpower the feedback voltage provided by the resistors R 1  and R 2  so as to direct the regulator  42  to output a relatively low value of output voltage on line  40  to the lamp  10 . As the ramp voltage decreases in magnitude, its effect is reduced so that the regulator  42  begins to increase the magnitude of the output voltage on line  40 . At the conclusion of the ramp voltage, the operation of the regulator  42  is controlled only by the feedback voltage provided by the resistors R 1  and R 2  so that the full value of the desired output voltage appears on line  40 . 
     In the operation of the feedforward circuit  54 , the leading edge of the step voltage is coupled by the capacitor C 3  to the diode D 2 , and then via the resistor R 3  to the feedback terminal  52 . The connection of the resistor R 3  to the junction of the resistors R 1  and R 2  provides the configuration of a voltage divider which reduces the magnitude of the ramp voltage to a small fraction of the input voltage appearing on line  36 . After the closure of the switch  44  to bring it into its second position, current flows through the capacitor C 3 , through the diode D 2 , and through the resistors R 3  and R 2  to ground, this current charging the capacitor C 3  to produce an ever increasing voltage drop across the capacitor C 3 . The increasing voltage drop across the capacitor C 3  results in a decreasing current through the capacitor C 3  and the resistor R 2  to provide the ramp voltage of decreasing amplitude at the feedback terminal  52 . At the conclusion of the energization of the lamp  10 , when the switch  44  is opened to bring it into its first position, the capacitor C 3  discharges by current flow through a series circuit consisting of the diode D 3 , the resistor R 4 , the switch  44  and ground. The value of the resistor R 4  is selected to provide for a discharge time which is sufficiently fast to ready the feedforward circuit  54  for the next interval of activation of the lamp  10 . 
     In the set of graphs shown in FIG. 4 respective ones of the graphs are identified by the legends  1 ,  2 ,  3  and  4  which correspond to the numbering of selected nodes  1 ,  2 ,  3  and  4  in the circuitry of FIG.  3 . Node  1  is at the junction of terminal  50 , the switch  44 , and capacitor C 3 . Node  2  is at the junction of the capacitor C 3 , resistor R 4  and diode D 2 . Node  3  and is at the junction of terminal  52 , resistor R 3 , resistor R 1  and resistor R 2 . Node  4  is at the junction of terminal  48 , the lamp  10 , resistor R 1  and capacitor C 2 . The first, the second, the third and the fourth graphs of FIG. 4 show the waveforms of the voltages appearing respectively at the nodes  1 ,  2 ,  3  and  4 . In each of the four graphs, voltage is shown along the vertical axis, and increasing time is shown toward the right along the horizontal axis. The voltage waveforms shown in all of the four graphs are presented in time registration with each other. 
     With respect to the waveform at node  1 , the voltage is zero when the node is grounded by the switch  44 . When the circuitry is turned ON by movement of the switch  44  to the second position, the voltage is equal to the voltage on line  36  as is outputted by the diode bridge rectifier  30 . These two voltage states are shown in the first graph. 
     With respect to the waveform at node  2 , the voltage is proportional to the current flowing through the capacitor C 3 , wherein the waveform begins with the operation of the switch  44  to turn the circuitry to the ON state. The graph shows that, as the capacitor C 3  charges, the current diminishes with time to approximate, during the initial stage of the charging, a ramp voltage. Eventually, the voltage passed by the capacitor C 3  drops below the level necessary to keep the diode D 2  in a state of conduction. From that point on, the ramp voltage no longer has an effect at node  3 . The charging time is significantly less than the ON time of the switch  44 . 
     With respect to the waveform at node  3 , there is a scaling of the voltage waveform of the second node to reduce its value, and a summation of the voltage of the second node with the voltage produced by the voltage-divider circuit of resistors R 1  and R 2 . Therefore, the initial stage (identified in FIG. 4 as the ramp period) of the voltage waveform at node  3  includes a ramp, while the balance of the waveform shows a voltage of substantially constant amplitude. The value of resistor R 4  is much smaller than the value of the resistor R 3  so that the discharging process of the capacitor C 3  requires less time than the charging process. 
     With respect in the waveform at node  4  there is shown the voltage outputted to the lamp  10  by the regulator  42  in response to the voltage (at node  3 ) fed back to the feedback terminal  52 . During the interval of time identified in FIG. 4 as the ramp period, there is a gradual rise in the amplitude of the voltage fed to the lamp  10  to provide a desired “soft” start which avoids a heavy inrush current to the lamp  10 . Thereafter, the voltage fed to the lamp  10  has a constant amplitude until the throwing of the switch  44  to the OFF state wherein node  1  is grounded. Thereupon, the regulator  42  terminates the voltage fed to the lamp  10 . 
     It is to be understood that the above described embodiments of the invention are illustrative only, and that modifications thereof may occur to those skilled in the art. Accordingly, this invention is not to be regarded as limited to the embodiments disclosed herein, but is to be limited only as defined by the appended claims.