Abstract:
A discharge lamp lighting device having a discharge lamp which has a pair of tungsten-made electrodes each having a protrusion formed at the tip thereof and an electric power feeding device which feeds an alternate current electric power to the discharge lamp, and is characterized in that the power feeding device has a function of switching between a first lighting mode in which a rated electric power is fed to the discharge lamp and a second lighting mode in which a lower electric power than the rated electric power is fed to the discharge lamp to light the discharge lamp, and therefore, can control electric power to be fed to the discharge lamp under such conditions that the average electric power change rate becomes 0.01-2.1 W/s during a mode switching period in which the switching from the first lighting mode into the second lighting mode is carried out.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a discharge lamp lighting apparatus which can be preferably used as a light source for a projector apparatus. 
     2. Description of Related Art 
     A projecting-type projector apparatus is required to project an image being uniform and having sufficient color rendering properties on a rectangular screen, and hence a short-arc type discharge lamp having a mercury vapor pressure of at least, for example, 150 atmospheres at the time of lighting is employed as a light source. 
     Therefore, recently, a projector apparatus having a dimming function capable of adjusting brightness of the screen according to the brightness of an usage environment or the type of the image to be projected has developed, and as a discharge lamp lighting apparatus used in such a projector apparatus, the one including a power supply apparatus configured to light a discharge lamp by switching the mode between a rated power lighting mode in which rated power is supplied to the discharge lamp and a low power lighting mode in which power lower than the rated power, for example, 80% of the rated power is supplied to the discharge lamp is known (see Japanese Patent No. 4274053 and corresponding U.S. Pat. No. 7,436,121). 
     However, in the discharge lamp lighting apparatus having such a structure, there is a problem that flicker may occur in the discharge lamp when switching from the rated power lighting mode to the low power lighting mode. 
     The reasons why the flicker occurs in the discharge lamp in the low power lighting mode when the mode is switched to the low power lighting mode in this manner seem to be as follows. 
     When the discharge lamp is lit by the rated power lighting mode, as illustrated in  FIG. 14(   a ), arcs A are formed between a pair of the electrodes  90 ,  91  of the discharge lamp, and hence evaporated electrode substance is accumulated by a halogen cycle at positions where original points of the arcs A at distal ends of the electrodes  90 ,  91  are positioned, so that projections p 1  according to the magnitude of the original points of the arcs A are formed. 
     Then, in a mode switching term in which the mode is switched from the rated power lighting mode to the low power lighting mode, as illustrated in  FIG. 14(   b ), the shape of the arcs A formed between the electrodes  90 ,  91  becomes narrower as the value of power supplied to the discharge lamp is lowered, so that the evaporated electrode substance is accumulated at positions on surfaces of the projections p 1  of the respective electrodes  90 ,  91  where the original points of the arcs A are positioned, whereby minute projections p 2  are formed on the surfaces of the projections p 1  of the respective electrodes  90 ,  91  as illustrated in  FIG. 14(   c ). 
     Therefore, since the shape of the arcs A formed between the electrodes  90 ,  91  is abruptly narrowed in the mode switching term in which the power supplied to the discharge lamp is lowered, the arcs A formed between the electrodes  90 ,  91  become unstable by the movement of the original points on the surfaces of the projections p 1  of the respective electrodes  90 ,  91  as illustrated by broken lines in  FIG. 14(   d ) and, consequently, a plurality of the minute projections p 2  are formed on the surfaces of the projections p 1  of the respective electrodes  90 ,  91  as illustrated in  FIG. 14(   e ). 
     In this manner, in a case where the discharge lamp is lit in the low power lighting mode in a state in which a plurality of the minute projections p 2  are formed at the distal ends of the electrodes  90 ,  91 , the original points of the arcs A move between the plurality of minute projections p 2  and, consequently, flicker occurs in the discharge lamp. 
     In the discharge lamp lighting apparatus of the related art having the light-dimming function, there is a problem that the distal ends of the respective electrodes are worn in the low power lighting mode, and hence the flicker occurs. 
     The reason why the wearing of the distal ends of the electrodes and the flicker occur in the discharge lamp in such a low power lighting mode seems to be as follows. 
     When the discharge lamp is lit, the electrode substance is evaporated from the electrodes by generation of high-temperature heat of the electrodes. However, the electrode substance evaporated by the halogen cycle accumulates on the surfaces of the electrodes. Then, when the discharge lamp is lit in the rated power lighting mode, evaporation of the electrode substance from the electrodes and accumulation of the evaporated electrode substance on the electrodes are balanced, so that wearing of the electrodes is suppressed. 
     Therefore, when the discharge lamp is it in the low power lighting mode, the amount of the evaporated electrode substance existing in the periphery of the electrodes is significantly reduced, and hence the accumulation of the electrode substance with respect to the electrodes is not sufficiently achieved. Therefore, evaporation of the electrode substance from the electrodes progresses, and consequently, the distal ends of the electrodes are worn, and hence the flicker occurs. 
     SUMMARY OF THE INVENTION 
     The present invention is based on the circumstances as described above, and it is an object of the present invention to provide a discharge lamp lighting apparatus which is capable of preventing or suppressing occurrence of flicker while the discharge lamp is lit in a second lighting mode in which AC power lower than rated power is supplied when the mode is switched from a first lighting mode in which AC power of the rated power is supplied to the second lighting mode. 
     Another object of the present invention is to provide a discharge lamp lighting apparatus which is capable of preventing or suppressing occurrence of wearing of electrodes of the discharge lamp and occurrence of flicker when the discharge lamp is lit in the a low power lighting mode in which AC power lower than the rated power of the discharge lamp is supplied. 
     Solution of the Problem 
     The discharge lamp lighting apparatus according to the present invention is a discharge lamp lighting apparatus including: a discharge lamp having a pair of electrodes formed of tungsten formed respectively with projections at distal ends thereof, and a power supply apparatus configured to supply AC power to the discharge lamp, wherein 
     the power supply apparatus has a function to light the discharge lamp by switching the mode between a first lighting mode in which rated power is supplied to the above-described discharge lamp and a second lighting mode in which power lower than the rated power is supplied to the discharge lamp, and is configured to control power to be supplied to the discharge lamp under the condition of an average power change ratio of 0.01 to 2.1 W/s in a mode switching term in which the mode is switched from the first lighting mode to the second lighting mode. 
     In the discharge lamp lighting apparatus described above, preferably, the power supply apparatus is configured to control the power to be supplied to the discharge lamp under the condition that the average power change ratio is equal to or lower than 2.1 W/s in the mode switching term in which the mode is switched from the second lighting mode to the first lighting mode. A configuration in which the power supply apparatus has a function to light the discharge lamp by switching the mode between the second lighting mode and a third lighting mode in which power lower than the supply power in the second lighting mode is supplied to the discharge lamp, and is configured to control the power to be supplied to the discharge lamp under the condition of an average power change ratio of 0.01 to 2.1 W/s in a mode switching term in which the mode is switched from the second lighting mode to the third lighting mode is also applicable. 
     In the discharge lamp lighting apparatus described above, the power supply apparatus is preferably configured to control the power to be supplied to the discharge lamp under the condition that the average power change ratio is equal to or lower than 2.1 W/s in the mode switching term in which the mode is switched from the third lighting mode to the second lighting mode. 
     Also, the discharge lamp lighting apparatus according to the present invention is a discharge lamp lighting apparatus including: a discharge lamp having a pair of electrodes formed respectively with projections at distal ends thereof; and a power supply apparatus configured to supply AC power to the discharge lamp, wherein 
     the power supply apparatus has a function to light the discharge lamp by switching the mode between a first lighting mode in which rated power is supplied to the above-described discharge lamp and a second lighting mode in which power lower than the rated power is supplied to the discharge lamp, and is configured to control the power to be supplied to the discharge lamp under the condition including the power change such that the power is lowered from a given power value P 1  equal to or lower than the supply power in the first lighting mode and equal to or higher than the supply power in the second lighting mode to a power value P 2  lower than the power value P 1 , and then is increased from the power value P 2  to a power value P 3  higher than the power value P 2  and lower than the power value P 1  in the mode switching term in which the mode is switched from the first lighting mode to the second lighting mode. 
     In the discharge lamp lighting apparatus described above, preferably, the power supply apparatus is configured to control the power to be supplied to the discharge lamp under the condition including twice or more power changes in which the power value is lowered from the power value P 1  to the power value P 2  lower than the power value P 1 , and then the power value is increased from the power value P 2  to the power value P 3  higher than the power value P 2  and lower than the power value P 1  in the mode switching term in which the mode is switched from the first lighting mode to the second lighting mode. 
     Also, the power supply apparatus may have a function to light the discharge lamp by switching the mode between the above-described second lighting mode and a third lighting mode in which power lower than the supply power in the second lighting mode is supplied to the discharge lamp, and be configured to control the power to be supplied to the discharge lamp under the condition including the power change such that the power value is lowered from a given power value P 1  equal to or lower than the supply power in the first lighting mode and equal to or higher than the supply power in the second lighting mode to the power value P 2  lower than the power value P 1 , and then is increased from the power value P 2  to a power value P 3  higher than the power value P 2  and lower than the power value P 1  in the mode switching term in which the mode is switched from the second lighting mode to the third lighting mode. 
     In the discharge lamp lighting apparatus described above, preferably, the power supply apparatus is configured to control the power to be supplied to the discharge lamp under the condition including twice or more power changes in which the power value is lowered from the power value P 1  to the power value P 2  lower than the power value P 1 , and then the power value is increased from the power value P 2  to the power value P 3  higher than the power value P 2  and lower than the power value P 1  in the mode switching term in which the mode is switched from the second lighting mode to the third lighting mode. 
     Also, the discharge lamp lighting apparatus according to the present invention is a discharge lamp lighting apparatus including: a discharge lamp; and a power supply apparatus configured to supply AC power to the discharge lamp, wherein the power supply apparatus has a function to light the discharge lamp by switching the mode between a rated power lighting mode in which rated power is supplied to the discharge lamp and a low power lighting mode in which power lower than the rated power is supplied to the discharge lamp, and is configured to supply basic AC power having a predetermined power value continuously to the discharge lamp, and supply superimposed power superimposed on this basic AC power cyclically, and simultaneously, controls the supply timing of the superimposed power, the power value of the superimposed power, or the supply time of the superimposed power according to a measured value of lighting voltage of the discharge lamp in the low power lighting mode. 
     In the discharge lamp lighting apparatus, preferably, the power supply apparatus is configured to control the supply timing of the superimposed power so as to supply the superimposed power to the discharge lamp at a shorter time interval when the measured value of the lighting voltage of the discharge lamp is higher than a predetermined reference value than when the measured value is lower than the predetermined reference value in the low power lighting mode. 
     Also, preferably, the power supply apparatus is configured to control the power value of the superimposed power so as to supply the superimposed power to the discharge lamp at a higher power value when the measured value of the lighting voltage of the discharge lamp is higher than the predetermined reference value than when the measured value is lower than the predetermined reference value in the low power lighting mode. 
     Also, preferably, the power supply apparatus is configured to control the supply time of the superimposed power so as to supply the superimposed power to the discharge lamp in a longer time when the measured value of the lighting voltage of the discharge lamp is higher than the predetermined reference value than when the measured value is lower than the predetermined reference value in the low power lighting mode. 
     The measured value of the lighting voltage of the discharge lamp may be measured either when the superimposed power is not supplied or when the superimposed power is supplied. 
     Advantageous Effects of Invention 
     According to a feature of the discharge lamp lighting apparatus of the invention, the power supply apparatus controls the power to be supplied to the discharge lamp under the condition in which the average power change ratio becomes 0.01 to 2.1 W/s in the mode switching term in which the mode is switched from the first lighting mode to the second lighting mode, whereby the original points of the arcs do not move along the surfaces of the projections of the respective electrodes and the stable arcs are formed, so that a plurality of minute projections are not formed on the surfaces of the projections of the respective electrodes, and hence occurrence of flicker may be prevented or suppressed while the discharge lamp is lit in the second lighting mode when the mode is switched from the first lighting mode to the second lighting mode. 
     According to another a feature of the discharge lamp lighting apparatus of the invention, the power supply apparatus controls the power to be supplied to the discharge lamp under the condition that the average power change ratio is equal to or lower than 2.1 W/s in the mode switching term in which the mode is switched from the second lighting mode to the first lighting mode, so that occurrence of cracks in the light-emitting tube of the discharge lamp may be prevented of suppressed. 
     According to a further a feature of the discharge lamp lighting apparatus of the invention, since the AC power to be supplied to the discharge lamp is controlled under the condition including the power change in which the power value is lowered from the power value P 1  to the power value P 2 , then is increased from the power value P 2  to the power value P 3  higher than the power value P 2  and lower than the power value P 1  in the mode switching term in which the mode is switched from the first lighting mode to the second lighting mode, even when a plurality of the minute projections are formed on the surfaces of the projections of the electrodes when the AC power to be supplied to the discharge lamp is lowered from the power value P 1  to the power value P 2 , a plurality of the minute projections are converged into one minute projection, whereby the electrodes are repaired when the AC power to be supplied to the discharge lamp is increased from the power value P 2  to the power value P 3 , so that the occurrence of flicker may be prevented or suppressed while the discharge lamp is lit in the second lighting mode. 
     According to yet another a feature of the discharge lamp lighting apparatus of the invention, the power supply apparatus supplies the superimposed power which is superimposed on the basic AC power of the predetermined power value cyclically to the discharge lamp, and controls the supply timing of the superimposed power, the power value of the superimposed power, or the supply time of the superimposed power in the low power lighting mode in which power lower than the rated power is supplied to the discharge lamp, so that the amount of the evaporated electrode substance existing in the periphery of the electrodes is increased, whereby accumulation of the evaporated electrode substance on the electrodes of the discharge lamp is accelerated, and hence occurrence of wearing of the electrodes of the discharge lamp and flicker may be prevented or suppressed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view illustrating a configuration of an example of a discharge lamp used in a discharge lamp lighting apparatus of the present invention. 
         FIG. 2  is an explanatory drawing illustrating a configuration of an example of a circuit of a power supply apparatus used in a discharge lamp lighting apparatus according to a first embodiment. 
         FIG. 3  is a graph illustrating an example of a change of value of power supplied to the discharge lamp in a case where the mode is switched from a first lighting mode to a second lighting mode in the discharge lamp lighting apparatus according to the first embodiment. 
         FIG. 4  is an explanatory drawing illustrating an example of a current waveform of AC power to be supplied to the discharge lamp in the discharge lamp lighting apparatus according to the first embodiment. 
         FIGS. 5(   a )- 5 ( c ) are explanatory views illustrating electrodes and a change of the shape of arcs of the discharge lamp in a mode switching term in which the mode is switched from the first lighting mode to the second lighting mode in the discharge lamp lighting apparatus according to the first embodiment. 
         FIG. 6  is a graph illustrating an example of a change of value of power supplied to the discharge lamp in the case where the mode is switched from the first lighting mode to the second lighting mode in a discharge lamp lighting apparatus according to a second embodiment. 
         FIGS. 7(   a )- 7 ( f ) are explanatory views illustrating electrodes and a change of the shape of arcs of the discharge lamp in the mode switching term in which the mode is switched from the first lighting to the second lighting mode in the discharge lamp lighting apparatus according to the second embodiment. 
         FIG. 8  is a graph illustrating, along with a change of lighting voltage of the discharge lamp, a change of value of power supplied to the discharge lamp in the low power lighting mode in an example of the power supply apparatus in a discharge lamp lighting apparatus according to a third embodiment. 
         FIG. 9  is a graph illustrating, along with the change of lighting voltage of the discharge lamp, the change of value of power supplied to the discharge lamp in the low power lighting mode in another example of the power supply apparatus in the discharge lamp lighting apparatus according to the third embodiment. 
         FIG. 10  is a graph illustrating, along with the change of lighting voltage of the discharge lamp, the change of value of power supplied to the discharge lamp in the low power lighting mode in still another example of the power supply apparatus in the discharge lamp lighting apparatus according to the third embodiment. 
         FIG. 11  is a graph illustrating another example of the change of value of power supplied to the discharge lamp in the case where the mode is switched from the first lighting mode to the second lighting mode in the discharge lamp lighting apparatus according to the first embodiment. 
         FIG. 12  is a graph illustrating another example of the change of value of power supplied to the discharge lamp in the case where the mode is switched from the first lighting mode to the second lighting mode in the discharge lamp lighting apparatus according to the second embodiment. 
         FIG. 13  is a graph illustrating the change of value of power supplied to a discharge lamp in Experimental Example 1 in an example according to the second embodiment. 
         FIGS. 14(   a )- 14 ( d ) are explanatory views illustrating electrodes and a change of the shape of arcs of the discharge lamp in the mode switching term in which the mode is switched from the first lighting to the second lighting mode in the discharge lamp lighting apparatus of the related art. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of a discharge lamp lighting apparatus of the present invention will be described below. 
     The discharge lamp lighting apparatus of the present invention is configured to be integrated, for example, in a projector apparatus having a dimming function, and includes a discharge lamp and a power supply apparatus configured to supply AC power to the discharge lamp. 
       FIG. 1  is an explanatory cross-sectional view illustrating a configuration of an example of the discharge lamp used in the discharge lamp lighting apparatus of the present invention. 
     A light-emitting tube  11  of this discharge lamp  10  includes a light-emitting portion  12  having an outline which defines a discharge space S in an interior thereof and has a substantially oval spherical shape, and sealed portions  13  extending continuously and integrally with both ends of the light-emitting portion  12  respectively and having a rod shape extending outward along a tube axis, and a pair of electrodes  14 ,  15  formed respectively of tungsten arranged in the interior of the light-emitting portion  12  of the light-emitting tube  11  so as to face each other. Specifically, a pair of the electrodes  14 ,  15  include rod-shaped shaft portions  14   b ,  15   b  extending along the direction toward the tube axis of the light-emitting tube  11 , substantially spherical head portions  14   a ,  15   a  formed continuously from respective distal ends of the shaft portions  14   b ,  15   b  and formed with projections p 1  at distal ends thereof, and coil portions  14   c ,  15   c  wound around rear end portions of the head portions  14   a ,  15   a  and distal end portions of the shaft portions  14   b ,  15   b , and the respective head portions  14   a ,  15   a  thereof are arranged so as to face each other, and proximal portions of the respective shaft portions  14   b ,  15   b  are held by being embedded in the respective sealed portions  13 . 
     Metal foils  16 ,  17  formed of molybdenum are hermetically embedded in interiors of the respective sealed portions  13  of the light-emitting tube  11 , and proximal ends of the shaft portions  14   b ,  15   b  of a pair of the electrodes  14 ,  15  are respectively welded and electrically connected to one of ends of each of the metal foils  16 ,  17 , and external leads  18 ,  19  projecting outward from outer ends of the sealed portions  13  are welded and electrically connected to the other ends of the metal foils  16 ,  17 . 
     The light-emitting tube  11  is formed of quartz glass, and for example, mercury, noble gas, and halogen are enclosed in the interior of the light-emitting portion  12  of the light-emitting tube  11 . 
     The mercury to be enclosed in the interior of the light-emitting portion  12  is for obtaining a required visible light wavelength, for example, radiant light having a wavelength of 360 to 780 nm, and the enclosed amount thereof is determined to be not lower than 0.15 mg/mm 3  in order to secure a high mercury vapor pressure not lower than 150 atmospheres, for example, at the time of lighting, and a high mercury vapor pressure not lower than 200 or 300 atmosphere may be obtained at the time of lighting by increasing the amount of enclosure of the mercury, so that a light source suitable for the projector apparatus may be realized. 
     The noble gas to be enclosed in the interior of the light-emitting portion  12  is for improving a lighting startability, and the enclosing pressure thereof is, for example, 10 to 26 kPa at static pressure. As the noble gas, argon gas may be preferably used. 
     The halogen enclosed in the interior of the light-emitting portion  12  is for forming halogen cycle in the interior of the light-emitting portion  12 , thereby suppressing tungsten as electrode substance from being adhered to an inner wall of the light-emitting portion  12 , and is enclosed in a form of a compound including mercury and other metals. The amount of enclosure of halogen is, for example, 1×10 −6  to 1×10 −2  μmol/mm 3 . Iodine, bromine, chlorine, and so on may be used as halogen. 
     In the interior of the light-emitting portion  12 , metal halide may be enclosed as still another charging medium. 
     To give a specific example of specifications of the discharge lamp  10  as described above, the maximum outer diameter of the light-emitting portion  12  of the light-emitting tube  11  is 10 mm and an inter-electrode distance of the same is 1.0 mm, an internal volume of the light-emitting portion  12  of the light-emitting tube  11  is 60 mm 3 , a rated voltage of the same is 75V, and a rated power of the same is 200 W. 
     In the discharge lamp  10 , a mercury vapor pressure in the interior of the light-emitting portion  12  of the light-emitting tube  11  while it is lit becomes, for example, not lower than 150 atmospheres, and in the projector apparatus in which the discharge lamp lighting apparatus is integrated therein, miniaturization of the entire apparatus is conducted, while a high light quantity is required, and hence the thermal condition in the interior of the light-emitting portion  12  of the light-emitting tube  11  of the discharge lamp  10  is quite strict, so that a bulb wall loading value of the lamp is, for example, 0.8 to 3.0 W/mm 2  and, more in detail, 2.0 W/mm 2 . 
     By having such a high mercury vapor pressure or a bulb wall loading value, radiant light having preferable color rendering properties may be obtained in the case of being used as the light source of the projector apparatus. 
     In the discharge lamp lighting apparatus according to a first embodiment, the power supply apparatus has a function to light the discharge lamp  10  by switching the mode between a first lighting mode in which rated power is supplied to the above-described discharge lamp  10  and a second lighting mode in which power lower than the rated power is supplied to the discharge lamp  10 , and is configured to control power to be supplied to the discharge lamp under the condition of an average power change ratio of 0.01 to 2.1 W/s, preferably, 0.08 to 0.77 W/s in a mode switching term in which the mode is switched from the first lighting mode to the second lighting mode. 
     Here, the power value to be supplied to the discharge lamp  10  in the second lighting mode is not specifically limited as long as being lower than the value of the rated power, and is selected normally within a range from 40 to 80% of the rated power. If the average power change ratio lower than 0.01 W/s in the mode switching term in which the mode is switched from the first lighting mode to the second lighting mode, lowering of the power to be supplied is continued for a long time, so that blacking of the light-emitting tube  11  of the discharge lamp  10  may result. In contrast, if the average power change ratio exceeds 2.1 W/s, the lowering of the power to be supplied brings about abruptly in a short time, so that the flicker may occur while the discharge lamp  10  is lit in the second lighting mode after the mode switching term. 
     Also, the power supply apparatus is preferably configured to control the power to be supplied to the discharge lamp  10  under the condition that the average power change ratio is equal to or lower than 2.1 W/s in a mode switching term in which the mode is switched from the second lighting mode to the first lighting mode. 
     If the average power change ratio exceeds 2.1 W/s in the mode switching term in which the mode is switched from the second lighting mode to the first lighting mode, increase of the power to be supplied brings about abruptly in a short time, so that cracking of the light-emitting tube  11  of the discharge lamp  10  may result. 
     Although the lower limit of the average power change ratio is not specifically limited in the mode switching term in which the mode is switched from the second lighting mode to the first lighting mode, the time required for switching the mode is increased as lowering of the average power change ratio, so that the average power change ratio is preferably not lower than 0.01 W/s. 
       FIG. 2  is an explanatory drawing illustrating a configuration of an example of a circuit of a power supply apparatus used in the discharge lamp lighting apparatus according to the first embodiment. A power supply apparatus  20  includes a chopper circuit U 1 , a full bridge circuit U 2 , a starter circuit U 3 , and a control unit U 4 . 
     The chopper circuit U 1  includes a switching element Qx connected to a plus-side power source terminal to which a direct current voltage Vdc is supplied, a reactor Lx, a diode Dx whose cathode-side terminal is connected to a connecting point between the switching element Qx and the reactor Lx at a position between the connecting point and a minus-side power source terminal, a smoothing capacitor Cx connected to an output-side terminal of the reactor Lx, and a current detecting unit Rx connected to a one-side terminal of the smoothing capacitor Cx and an anode-side terminal of the diode Dx. The switching element Qx is configured to be driven by a drive circuit Gx operated on the basis of a signal output from the control unit U 4 , and turned ON/OFF by a predetermined duty, and the power to be supplied to the discharge lamp  10  is controlled by the duty. 
     The full bridge circuit U 2  includes four switching elements Q 1  to Q 4  connected in a bridge pattern. The switching elements Q 1  to Q 4  are driven by drive circuits G 1  to G 4  corresponding to the respective switching elements Q 1  to Q 4  being operated on the basis of a signal output from a full bridge control circuit Gw, and a square wave-shaped AC voltage is generated between a connecting point between the switching elements Q 1 , Q 2  and a connecting point between the switching elements Q 3 , Q 4  by turning the switching elements Q 1 , Q 4  and the switching elements Q 2 , Q 3  arranged at opposing corners in an ON state alternately. 
     The starter circuit U 3  includes a coil Lh and a capacitor Ch. In the starter circuit U 3  configured as described above, a resonant frequency of a resonant circuit of the coil Lh and the capacitor Ch is output from the full bridge circuit U 2 , so that a high voltage may be generated from the capacitor Ch by a resonant effect thereof. Therefore, the starter circuit U 3  is operated at a high frequency only at the time of starting of the discharge lamp  10 , whereby a high voltage is applied between a pair of the electrodes of the discharge lamp  10  and the discharge lamp  10  is lit. 
     In the power supply apparatus  20  as described above, the power to be supplied to the discharge lamp  10  is set by ON/OFF duty of the switching element Qx of the chopper circuit U 1 . The switching element Qx of the chopper circuit U 1  is turned ON and OFF by a predetermined duty of the drive circuit Gx on the basis of the signal output from the control unit U 4 , whereby the power to be supplied to the discharge lamp  10  is adjusted. When the power value to be supplied to the discharge lamp  10  is increased for example, the power value to be supplied to the discharge lamp  10  is controlled by the drive circuit Gx so as to match a preset reference voltage value by lowering the duty or the like. 
     In this manner, the control unit U 4  has a function to control the power value to be supplied to the discharge lamp  10  so as to match the reference power value, and has a function to change the power value to be supplied to the discharge lamp  10  not instantaneously, but at a predetermined power change ratio when changing the lighting mode of the discharge lamp  10  at the time of being steadily lit. 
     More specifically, a predetermined power value is set to a power setting unit  50  in the control unit U 4 . For example, a rated power value is set to the power setting unit  50  when the discharge lamp  10  is lit in the first lighting mode, and a power value lower than the rated power value is set to the power setting unit  50  when the discharge lamp  10  is lit in the second lighting mode. The power setting unit  50  may be provided in an interior of the power supply apparatus  20  or on the outside of the power supply apparatus  20 , for example, in the interiors of an apparatuses other than the discharge lamp lighting apparatus in the projector apparatus. 
     A power change ratio when changing the power in the power setting unit  50  is set in a power change ratio setting unit  51 . 
     A set power signal from the power setting unit  50 , a change ratio signal from the power change ratio setting unit  51 , and a time signal from a timer circuit  53  are input to a reference power setting unit  52 , and a reference power signal which is changed with elapsed time is output on the basis of these signals. For example, when the rated power value is initially set in the power setting unit  50  and a low power value lower than the rated power value is set at a certain time point, in the reference power setting unit  52 , the reference power value is changed from the rated power value to the low power value while receiving the time signal from the timer circuit  53  on the basis of the power change ratio set in the power change ratio setting unit  51 . For example, when the rated power value is 230 W, the low power value is 180 W, and the power change ratio is 2.00 W/s, the reference power value is changed from the rated power value (230 W) to the low power value (180 W) by spending 25 seconds, and subsequently, the reference power setting unit  52  outputs a reference power signal having the low power value (180 W) unless otherwise a new power value is set in the power setting unit  50 . 
     Signals from a voltage detecting unit Vx and the current detecting unit Rx are input to a power calculating circuit  54 , and a power value of a lighting power is calculated on the basis of these signals, the obtained lighting power signal is input to a comparing unit  55 , and in this comparing unit  55 , the lighting power signal is compared with the reference power signal from the power setting unit  50 . The signal output from the comparing unit  55  is input to the drive circuit Gx of the switching element Qx via a PWM circuit  56 . 
     The full bridge circuit U 2  contributes to a lighting frequency of the discharge lamp  10  as described above, and the respective drive circuits G 1  to G 4  are driven so as to achieve the frequency set in the full bridge circuit U 2 . 
     In the discharge lamp lighting apparatus according to the first embodiment, the discharge lamp  10  is lit by the supply of the AC power from the power supply apparatus  20  to the discharge lamp  10 , and the power to be supplied to the discharge lamp  10  by the power supply apparatus  20  is controlled, for example, as described below. 
       FIG. 3  is a graph illustrating an example of a change of value of power supplied to the discharge lamp in a case where the mode is switched from the first lighting mode to the second lighting mode in the discharge lamp lighting apparatus according to the first embodiment. In this graph, the vertical axis indicates a ratio of the power value to be actually supplied to the discharge lamp  10  with respect to the rated power value of the discharge lamp  10  (the ratio of the supply power value assuming that the rated power value is determined to be 100%), and the lateral axis indicates a lighting time of the discharge lamp  10 . 
     First of all, when the power supply apparatus  20  is driven in a state in which the first lighting mode is selected as the lighting mode, power in the first lighting mode, specifically, AC power having a power value corresponding to the rated power of the discharge lamp  10  is supplied from the power supply apparatus  20  to the discharge lamp  10 , whereby the discharge lamp  10  is lit. Subsequently, if the lighting mode is changed from the first lighting mode to the second lighting mode in the power supply apparatus  20  when a time T 1  is elapsed since the lighting of the discharge lamp  10  in the first lighting mode is started, the power value of the AC power to be supplied from the power supply apparatus  20  is lowered at a predetermined average power change ratio selected from a range of 0.01 to 2.1 W/s in the mode switching term in which the mode is switched from the first lighting mode to the second lighting mode, and when a time T 2  is elapsed from the change of the lighting mode from the first lighting mode to the second lighting mode, the power value of AC power to be supplied to the discharge lamp  10  reaches the power value of the power in the second lighting mode, and the lighting of the discharge lamp  10  in the second lighting mode is started. 
     Here, to give a detailed example of the length of the mode switching term, that is, the time T 2  taken until the power value supplied to the discharge lamp  10  is lowered from the value of the supply power in the first lighting mode to the value of the supply power in the second lighting mode, T 2 =(200 W−120 W)÷2.0 W/s=40 s is satisfied, where a rated power value of the discharge lamp  10  is 200 W, the value of the supply power in the second lighting mode is 120 W (60% of the rated power), and the average power change ratio in the mode switching term is 2.0 W/s, for example. 
     Also, when the lighting mode is changed from the second lighting mode to the first lighting mode in the power supply apparatus  20 , the power value of the AC power to be supplied from the power supply apparatus  20  is increased at the predetermined average power change ratio equal to or lower than 2.1 W/s in the mode switching term in which the mode is switched from the second lighting mode to the first lighting mode, and subsequently, the power value of the AC power to be supplied to the discharge lamp  10  reaches the power value of the power in the first lighting mode, and the lighting of the discharge lamp  10  in the first lighting mode is started. 
     In the description given above, a current waveform of the AC power to be supplied to the discharge lamp  10  preferably has a form in which a low-frequency component having a frequency lower than a basic frequency component is cyclically inserted into the basic frequency component. 
     An example of the current waveform of the AC power to be supplied to the discharge lamp  10  will be illustrated in  FIG. 4 . In this drawing, the vertical axis indicates the current power value of the power to be supplied to the discharge lamp  10 , and the lateral axis indicates the lighting time of the discharge lamp  10 . In this current waveform, a low-frequency component F 2  having a frequency lower than the basic frequency component is cyclically inserted into a basic frequency component F 1 . 
     The frequency of the basic frequency component F 1  is selected, for example, from a range from 60 to 1000 Hz. In contrast, the frequency of the low-frequency component F 2  is a frequency lower than the frequency of the basic frequency component F 1 , and is selected, for example, from a range from 5 to 200 Hz. 
     The low-frequency component F 2  may have a length of half a cycle, and an insertion interval of the low-frequency component F 2  (the time interval from when a certain low frequency component is inserted until when the next low-frequency component is inserted) is preferably not more than 120 seconds and, more preferably, from 0.01 to 120 seconds. Specific frequencies of the basic frequency component F 1  and the low-frequency component F 2 , an insertion interval of the low-frequency component F 2 , and the amplitude of the low-frequency component F 2  are selected as needed by considering the design of the discharge lamp  10  to be used, specifically, a relation with respect to a thermal design of the electrodes  14 ,  15  or the values of the power in the respective lighting modes or the like. 
     To give a specific example of the current waveform, the frequency of the basic frequency component F 1  is 370 Hz, the length of the one basic frequency component F 1  is 37.5 cycle, the frequency of the low-frequency component F 2  is 90 Hz, and the length of the one low-frequency component F 2  is 1 cycle. 
     In the discharge lamp lighting apparatus according to the first embodiment, when the power in the first lighting mode is supplied from the power supply apparatus  20  to the discharge lamp  10 , arcs A are formed between a pair of the electrodes  14 ,  15  of the discharge lamp  10  as the projections p 1  formed at the distal ends of the head portions  14   a ,  15   a  of the respective electrodes  14 ,  15  as original points as illustrated in  FIG. 5(   a ). Then, in the mode switching term in which the mode is switched from the first lighting mode to the second lighting mode, as illustrated in  FIG. 5(   b ), the shape of the arcs A formed between the electrodes  14 ,  15  becomes narrower as the value of power supplied to the discharge lamp is lowered, so that the evaporated electrode substance is accumulated at positions on surfaces of the projections p 1  of the respective electrodes  14 ,  15  where the original points of the arcs A are positioned by a halogen cycle, whereby minute projections p 2  are formed on the surfaces of the projections p 1  on the respective electrodes  14 ,  15  as illustrated in  FIG. 5(   c ). 
     Therefore, according to the discharge lamp lighting apparatus of the first embodiment, the power to be supplied to the discharge lamp  10  is controlled by the power supply apparatus  20  under the condition in which the average power change ratio becomes a value from 0.01 to 2.1 W/s in the mode switching term in which the mode is switched from the first lighting mode to the second lighting mode, whereby the original points of the arcs A do not move on the surfaces of the projections p 1  of the respective electrodes  14 ,  15 , and the stable arcs A are formed, so that a plurality of the minute projections p 2  are not formed on the surfaces of the projections p 1  on the respective electrodes  14 ,  15 , and hence occurrence of the flicker may be prevented or suppressed while the discharge lamp  10  is lit in the second lighting mode. 
     In the discharge lamp lighting apparatus according to a second embodiment, the power supply apparatus has a function to light the discharge lamp  10  by switching the mode between the first lighting mode in which rated power is supplied to the above-described discharge lamp  10  and the second lighting mode in which the power lower than the rated power is supplied to the discharge lamp  10 , and is configured to control the power to be supplied to the discharge lamp  10  under the condition including the power change such that the power value is lowered from a given power value P 1  equal to or lower than the supply power in the first lighting mode and equal to or higher than the supply power in the second lighting mode to a power value P 2  lower than the power value P 1  in the mode switching term in which the mode is switched from the first lighting mode to the second lighting mode, and then is increased from the power value P 2  to a power value P 3  higher than the power value P 2  and lower than the power value P 1  (hereinafter, referred to as “specific power change”). Here, the power value to be supplied to the discharge lamp  10  in the second lighting mode is not specifically limited as long as being lower than the value of the rated power, and is selected normally within a range from 40 to 80% of the rated power. 
     Although the specific power change has only to be performed at least once in the mode switching term in which the mode is switched from the first lighting mode to the second lighting mode, it is preferably performed twice or more. In particular, when the difference in power value between the supply power in the first lighting mode and the supply power in the second lighting mode is significant, it is preferably performed three times or more little by little. 
     In the specific power change, the power value P 1  may be determined arbitrarily without being specifically limited as long as being values equal to and lower than the supply power in the first lighting mode and equal to and higher than the supply power in the second lighting mode. The power value P 2  has only to be a value lower than the power value P 1 , and may be a value lower than the supply power in the second lighting mode, for example. However, a difference (P 1 −P 2 ) between the power value P 1  and the power value P 2  is preferably on the order of 5 to 50% of the power value of the supply power in the first lighting mode. 
     The power value P 3  has only to be a value higher than the power value P 2  and lower than the power value P 1 , and a difference (P 3 -P 2 ) between the power value P 3  and the power value P 2  is preferably on the order of 2 to 50% of the difference (P 1 -P 2 ) between the power value P 1  and the power value P 2 . 
     When the specific power change is performed twice or more, the last specific power change is preferably the power change which is lowered from the power value P 1  to the power value P 2  which is a lower value than the supply power in the second lighting mode and then is increased to the power value P 2  which corresponds to the supply power in the second lighting mode. 
     Also, the average power change ratio when the power value to be supplied to the discharge lamp  10  is lowered from the power value P 1  to the power value P 2  is preferably from 0.1 to 10 W/s. 
     Also, the average power change ratio when the power value to be supplied to the discharge lamp  10  is increased from the power value P 2  to the power value P 3  is preferably from 0.1 to 10 W/s. 
     In the discharge lamp lighting apparatus according to the second embodiment, the discharge lamp  10  is lit by the supply of the AC power from the power supply apparatus to the discharge lamp  10 , and the power value to be supplied to the discharge lamp  10  by the power supply apparatus is controlled, for example, as described below. 
       FIG. 6  is a graph illustrating an example of the change of value of power supplied to the discharge lamp in the case where the mode is switched from the first lighting mode to the second lighting mode in the discharge lamp lighting apparatus according to the second embodiment. In this graph, the vertical axis indicates a ratio of the power value to be actually supplied to the discharge lamp  10  with respect to the rated power value of the discharge lamp  10  (the ratio of the supply power value assuming that the rated power value is determined to be 100%), and the lateral axis indicates the lighting time of the discharge lamp  10 . 
     First of all, when the power supply apparatus is driven in a state in which the first lighting mode is selected as the lighting mode, power in the first lighting mode, specifically, AC power having a power value corresponding to the rated power of the discharge lamp  10  is supplied from the power supply apparatus to the discharge lamp  10 , whereby the discharge lamp  10  is lit. Subsequently, if the lighting mode is changed from the first lighting mode to the second lighting mode in the power supply apparatus when the time T 1  is elapsed since the lighting of the discharge lamp  10  in the first lighting mode is started, the power to be supplied to the discharge lamp  10  is lowered while performing the power change such that the power value is lowered from the given power value P 1  which is lower than the supply power in the first lighting mode and equal to or higher than the supply power in the second lighting mode to the power value P 2  lower than the power value P 1 , and then, is increased from the power value P 2  to the power value P 3  higher than the power value P 2  and lower than the power value P 1  in the mode switching term in which the mode is switched from the first lighting mode to the second lighting mode, and when the time T 2  is elapsed since the change of the lighting mode from the first lighting mode to the second lighting mode, the power value of AC power to be supplied to the discharge lamp  10  reaches the power value of the power in the second lighting mode, and the lighting of the discharge lamp  10  in the second lighting mode is started. 
     In the description given above, the current waveform of the AC power to be supplied to the discharge lamp  10  is preferably a form in which a low-frequency component having a lower frequency than a basic frequency component is cyclically inserted into the basic frequency component. 
     As an example the current waveform of the AC power supplied to the discharge lamp  10 , the one exemplified in conjunction with the discharge lamp lighting apparatus according to the first embodiment, that is, those illustrated in  FIG. 4  may be presented. 
     The frequency of the basic frequency component F 1  is selected, for example, from a range from 60 to 1000 Hz. In contrast, the frequency of the low-frequency component F 2  is the frequency lower than the frequency of the basic frequency component F 1 , and is selected, for example, from a range from 5 to 200 Hz. 
     The low-frequency component may have a length of half a cycle, and an insertion interval of the low-frequency component (the time interval from when a certain low frequency component is inserted until when the next low-frequency component is inserted) is preferably not more than 120 seconds and, more preferably, from 0.01 to 120 seconds. 
     Specific frequencies of the basic frequency component F 1  and the low-frequency component F 2 , an insertion interval of the low-frequency component F 2 , and the amplitude of the low-frequency component F 2  are selected as needed by considering the design of the discharge lamp  10  to be used, specifically, the relation with respect to the thermal design of the electrodes  14 ,  15  or the values of the power in the respective lighting modes and the like. 
     To give a specific example of the current waveform, the frequency of the basic frequency component F 1  is 370 Hz, the length of the one basic frequency component F 1  is 37.5 cycle, the frequency of the low-frequency component F 2  is 90 Hz, and the length of the one low-frequency component F 2  is 1 cycle. 
     In the discharge lamp lighting apparatus according to the second embodiment, when the power in the first lighting mode is supplied from the power supply apparatus to the discharge lamp  10 , the arcs A are formed between a pair of the electrodes  14 ,  15  of the discharge lamp  10  as the projections p 1  formed at the distal ends of the head portions  14   a ,  15   a  of the respective electrodes  14 ,  15  as original points as illustrated in  FIG. 7(   a ). 
     Then, in the mode switching term in which the mode is switched from the first lighting mode to the second lighting mode, when the power to be supplied to the discharge lamp  10  is lowered from the power value P 1  to the power value P 2 , as illustrated in  FIG. 7(   b ), the shape of the arcs A formed between the electrodes  14 ,  15  becomes narrower, so that the evaporated electrode substance is accumulated at positions on the surfaces of the projections p 1  of the respective electrodes  14 ,  15  where the original points of the arcs A are positioned by the halogen cycle, whereby the minute projections p 2  are formed on the surfaces of the projections p 1  of the respective electrodes  14 ,  15  as illustrated in  FIG. 7(   c ). At this time, since the shape of the arcs A formed between the electrodes  14 ,  15  is abruptly narrowed, the arcs A formed between the electrodes  14 ,  15  become unstable by the movement of the original points on the surfaces of the projections p 1  of the respective electrodes  14 ,  15  as illustrated in  FIG. 7(   d ) and, consequently, a plurality of the minute projections p 2  are formed on the surfaces of the projections p 1  of the respective electrodes  14 ,  15  as illustrated in  FIG. 7(   e ). Therefore, when the power to be supplied to the discharge lamp  10  is increased from the power value P 2  to the power value P 3 , as illustrated in  FIG. 7(   t ), the shape of the arcs A formed between the electrodes  14 ,  15  becomes thicker and a plurality of the minute projections p 2  are melted, whereby the minute projections p 2  converge into one as illustrated in  FIG. 7(   g ). 
     In this manner, according to the discharge lamp lighting apparatus of the second embodiment, since the AC power to be supplied to the discharge lamp  10  is controlled after lowering from the power value P 1  to the power value P 2  under the condition including the power change increased from the power value P 2  to the power value P 3  higher than the power value P 2  and lower than the power value P 1  in the mode switching term in which the mode is switched from the first lighting mode to the second lighting mode, even though a plurality of the minute projections p 2  are formed on the surfaces of the projections p 1  of the electrodes  14 ,  15  when the AC power to be supplied to the discharge lamp  10  is lowered from the power value P 1  to the power value P 2 , a plurality of the minute projections p 2  are converged into one minute projection p 2  and the electrodes  14 ,  15  are repaired when the AC power to be supplied to the discharge lamp  10  is increased from the power value P 2  to the power value P 3 , so that the occurrence of flicker may be prevented or suppressed while the discharge lamp  10  is lit in the second lighting mode. 
     In the discharge lamp lighting apparatus according to a third embodiment, the power supply apparatus has a function to light the discharge lamp  10  by switching the mode between the rated power lighting mode in which the rated power is supplied to the above-described discharge lamp  10  and the low power lighting mode in which power lower than the rated power is supplied to the discharge lamp  10 , and is configured to supply basic AC power of the predetermined power value continuously to the discharge lamp  10  in the low power lighting mode, and supply superimposed power superimposed on this basic AC power cyclically. 
     Here, the power value of the basic AC power in the low power lighting mode is not specifically limited as long as being lower than the value of the rated power, and is selected normally within a range from 40 to 80% of the rated power. 
     The current waveform of the AC power to be supplied to the discharge lamp  10  is preferably a form in which a low-frequency component having a lower frequency than a basic frequency component is cyclically inserted into the basic frequency component. 
     As an example of the current waveform of the AC power supplied to the discharge lamp  10 , the one exemplified in conjunction with the discharge lamp lighting apparatus according to the first embodiment, that is, those illustrated in  FIG. 4  may be presented. 
     The frequency of the basic frequency component F 1  is selected, for example, from a range from 60 to 1000 Hz. In contrast, the frequency of the low-frequency component F 2  is a frequency lower than the frequency of the basic frequency component F 1 , and is selected, for example, from a range from 5 to 200 Hz. 
     The low-frequency component may have a length of half a cycle, and an insertion interval of the low-frequency component (the time interval from when a certain low frequency component is inserted until when the next low-frequency component is inserted) is preferably not more than 120 seconds and, more preferably, from 0.01 to 120 seconds. 
     Detailed frequencies of the basic frequency component F 1  and the low-frequency component F 2 , an insertion interval of the low-frequency component F 2 , and the amplitude of the low-frequency component F 2  are selected as needed by considering the design of the discharge lamp  10  to be used, specifically, a relation with respect to a thermal design of the electrodes  14 ,  15  or the values of the power in the respective lighting modes. 
     Also, the basic frequency component of the basic AC power in the low power lighting mode is preferably a frequency higher than the basic frequency component of the AC current in the rated power lighting mode and, for example, is preferably 1.1 to 10 times the frequency of the basic frequency component of the AC current in the rated power lighting mode. 
     Also, the low frequency component of the superimposed power in the low power lighting mode is preferably a frequency lower than the low frequency component in the rated power lighting mode and, for example, is preferably 10 to 95% of the frequency of the low frequency component of the rated power. 
     To give a specific example, when the rated power of the discharge lamp  10  is 230 W, and the basic AC power in the low power lighting mode is 75% of the rated power, the basic frequency component of the AC current in the rated voltage lighting mode is 370 Hz, the low-frequency component is 46.25 Hz, the basic frequency component of the basic AC power in the low power lighting mode is 518 Hz, and the low-frequency component of the superimposed power is 37 Hz. 
     Then, the supply timing of the superimposed power, the power value of the superimposed power, or the supply time of the superimposed power is controlled by the power supply apparatus according to a measured value of a lighting voltage of the discharge lamp  10 . Here, the measured value of the lighting voltage of the discharge lamp  10  may be a value measured when the superimposed power is supplied (hereinafter, referred to as “when the power is superimposed”), or may be a value measured when the superimposed power is not supplied (hereinafter, referred to as “when the power is not superimposed”). 
       FIG. 8  is a drawing illustrating a change of value of power supplied to the discharge lamp in the low power lighting mode in an example of the power supply apparatus along with the change of lighting voltage of the discharge lamp. 
     The power supply apparatus of this example is configured to control the supply timing of the superimposed power according to the measured value of the lighting voltage of the discharge lamp  10  in the low power lighting mode. To give a specific description, when the measured value of the lighting voltage of the discharge lamp  10  is lower than a predetermined reference value V 0 , the basic AC power having a power value P 1  is supplied to the discharge lamp  10  and, simultaneously, the superimposed power having a power value P 2  is supplied by being superimposed on the basic AC power cyclically for a predetermined supply time T at a predetermined time interval t 1 . When the measured value of the lighting voltage of the discharge lamp  10  is increased beyond the reference value V 0 , the superimposed power having the power value P 2  is supplied to the discharge lamp  10  by being superimposed on the basic AC power cyclically for the predetermined supply time T at the predetermined time interval t 2  which is shorter than the time interval t 2 , that is, the time interval of the superimposed power is changed from t 1  to t 2  which is shorter than t 1 . When the measured value of the lighting voltage of the discharge lamp  10  is lowered again beyond the reference value V 0 , the superimposed power of the power value P 2  is supplied to the discharge lamp  10  by being superimposed on the basic AC power cyclically for a predetermined supply time T at a predetermined time interval t 1 , that is, the time interval of the superimposed power is changed from t 2  to t 1 . 
     In the description given above, the reference value V 0  of the lighting voltage is set as needed considering the specifications of the discharge lamp  10 , the power value P 1  of the basic AC power, power value P 2  of the superimposed power, the supply time T of the superimposed power, the time intervals t 1 , t 2  of the superimposed power and so on supplied to the discharge lamp  10  in the low power lighting mode. For example, it is preferable to make the set value of the reference value V 0  of the lighting voltage higher as the set values of the time intervals t 1 , t 2  of the superimposed power are increased. 
     The time interval t 1  of the superimposed power when the lighting voltage is lower than the reference value V 0  is preferably set within a range from 1 to 60 minutes, for example, although it depends on the basic AC power P 1 , the power value P 2  of the superimposed power, and the supply time T of the superimposed power, and so on. 
     The time interval t 2  of the superimposed power when the lighting voltage is higher than the reference value V 0  only needs to be shorter than the time interval t 1 , and is set as needed considering the power value P 1  of the basic AC power, power value P 2  of the superimposed power, the supply time T of the superimposed power, the reference value V 0  of the lighting voltage, and so on. For example, it is preferable to make the set value of the time interval t 2  of the superimposed power shorter as the set value of the reference value V 0  of the lighting voltage is increased. 
     The supply time T of the superimposed power is preferably selected within a range from 1 to 10 minutes, for example, although it depends on the power value P 1  of the basic AC power, the power value P 2  of the superimposed power, and the time interval t 1  of the superimposed power and the like. 
     The power value P 2  of the superimposed power is preferably set within a range from 65 to 120% of the rated power, for example, although it depends on the power value P 1  of the basic AC power, the supply time T of the superimposed power, the time interval t 1  of the superimposed power, and so on. 
     A further specific example will be given as follows. 
     Superimposed power of 173 W is supplied to supply power of 230 W in the rated power lighting mode and supply power of 138 W in the low power lighting mode, and parameters of the supply power and the superimposed power in the low power lighting mode are set according to a lamp voltage in either mode. 
     When the lamp voltage is lower than the reference value of the lighting voltage, the time interval of the superimposed power is elongated, and when the lamp voltage is higher than the above-described reference value, the time interval of the superimposed power is shortened. For example, when the lamp voltage is lower than a reference value of 100V, the time interval of the superimposed power is set to 15 minutes, and when the lamp voltage is equal to or higher than the reference value of 100 V, the time interval of the superimposed power is set to 5 minutes. 
     For reference, the supply time of the superimposed power is 2 minutes in both cases. 
     Alternatively, when the lamp voltage is lower than the reference value of the lighting voltage, the supply time of the superimposed power is shortened, and when the lamp voltage is higher than the reference value, the supply time of the superimposed power is elongated. For example, when the lamp voltage is lower than the reference value of 100 V, the supply time of the superimposed power is set to 2 minutes, and when the lamp voltage is equal to or higher than the reference value of 100 V, the supply time of the superimposed power is set to 4 minutes. 
     Also, the time interval of the superimposed power is 15 minutes in both cases. 
     To give a specific example of the set value of the supply power in the low power lighting mode in such a power supply apparatus, when the rated power of the discharge lamp  10  is 230 W, the power value P 1  of the basic AC power is 138 W (60% of the rated power), the power value P 2  of the superimposed power is 35 W, the supply time of the superimposed power is 2 minutes, the time interval t 1  of the superimposed power is 15 minutes, the time interval t 2  of the same is 5 minutes, and the reference value V 0  of the lighting voltage measured when the power is superimposed is 80 V. 
       FIG. 9  illustrates the change of value of power supplied to the discharge lamp in the low power lighting mode in another example of the power supply apparatus in the discharge lamp lighting apparatus according to the third embodiment along with the change of the lighting voltage of the discharge lamp. 
     The power supply apparatus of this example is configured to control the power value of the superimposed power according to the measured value of the lighting voltage of the discharge lamp  10  in the low power lighting mode. To give a specific description, when the measured value of the lighting voltage of the discharge lamp  10  is lower than the predetermined reference value V 0 , the basic AC power having the power value P 1  is supplied to the discharge lamp  10  and, simultaneously, the superimposed power having the power value P 2  is supplied by being superimposed on the basic AC power cyclically for the predetermined supply time T at a predetermined time interval t. When the measured value of the lighting voltage of the discharge lamp  10  is increased beyond the predetermined reference value V 0 , the superimposed power having the predetermined power value P 3  higher than the power value P 2  is supplied to the discharge lamp  10  by being superimposed on the basic AC power cyclically for the predetermined supply time T at the time interval t, that is, the power value of the superimposed power is changed from P 2  to P 3  higher than P 2 . Also, when the measured value of the lighting voltage of the discharge lamp  10  is lowered again beyond the predetermined reference value V 0 , the superimposed power having the predetermined power value P 2  is supplied to the discharge lamp  10  by being superimposed on the basic AC power cyclically for the predetermined supply time T at the time interval t, that is, the power value of the superimposed power is changed from P 3  to P 2 . 
     To give a specific example of the set value of the supply power in the low power lighting mode in such a power supply apparatus, when the rated power of the discharge lamp  10  is 230 W, the power value P 1  of the basic AC power is 138 W (60% of the rated power), the power value P 2  of the superimposed power is 35 W, the power value P 3  of the same is 92 W, the supply time T of the superimposed power is 2 minutes, the time interval t of the superimposed power is 15 minutes, and the reference value V 0  of the lighting voltage measured when the power is not superimposed is 70 V 
       FIG. 10  is a drawing illustrating a change of value of power supplied to the discharge lamp in the low power lighting mode in still another example of the power supply apparatus in the discharge lamp lighting apparatus according to the third embodiment along with the change of the lighting voltage of the discharge lamp. 
     The power supply apparatus of this example is configured to control the supply time of the superimposed power according to the measured value of the lighting voltage of the discharge lamp  10  in the low power lighting mode. To give a specific description, when the measured value of the lighting voltage of the discharge lamp  10  is lower than the predetermined reference value V 0 , the basic AC power having the power value P 1  is supplied to the discharge lamp  10  and, simultaneously, the superimposed power having the power value P 2  is supplied by being superimposed on the basic AC power cyclically for a predetermined supply time T 1  at the predetermined time interval t. When the measured value of the lighting voltage of the discharge lamp  10  is increased beyond the predetermined reference value V 0 , the superimposed power having the power value P 2  is supplied to the discharge lamp  10  by being superimposed on the basic AC power cyclically for a predetermined supply time T 2  which is longer than the supply time T 1  at the predetermined time interval t, that is, the supply time of the superimposed power is changed from T 1  to T 2  which is longer than T 1 . Also, when the measured value of the lighting voltage of the discharge lamp  10  is lowered again beyond the predetermined reference value V 0 , the superimposed power having the predetermined power value P 2  is supplied to the discharge lamp  10  by being superimposed on the basic AC power cyclically for the predetermined supply time T 1  at the predetermined time interval t, that is, the supply time of the superimposed power is changed from T 2  to T 1 . 
     To give a specific example of the set value of the supply power in the low power lighting mode in such a power supply apparatus, when the rated power of the discharge lamp  10  is 230 W, the power value P 1  of the basic AC power is 138 W (60% of the rated power), the power value P 2  of the superimposed power is 35 W, the supply time T 1  of the superimposed power is 2 minutes, the supply time T 2  of the same is 4 minutes, the time interval t of the superimposed power is 15 minutes, and the reference value V 0  of the lighting voltage measured when the power is not superimposed is 70 V. 
     According to the discharge lamp lighting apparatus of the third embodiment, the superimposed power which is superimposed on the basic AC power having the predetermined power value is cyclically supplied to the discharge lamp  10 , and the supply timing of the superimposed power, the power value of the superimposed power, or the supply time of the superimposed power is controlled in the low power lighting mode in which power lower than the rated power is supplied to the discharge lamp  10  by the power supply apparatus, so that the amount of the evaporated electrode substance existing in the periphery of the electrodes  14 ,  15  of the discharge lamp  10  is increased, whereby accumulation of the evaporated electrode substance on the electrodes  14 ,  15  of the discharge lamp  10  is accelerated, and hence occurrence of wearing and flicker of the electrodes  14 ,  15  of the discharge lamp  10  may be prevented or suppressed. 
     The discharge lamp lighting apparatus of the present invention is not limited to the above-described embodiments, and various modifications may be made. For example, the form of the electrodes of the discharge lamp is not limited to those illustrated in  FIG. 1  and, for example, may be those having head portions of a truncated conical shape. 
     In the first embodiment, for example, the power supply apparatus may have a function to light the discharge lamp by switching the mode between the above-described second lighting mode and a third lighting mode in which power lower than the supply power in the second lighting mode is supplied to the discharge lamp. 
     In the discharge lamp lighting apparatus as described above, the power supply apparatus is preferably configured to control the power to be supplied to the discharge lamp under the condition that the average power change ratio becomes a value from 0.01 to 2.1 W/s in the mode switching term in which the mode is switched from the second lighting mode to the third lighting mode and, in addition, is preferably configured to control the power to be supplied to the discharge lamp  10  under the condition that the average power change ratio is equal to or lower than 2.1 W/s in the mode switching term in which the mode is switched from the third lighting mode to the second lighting mode. 
     Also, the power supply apparatus does not need to lower the power to be supplied to the discharge lamp under the condition that the power change ratio is always constant in the mode switching term in which the mode is switched from the first lighting mode to the second lighting mode and, as illustrated in  FIG. 11 , the power to be supplied to the discharge lamp may be controlled so as to be lowered from the power value in the first lighting mode to the power value in the second lighting mode while repeating lowering of the power value and maintenance of the power value alternately. 
     Also, in the mode switching term in which the mode is switched from the second lighting mode to the first lighting mode, the power to be supplied to the discharge lamp may be controlled so as to increase from the power value in the second lighting mode to the power value in the first lighting mode while repeating increase of the power value and maintenance of the power value alternately. 
     Furthermore, when the power supply apparatus is configured to have a function to light the discharge lamp by switching the mode between the second lighting mode and the third lighting mode in which the power lower than the supply power in the second lighting mode is supplied to the discharge lamp, the power to be supplied to the discharge lamp may be controlled so as to be lowered from the power value in the second lighting mode to the power value in the third lighting mode, or to be increased from the power value in the third lighting mode to the power value in the second lighting mode while repeating lowering of the power value and maintenance of the power value alternately, or repeating increase of the power value and maintenance of the power value alternately in the mode switching term in which the mode is switched from the second lighting mode to the third lighting mode or in the mode switching term in which the mode is switched from the third lighting mode to the second lighting mode. 
     When performing the power control as described above, the power to be supplied to the discharge lamp has only to be controlled under the condition that the average power change ratio in the respective mode switching terms, that is, in a term from t 1  to t 2  in  FIG. 11  becomes a value from 0.01 to 2.1 W/s. 
     Also, the power supply apparatus is not limited to the configuration illustrated in  FIG. 2 , but various circuit configurations are applicable. 
     In the second embodiment, for example, the power supply apparatus may have a function to light the discharge lamp by switching the mode between the above-described second lighting mode and the third lighting mode in which the power lower than the supply power in the second lighting mode is supplied to the discharge lamp. 
     In the discharge lamp lighting apparatus as described above, the power supply apparatus is preferably configured to control the power to be supplied to the discharge lamp under the condition including the power change such that the power value is lowered from a given power value P 1  equal to or lower than the supply power in the second lighting mode and equal to or higher than the supply power in the third lighting mode to the power value P 2  lower than the power value P 1 , and then is increased from the power value P 2  to the power value P 3  higher than the power value P 2  and lower than the power value P 1  in a switching term in which the mode is switched from the second lighting mode to the third lighting mode. 
     Also, in the power supply apparatus, the power change increasing from the power value P 2  to the power value P 3  does not have to be performed continuously for the power change lowering from the power value P 1  to the power value P 2  in the mode switching term in which the mode is switched from the first lighting mode to the second lighting mode and, as illustrated in  FIG. 12  for example, the power value may be lowered from the power value P 1  to the power value P 2 , be maintained at the power value P 2 , and then be increased from the power value P 2  to the power value P 3 . 
     Also, when the power supply apparatus is configured to have a function to light the discharge lamp by switching the mode to or from the third lighting mode in which the power lower than the supply power in the second lighting mode is supplied to the discharge lamp, the power value may be lowered from the power value P 1  to the power value P 2 , be maintained at the power value P 2 , and then be increased from the power value P 2  to the power value P 3  in the mode switching term in which the mode is switched from the second switching mode to the third lighting mode. 
     In the third embodiment, although the power supply apparatus is configured to control one of the supply timing of the superimposed power, the power value of the superimposed power, and the supply time of the superimposed power all in accordance with the measured value of the lighting voltage of the discharge lamp  10 , the power supply apparatus may be configured to control two of the supply timing of the superimposed power, the power value of the superimposed power, and the supply time of the superimposed power, or to control all of the supply timing of the superimposed power, the power value of the superimposed power, and the supply time of the superimposed power. 
     Also, the power supply apparatus may be configured to have two or more preset reference values of the lighting voltage of the discharge lamp  10  and control the supply timing of the superimposed power, the power value of the superimposed power, or the supply time of the superimposed power on the basis of the respective reference values. 
     The power supply apparatus may be configured to be set to zero as the supply of the superimposed power (a state in which the superimposed power is not supplied) when the measured value of the lighting voltage of the discharge lamp is lower than the predetermined reference value. 
     Manufacture of Discharge Lamp 
     According to the configuration illustrated in  FIG. 1 , a discharge lamp (A) and a discharge lamp (B) having the following specifications were manufactured. 
     Discharge Lamp (A): 
     The light-emitting tube was formed of quartz glass, and the maximum diameter of the light-emitting portion was 10 mm, and the internal volume of the light-emitting portion was 65 mm 3 . 
     The respective electrodes were formed of tungsten, and the inter-electrode distance was 1.0 mm. 
     0.3 mg/mm 3  of mercury, 13 kPa of argon gas at a static pressure, and halogen (Br) of 4.0×10 −4  μmol/mm 3  were enclosed in the interior of the light-emitting tube. 
     The rated power of the discharge lamp (A) was 230 W, the rated voltage was 80 V, and the bulb wall loading value was 2.5 W/mm 2 . 
     Discharge Lamp (B): 
     The light-emitting tube was formed of quartz glass, and the maximum diameter of the light-emitting portion was 9.4 mm, and the internal volume of the light-emitting portion was 50 mm 3 . 
     The respective electrodes were formed of tungsten, and the inter-electrode distance was 0.7 mm. 
     0.3 mg/mm 3  of mercury, 13 kPa of argon gas at a static pressure, and halogen (Br) of 5.0×10 −4  μmol/mm 3  were enclosed in the light-emitting tube. 
     The rated power of the discharge lamp (B) was 180 W, the rated voltage was 65 V, and the bulb wall loading value was 2.5 W/mm 2 . 
     Examples of First Embodiment 
     Experimental Example 1 
     The discharge lamp (A) was fit by supplying AC power (power in which one cycle of a lower frequency component in 90 Hz was inserted at every 37.5 cycle of a basic frequency component in 370 Hz) of 230 W (rated power value), the value of power to be supplied to the discharge lamp (A) was lowered from 230 W to 138 W (60% of the rated power value) at an average power change ratio shown in Table 1, then light radiated from the discharge lamp (A) was visually observed to inspect the conditions of occurrence of flicker, then if the occurrence of the flicker was not visually recognized, luminance of an irradiated surface was measured by using a measuring instrument such as illuminance meter, and evaluated as “very good” if a rate of variability of an illuminance value was within ±2%, “good” if the rate of variability exceeds the range of ±2% but within ±3%, and “bad” if the occurrence of the flicker was visually recognized. 
     The operation described above was performed repeatedly by five times, and then the light-emitting tube of the discharge lamp (A) was observed to inspect the conditions of occurrence of blackening, and evaluated as “good” if occurrence of blackening was not recognized, and “bad” if the occurrence of blackening was recognized. 
     The results of the evaluation described above will be shown in Table 1 given below. 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Power change ratio (W/s) 
               
             
          
           
               
                   
                 9.20 
                 3.07 
                 2.04 
                 1.53 
                 1.02 
                 0.77 
                 0.51 
                 0.44 
                 0.38 
                 0.31 
                 0.15 
                 0.08 
                 0.05 
                 0.03 
               
               
                   
                   
               
             
          
           
               
                 Time of mode 
                 10 
                 30 
                 45 
                 60 
                 90 
                 120 
                 180 
                 210 
                 240 
                 300 
                 600 
                 1200 
                 1800 
                 3600 
               
               
                 switching 
               
               
                 term (s) 
               
               
                 Condition of 
                 bad 
                 bad 
                 good 
                 good 
                 good 
                 very 
                 very 
                 very 
                 very 
                 very 
                 very 
                 very 
                 good 
                 good 
               
               
                 occurrence of 
                   
                   
                   
                   
                   
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
               
               
                 flicker 
               
               
                 Condition of 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
               
               
                 occurrence of 
               
               
                 blackening 
               
               
                   
               
             
          
         
       
     
     Experimental Example 2 
     The discharge lamp (A) was it by supplying AC power (power in which one cycle of a lower frequency component in 90 Hz was inserted at every 37.5 cycle of a basic frequency component in 370 Hz) of 184 W (80% of the rated power value), the value of power to be supplied to the discharge lamp (A) was lowered from 184 W to 115 W (50% of the rated power value) at an average power change ratio shown in Table 2, then light radiated from the discharge lamp (A) was visually observed to inspect the conditions of occurrence of flicker, then if the occurrence of the flicker was not visually recognized, luminance of an irradiated surface was measured by using a measuring instrument such as illuminance meter, and evaluated as “very good” if a rate of variability of the illuminance value was within ±2%, “good” if the rate of variability exceeds the range of ±2% but within ±3%, and “bad” if the occurrence of the flicker was visually recognized. 
     The operation described above was performed repeatedly by five times, and then the light-emitting tube of the discharge lamp (A) was observed to inspect the conditions of occurrence of blackening, and evaluated as “good” if occurrence of blackening was not recognized, and “bad” if the occurrence of blackening was recognized. 
     The results of the evaluation described above will be shown in Table 2 given below. 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Power change ratio (W/s) 
               
             
          
           
               
                   
                 6.90 
                 2.30 
                 1.97 
                 1.15 
                 0.77 
                 0.58 
                 0.38 
                 0.33 
                 0.29 
                 0.23 
                 0.12 
                 0.06 
                 0.04 
                 0.02 
               
               
                   
                   
               
             
          
           
               
                 Time of mode 
                 10 
                 30 
                 45 
                 60 
                 90 
                 120 
                 180 
                 210 
                 240 
                 300 
                 600 
                 1200 
                 1800 
                 3600 
               
               
                 switching 
               
               
                 term (s) 
               
               
                 Condition of 
                 bad 
                 bad 
                 good 
                 good 
                 very 
                 very 
                 very 
                 very 
                 very 
                 very 
                 very 
                 good 
                 good 
                 good 
               
               
                 occurrence of 
                   
                   
                   
                   
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
               
               
                 flicker 
               
               
                 Condition of 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
               
               
                 occurrence of 
               
               
                 blackening 
               
               
                   
               
             
          
         
       
     
     Experimental Example 3 
     The discharge lamp (A) was lit by supplying AC power (power in which one cycle of a lower frequency component in 90 Hz was inserted at every 37.5 cycle of a basic frequency component in 370 Hz) of 184 W (80% of the rated power value), the value of power to be supplied to the discharge lamp (A) was lowered from 184 W to 138 W (60% of the rated power value) at an average power change ratio shown in Table 3 given below, then light radiated from the discharge lamp (A) was visually observed to inspect the conditions of occurrence of flicker, then if the occurrence of the flicker was not visually recognized, luminance of an irradiated surface was measured by using a measuring instrument such as illuminance meter, and evaluated as “very good” if a rate of variability of the illuminance value was within ±2%, “good” if the rate of variability exceeds the range of ±2% but within ±3%, and “bad” if the occurrence of the flicker was visually recognized. 
     The operation described above was performed repeatedly by five times, and then the light-emitting tube of the discharge lamp (A) was observed to inspect the conditions of occurrence of blackening and, evaluated as “good” if occurrence of blackening was not recognized, and “bad” if the occurrence of blackening was recognized. 
     The results of the evaluation described above will be shown in Table 3 given below. 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
             
             
               
                   
                   
               
               
                   
                 Power change ratio (W/s) 
               
             
          
           
               
                   
                 4.60 
                 2.30 
                 1.53 
                 0.77 
                 0.51 
                 0.38 
                 0.26 
                 0.22 
                 0.19 
                 0.15 
                 0.08 
                 0.04 
                 0.03 
                 0.013 
               
               
                   
                   
               
             
          
           
               
                 Time of mode 
                 10 
                 30 
                 45 
                 60 
                 90 
                 120 
                 180 
                 210 
                 240 
                 300 
                 600 
                 1200 
                 1800 
                 3600 
               
               
                 switching 
               
               
                 term (s) 
               
               
                 Condition of 
                 bad 
                 bad 
                 good 
                 very 
                 very 
                 very 
                 very 
                 very 
                 very 
                 very 
                 very 
                 good 
                 good 
                 good 
               
               
                 occurrence of 
                   
                   
                   
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
               
               
                 flicker 
               
               
                 Condition of 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
               
               
                 occurrence of 
               
               
                 blackening 
               
               
                   
               
             
          
         
       
     
     Experimental Example 4 
     The discharge lamp (A) was lit by supplying AC power (power in which one cycle of a lower frequency component in 90 Hz was inserted at every 37.5 cycle of a basic frequency component in 370 Hz) of 161 W (70% of the rated power value), the value of power to be supplied to the discharge lamp (A) was lowered from 184 W to 138 W (60% of the rated power value) at an average power change ratio shown in Table 4 given below, then light radiated from the discharge lamp (A) was visually observed to inspect the conditions of occurrence of flicker, then if the occurrence of the flicker was not visually recognized, luminance of an irradiated surface was measured by using a measuring instrument such as illuminance meter, and evaluated as “very good” if a rate of variability of the illuminance value was within ±2%, “good” if the rate of variability exceeds the range of ±2% but within ±3%, and “bad” if the occurrence of the flicker was visually recognized. 
     The operation described above was performed repeatedly by five times, and then the light-emitting tube of the discharge lamp (A) was observed to inspect the conditions of occurrence of blackening, and evaluated as “good” if occurrence of blackening was not recognized, and “bad” if the occurrence of blackening was recognized. 
     The results of the evaluation described above will be shown in Table 4 given below. 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
             
             
               
                   
                   
               
               
                   
                 Power change ratio (W/s) 
               
             
          
           
               
                   
                 2.30 
                 1.53 
                 0.77 
                 0.38 
                 0.26 
                 0.19 
                 0.13 
                 0.10 
                 0.10 
                 0.08 
                 0.04 
                 0.02 
                 0.013 
                 0.006 
               
               
                   
                   
               
             
          
           
               
                 Time of mode 
                 10 
                 30 
                 45 
                 60 
                 90 
                 120 
                 180 
                 210 
                 240 
                 300 
                 600 
                 1200 
                 1800 
                 3600 
               
               
                 switching 
               
               
                 term (s) 
               
               
                 Condition of 
                 bad 
                 good 
                 very 
                 very 
                 very 
                 very 
                 very 
                 very 
                 very 
                 very 
                 good 
                 good 
                 good 
                 good 
               
               
                 occurrence of 
                   
                   
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
               
               
                 flicker 
               
               
                 Condition of 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 bad 
               
               
                 occurrence of 
               
               
                 blackening 
               
               
                   
               
             
          
         
       
     
     Experimental Example 5 
     The discharge lamp (B) was lit by supplying AC power (power in which one cycle of a lower frequency component in 90 Hz was inserted at every 37.5 cycle of a basic frequency component in 370 Hz) of 180 W (rated power value), the value of power to be supplied to the discharge lamp (B) was lowered from 180 W to 108 W (60% of the rated power value) at an average power change ratio shown in Table 5 given below, then light radiated from the discharge lamp (B) was visually observed to inspect the conditions of occurrence of flicker, then if the occurrence of the flicker was not visually recognized, luminance of an irradiated surface was measured by using a measuring instrument such as illuminance meter, and evaluated as “very good” if a rate of variability of the illuminance value was within ±2%, “good” if the rate of variability exceeds the range of ±2% but within ±3%, and “bad” if the occurrence of the flicker was visually recognized. 
     The operation described above was performed repeatedly by five times, and then the light-emitting tube of the discharge lamp (B) was observed to inspect the conditions of occurrence of blackening, and evaluated as “good” if occurrence of blackening was not recognized, and “bad” if the occurrence of blackening was recognized. 
     The results of the evaluation described above will be shown in Table 5 given below. 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 5 
               
             
             
               
                   
                   
               
               
                   
                 Power change ratio (W/s) 
               
             
          
           
               
                   
                 7.20 
                 2.40 
                 2.06 
                 1.20 
                 0.80 
                 0.60 
                 0.40 
                 0.34 
                 0.30 
                 0.24 
                 0.12 
                 0.06 
                 0.04 
                 0.02 
               
               
                   
                   
               
             
          
           
               
                 Time of mode 
                 10 
                 30 
                 45 
                 60 
                 90 
                 120 
                 180 
                 210 
                 240 
                 300 
                 600 
                 1200 
                 1800 
                 3600 
               
               
                 switching 
               
               
                 term (s) 
               
               
                 Condition of 
                 bad 
                 bad 
                 good 
                 good 
                 good 
                 very 
                 very 
                 very 
                 very 
                 very 
                 very 
                 very 
                 good 
                 good 
               
               
                 occurrence of 
                   
                   
                   
                   
                   
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
               
               
                 flicker 
               
               
                 Condition of 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
               
               
                 occurrence of 
               
               
                 blackening 
               
               
                   
               
             
          
         
       
     
     As is apparent from the results in Table 1 to Table 5, it was found that when the power to be supplied to the discharge lamp was lowered under the condition that the average power change ratio became 0.01 to 2.1 W/s, the flicker did not occur and even when lowering of the power to be supplied to the discharge lamp under the condition described above was performed repeatedly, blackening of the light-emitting tube of the discharge lamp did not occur. 
     Experimental Example 6 
     The discharge lamp (A) was lit by supplying AC power (power in which one cycle of a lower frequency component in 90 Hz was inserted at every 37.5 cycle of a basic frequency component in 370 Hz) of 138 W (60% of the rated power value), the value of power to be supplied to the discharge lamp (A) was increased from 138 W to 230 W (the rated power value) at an average power change ratio shown in Table 6 given below, then the light-emitting tube was visually observed to inspect the conditions of occurrence of cracks, and evaluated as “good” if no crack occurred in the light-emitting tube, and “bad” if the crack occurred in the light-emitting tube. 
     The operation described above was performed repeatedly by five times, and then the light-emitting tube of the discharge lamp (A) was observed to inspect the conditions of occurrence of blackening, and evaluated as “good” if occurrence of blackening was not recognized, and “bad” if the occurrence of blackening was recognized. 
     The results of the evaluation described above will be shown in Table 6 given below. 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 6 
               
             
             
               
                   
                   
               
               
                   
                 Power change ratio (W/s) 
               
             
          
           
               
                   
                 9.20 
                 3.07 
                 2.04 
                 1.53 
                 1.02 
                 0.77 
                 0.51 
                 0.44 
                 0.38 
                 0.31 
                 0.15 
                 0.08 
                 0.05 
                 0.03 
               
               
                   
                   
               
             
          
           
               
                 Time of mode 
                 10 
                 30 
                 45 
                 60 
                 90 
                 120 
                 180 
                 210 
                 240 
                 300 
                 600 
                 1200 
                 1800 
                 3600 
               
               
                 switching 
               
               
                 term (s) 
               
               
                 Condition of 
                 bad 
                 bad 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
               
               
                 occurrence of 
               
               
                 crack 
               
               
                 Condition of 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
               
               
                 occurrence of 
               
               
                 blackening 
               
               
                   
               
             
          
         
       
     
     Experimental Example 7 
     The discharge lamp (A) was it by supplying AC power (power in which one cycle of a lower frequency component in 90 Hz was inserted at every 37.5 cycle of a basic frequency component in 370 Hz) of 115 W (50% of the rated power value), the value of power to be supplied to the discharge lamp (A) was increased from 115 W to 184 W (80% of the rated power value) at an average power change ratio shown in Table 7 given below, then the light-emitting tube was visually observed to inspect the conditions of occurrence of cracks, and evaluated as “good” if no crack occurred in the light-emitting tube, and “bad” if the crack occurred in the light-emitting tube. 
     The operation described above was performed repeatedly by five times, and then the light-emitting tube of the discharge lamp (A) was observed to inspect the conditions of occurrence of blackening, and evaluated as “good” if occurrence of blackening was not recognized, and “bad” if the occurrence of blackening was recognized. 
     The results of the evaluation described above will be shown in Table 7 given below. 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 7 
               
             
             
               
                   
                   
               
               
                   
                 Power change ratio (W/s) 
               
             
          
           
               
                   
                 6.90 
                 2.30 
                 1.97 
                 1.15 
                 0.77 
                 0.58 
                 0.38 
                 0.33 
                 0.29 
                 0.23 
                 0.12 
                 0.06 
                 0.04 
                 0.02 
               
               
                   
                   
               
             
          
           
               
                 Time of mode 
                 10 
                 30 
                 45 
                 60 
                 90 
                 120 
                 180 
                 210 
                 240 
                 300 
                 600 
                 1200 
                 1800 
                 3600 
               
               
                 switching 
               
               
                 term (s) 
               
               
                 Condition of 
                 bad 
                 bad 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
               
               
                 occurrence of 
               
               
                 crack 
               
               
                 Condition of 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
               
               
                 occurrence of 
               
               
                 blackening 
               
               
                   
               
             
          
         
       
     
     Experimental Example 8 
     The discharge lamp (A) was lit by supplying AC power (power in which one cycle of a lower frequency component in 90 Hz was inserted at every 37.5 cycle of a basic frequency component in 370 Hz) of 138 W (60% of the rated power value), the value of power to be supplied to the discharge lamp (A) was increased from 138 W to 184 W (80% of the rated power value) at an average power change ratio shown in Table 8 given below, then the light-emitting tube was visually observed to inspect the conditions of occurrence of cracks, and evaluated as “good” if no crack occurred in the light-emitting tube, and “bad” if the crack occurred in the light-emitting tube. 
     The operation described above was performed repeatedly by five times, and then the light-emitting tube of the discharge lamp (A) was observed to inspect the conditions of occurrence of blackening, and evaluated as “good” if occurrence of blackening was not recognized, and “bad” if the occurrence of blackening was recognized. 
     The results of the evaluation described above will be shown in Table 8 given below. 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 8 
               
             
             
               
                   
                   
               
               
                   
                 Power change ratio (W/s) 
               
             
          
           
               
                   
                 4.60 
                 2.30 
                 1.53 
                 0.77 
                 0.51 
                 0.38 
                 0.26 
                 0.22 
                 0.19 
                 0.15 
                 0.08 
                 0.04 
                 0.03 
                 0.013 
               
               
                   
                   
               
             
          
           
               
                 Time of mode 
                 10 
                 30 
                 45 
                 60 
                 90 
                 120 
                 180 
                 210 
                 240 
                 300 
                 600 
                 1200 
                 1800 
                 3600 
               
               
                 switching 
               
               
                 term (s) 
               
               
                 Condition of 
                 bad 
                 bad 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
               
               
                 occurrence of 
               
               
                 crack 
               
               
                 Condition of 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
               
               
                 occurrence of 
               
               
                 blackening 
               
               
                   
               
             
          
         
       
     
     Experimental Example 9 
     The discharge lamp (A) was lit by supplying AC power (power in which one cycle of a lower frequency component in 90 Hz was inserted at every 37.5 cycle of a basic frequency component in 370 Hz) of 138 W (60% of the rated power value), the value of power to be supplied to the discharge lamp (A) was increased from 138 W to 161 W (70% of the rated power value) at an average power change ratio shown in Table 9 given below, then the light-emitting tube was visually observed to inspect the conditions of occurrence of cracks, and evaluated as “good” if no crack occurred in the light-emitting tube, and “bad” if the crack occurred in the light-emitting tube. 
     The operation described above was performed repeatedly by five times, and then the light-emitting tube of the discharge lamp (A) was observed to inspect the conditions of occurrence of blackening, and evaluated as “good” if occurrence of blackening was not recognized, and “bad” if the occurrence of blackening was recognized. 
     The results of the evaluation described above will be shown in Table 9 given below. 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 9 
               
             
             
               
                   
                   
               
               
                   
                 Power change ratio (W/s) 
               
             
          
           
               
                   
                 2.30 
                 1.53 
                 0.77 
                 0.38 
                 0.26 
                 0.19 
                 0.13 
                 0.10 
                 0.10 
                 0.08 
                 0.04 
                 0.02 
                 0.013 
                 0.006 
               
               
                   
                   
               
             
          
           
               
                 Time of mode 
                 10 
                 30 
                 45 
                 60 
                 90 
                 120 
                 180 
                 210 
                 240 
                 300 
                 600 
                 1200 
                 1800 
                 3600 
               
               
                 switching 
               
               
                 term (s) 
               
               
                 Condition of 
                 bad 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
               
               
                 occurrence of 
               
               
                 crack 
               
               
                 Condition of 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 bad 
               
               
                 occurrence of 
               
               
                 blackening 
               
               
                   
               
             
          
         
       
     
     Experimental Example 10 
     The discharge lamp (B) was it by supplying AC power (power in which one cycle of a lower frequency component in 90 Hz was inserted at every 37.5 cycle of a basic frequency component in 370 Hz) of 108 W (60% of the rated power value), the value of power to be supplied to the discharge lamp (B) was increased from 108 W to 180 W (the rated power value) at an average power change ratio shown in Table 10 given below, then the light-emitting tube was visually observed to inspect the conditions of occurrence of cracks, and evaluated as “good” if no crack occurred in the light-emitting tube, and “bad” if the crack occurred in the light-emitting tube. The result is shown in Table 10 given below. 
     The operation described above was performed repeatedly by five times, and then the light-emitting tube of the discharge lamp (B) was observed to inspect the conditions of occurrence of blackening, and evaluated as “good” if occurrence of blackening was not recognized, and “bad” if the occurrence of blackening was recognized. 
     The results of the evaluation described above will be shown in Table 10 given below. 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 10 
               
             
             
               
                   
                   
               
               
                   
                 Power change ratio (W/s) 
               
             
          
           
               
                   
                 7.20 
                 2.40 
                 2.06 
                 1.20 
                 0.80 
                 0.60 
                 0.40 
                 0.34 
                 0.30 
                 0.24 
                 0.12 
                 0.06 
                 0.04 
                 0.02 
               
               
                   
                   
               
             
          
           
               
                 Time of mode 
                 10 
                 30 
                 45 
                 60 
                 90 
                 120 
                 180 
                 210 
                 240 
                 300 
                 600 
                 1200 
                 1800 
                 3600 
               
               
                 switching 
               
               
                 term (s) 
               
               
                 Condition of 
                 bad 
                 bad 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
               
               
                 occurrence of 
               
               
                 crack 
               
               
                 Condition of 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
               
               
                 occurrence of 
               
               
                 blackening 
               
               
                   
               
             
          
         
       
     
     As is apparent from the results in Table 6 to Table 10, it was found that when the power to be supplied to the discharge lamp was increased under the condition that the average power change ratio became 0.01 to 2.1 W/s, the cracks did not occur in the light-emitting tube, and even when lowering of the power to be supplied to the discharge lamp under the condition described above was performed repeatedly, blackening of the light-emitting tube of the discharge lamp did not occur. 
     Examples of Second Embodiment 
     Experimental Example 1 
     The discharge lamp (A) was lit by supplying AC power (power in which one cycle of a lower frequency component in 90 Hz was inserted at every 37.5 cycle of a basic frequency component in 370 Hz) having a power of 75% of the rated power value (172.5 W), the power to be supplied to the discharge lamp (A) was lowered by 7% of the rated power (16.1 W) during four seconds (average power change ratio of 4.025 W/s), then the power change (the power change of lowering by 5% of the rated power (11.5 W) in the cycle of 5 seconds) to increase the power by 2% of the rated power (4.6 W) (average power change ratio of 4.6 W) in one second was repeated three times, whereby the power was lowered to 60% of the rated power value (132 W). A graph of change of the value of the power to be supplied to the discharge lamp in Experimental Example 1 is illustrated in  FIG. 13 . 
     When light radiated from the discharge lamp (A) was visually observed in a state in which the power to be supplied to the discharge lamp (A) is maintained at 60% of the rated power value (132 W), occurrence of the flicker was not recognized. 
     Comparative Experimental Example 1 
     When the discharge lamp (A) was lit by supplying AC power (power in which one cycle of a lower frequency component in 90 Hz was inserted at every 37.5 cycle of a basic frequency component in 370 Hz) having a power of 75% of the rated power value (172.5 W), and the power to be supplied to the discharge lamp (A) was lowered by 15% of the rated power (34.5 W) during 30 seconds (average power change ratio of 2.3 W/s), whereby the power value was lowered to 60% of the rated power value (132 W), and when the light radiated from the discharge lamp (A) was observed in this state, occurrence of the flicker was recognized. 
     Examples of Third Embodiment 
     Experimental Example 1 
     When the discharge lamp (A) was lit for 200 hours by supplying a basic AC power and a superimposed power to the discharge lamp (A) under the condition given below, then when light radiated from the discharge lamp (A) after having lit the discharge lamp (A) for 200 hours was visually observed, occurrence of flicker was not recognized, and when the electrodes of the discharge lamp (A) were observed, no abnormality was recognized.
     Basic AC Power   Current Frequency: 740 Hz   Power Value: 138 W (60% of the rated power value)   Superimposed Power   Current Frequency: 518 Hz   Superimposed Power Value: 35 W (sum of the basic AC power and the superimposed power was 173 W)   Supply Time: 2 minutes   Time Interval:   

     When the measured value of the lighting voltage when the superimposed power was supplied was equal to or lower than 80 V; 15 minutes 
     When the measured value of the lighting voltage when the superimposed power was supplied exceeded 80 V; 5 minutes 
     Experimental Example 2 
     When the discharge lamp (A) was lit for 200 hours by supplying a basic AC power and a superimposed power to the discharge lamp (A) under the condition given below, then when light radiated from the discharge lamp (A) after having lit the discharge lamp (A) for 200 hours was visually observed, occurrence of flicker was not recognized, and when the electrodes of the discharge lamp (A) were observed, no abnormality was recognized.
     Basic AC Power   Current Frequency: 740 Hz   Power Value: 138 W (60% of the rated power value)   Superimposed Power   Current Frequency: 518 Hz   Superimposed Power Value: 35 W (sum of the basic AC power and the superimposed power was 173 W)   Supply Time: 2 minutes   Time Interval:   

     15 minutes when the measured value of the lighting voltage when the superimposed power was supplied was lower than 80 V; 
     10 minutes when the measured value of the lighting voltage when the superimposed power was supplied was equal to or larger than 80 V and lower than 100 V; 
     15 minutes when the measured value of the lighting voltage when the superimposed power was supplied was equal to or higher than 100 V; 
     Experimental Example 3 
     When the discharge lamp (A) was lit for 200 hours by supplying the basic AC power and the superimposed power to the discharge lamp (A) under the condition given below, then when light radiated from the discharge lamp (A) after having lit the discharge lamp (A) for 200 hours was visually observed, occurrence of flicker was not recognized, and when the electrodes of the discharge lamp (A) were observed, no abnormality was recognized.
     Basic AC Power   Current Frequency: 740 Hz   Power Value: 138 W (60% of the rated power value)   Superimposed Power   Current Frequency: 518 Hz   Superimposed Power Value: 35 W (sum of the basic AC power and the superimposed power was 173 W)   Time Interval:   

     5 minutes when the measured value of the lighting voltage when the superimposed power was not supplied was lower than 75 V; 
     15 minutes when the measured value of the lighting voltage when the superimposed power was not supplied was equal to or higher than 75 V; 
     Supply Time: 2 minutes 
     Experimental Example 4 
     When the discharge lamp (A) was lit for 200 hours by supplying the basic AC power and the superimposed power to the discharge lamp (A) under the condition given below, then when light radiated from the discharge lamp (A) after having lit the discharge lamp (A) for 200 hours was visually observed, occurrence of flicker was not recognized, and when the electrodes of the discharge lamp (A) were observed, no abnormality was recognized.
     Basic AC Power   Current Frequency: 740 Hz   Power Value: 138 W (60% of the rated power value)   Superimposed Power   Current Frequency: 518 Hz   Superimposed Power Value: 35 W (sum of the basic AC power and the superimposed power was 173 W)   Supply Time:   

     2 minutes when the measured value of the lighting voltage when the superimposed power was not supplied was lower than 70 V; 
     4 minutes when the measured value of the lighting voltage when the superimposed power was not supplied was equal to or higher than 70 V; 
     Time Interval: 15 minutes 
     Experimental Example 5 
     When the discharge lamp (A) was lit for 200 hours by supplying a basic AC power and a superimposed power to the discharge lamp (A) under the condition given below, then when light radiated from the discharge lamp (A) after having lit the discharge lamp (A) for 200 hours was visually observed, occurrence of flicker was not recognized, and when the electrodes of the discharge lamp (A) were observed, no abnormality was recognized.
     Basic AC Power   Current Frequency: 740 Hz   Power Value: 138 W (60% of the rated power value)   Superimposed Power   Current Frequency: 518 Hz   Superimposed Power Value:   

     When the measured value of the lighting voltage when the superimposed power was not supplied was lower than 70 V; 35 V (the sum of the basic AC power and the superimposed power was 173 W) 
     When the measured value of the lighting voltage when the superimposed power was not supplied exceeded 70 V; 92 V (the sum of the basic AC power and the superimposed power was 230 W)
     Time Interval: 15 minutes   Supply Time: 2 minutes   

     Experimental Example 6 
     When the discharge lamp (A) was lit for 200 hours by supplying the basic AC power and the superimposed power to the discharge lamp (A) under the condition given below, then when light radiated from the discharge lamp (A) after having lit the discharge lamp (A) for 200 hours was visually observed, occurrence of flicker was not recognized, and when the electrodes of the discharge lamp (A) were observed, no abnormality was recognized.
     Basic AC Power   Current Frequency: 740 Hz   Power Value: 138 W (60% of the rated power value)   Superimposed Power   Current Frequency: 518 Hz   Superimposed Power Value:   

     When the measured value of the lighting voltage when the superimposed power was not supplied was lower than 70 V; 35 V (the sum of the basic AC power and the superimposed power was 173 W) 
     When the measured value of the lighting voltage when the superimposed power was not supplied exceeded 70 V; 92 V (the sum of the basic AC power and the superimposed power was 230 W)
     Time Interval: 15 minutes   Supply Time: 2 minutes   

     When the measured value of the lighting voltage when the superimposed power was not supplied was equal to or lower than 70 V; 3 minutes 
     When the measured value of the lighting voltage when the superimposed power was not supplied exceeded 70 V; 4 minutes 
     Experimental Example 7 
     When the discharge lamp (A) was lit for 200 hours by supplying a basic AC power and a superimposed power to the discharge lamp (A) under the condition given below, then when light radiated from the discharge lamp (A) after having lit the discharge lamp (A) for 200 hours was visually observed, occurrence of flicker was not recognized, and when the electrodes of the discharge lamp (A) were observed, no abnormality was recognized.
     Basic AC Power   Current Frequency: 740 Hz   Power Value: 138 W (60% of the rated power value)   Superimposed Power   Current Frequency: 518 Hz   Superimposed Power Value: 35 V   Time Interval: 5 minutes at the shortest   Supply Time:   

     When the measured value of the lighting voltage when the superimposed power was not supplied was equal to or lower than 70 V; 0 minute 
     When the measured value of the lighting voltage when the superimposed power was not supplied exceeded 70 V; 2 minutes 
     Comparative Experimental Example 1 
     When light radiated from the discharge lamp (A) was visually observed after having lit the discharge lamp (A) for 200 hours by supplying AC power having a power value of 138 W (60% of the rated power value) in a current frequency of 740 Hz to the discharge lamp (A), occurrence of the flicker was recognized. Also, when the electrodes of the discharge lamp (A) were observed, wearing of the distal ends was recognized.