Lighting system

1. A lighting system including:

a metal halide lamp having an arc tube housed within an outer tube; and

a lamp lighting device for operating the metal halide lamp,

the lamp lighting device comprising:

a power supply unit operable to supply an electric power to the metal halide lamp;

a detecting unit operable to detect an electrical property of the metal halide lamp;

a judging unit operable to judge, based on the detected electrical property, whether a currently occurring discharge is in either of (i) an outer-tube discharge state in which a discharge occurs inside the outer tube but outside the arc tube and (ii) a state preceding the outer-tube discharge state; and

an instructing unit operable to instruct the power supply unit to suspend or reduce the electric power supply to the metal halide lamp upon a judgment by the judging unit that the currently occurring discharge is in either of the outer-tube discharge state and the preceding state.

2. The lighting system according to claim 1 , wherein

the judging unit is operable to judge that the currently occurring discharge is in either of the outer-tube discharge state and the preceding state, if the detected electrical property differs at least from an electrical property of the metal halide lamp under normal operation.

3. The lighting system according to claim 1 , wherein

the judging unit is operable to judge that the currently occurring discharge is in either of the outer-tube discharge state and the preceding state, based on a characteristic specific to a discharge in the outer-tube discharge state or the preceding state.

4. The lighting system according to claim 3 , wherein

the electrical property detected by the detecting unit is at least either of a lamp current and a lamp voltage.

5. The lighting system according to claim 3 , wherein

the specific characteristic is that the discharge repeats fading-out and re-starting, and

the judging unit is operable to count, based on the detected electrical property, occurrences of fading-out or re-starting, and to judge that the currently occurring discharge is in either of the outer-tube discharge state and the preceding state if the count reaches a predetermined count.

6. The lighting system according to claim 3 , wherein

the specific characteristic is that the electrical property fluctuates widely, and

the judging unit is operable to count fluctuations of the detected electrical property beyond a reference level that is determined based on an electrical property of the metal halide lamp under normal operation, and to-judge that the currently occurring discharge is in the outer-tube discharge state if the count reaches a predetermined count.

7. The lighting system according to claim 3 , wherein

the power supply unit is operable to supply an alternating current power,

the specific characteristic is that a half-wave discharge occurs, and

the judging unit is operable to judge that the currently occurring discharge is in the outer-tube discharge state if the detecting unit repeatedly detects an electrical property specific to a half-wave discharge.

8. The lighting system according to claim 7 , wherein

the judging unit is operable to judge that an electrical property specific to a half-wave discharge is repeatedly detected, if a duration or occurrences of the half-wave discharge reaches a predetermined value.

9. The lighting system according to claim 3 , wherein

the specific characteristic is that an outer-tube discharge stably continues, and

the judging unit is operable to count a number of times that the detected electrical property exceeds a predetermined upper limit or that the detected electrical property is below a predetermined lower limit, and to judge that the currently occurring discharge is in the outer-tube discharge state if the count reaches- a predetermined count.

10. The lighting system according to claim 3 , wherein.

the specific characteristic is that an outer-tube discharge stably continues, and

the judging unit is operable-to measure a duration that the detected electrical property exceeds a predetermined upper limit or that the detected electrical property is below a predetermined lower limit, and to judge that the currently occurring discharge is in the outer-tube discharge state if the measured duration reaches a predetermined duration.

11. The lighting system according to claim 3 , wherein

the power supply unit is operable to supply an alternating current power that is cyclically reversed between positive and negative polarities,

the detecting unit is operable to detect the electrical property for the respective polarities, and

the judging unit is operable to make a judgment based on the electrical property for the respective polarities.

12. The lighting system according to claim 3 , wherein

the power supply unit is operable to supply an alternating current power that is cyclically reversed between positive and negative polarities,

the specific characteristic is that the electrical property abruptly changes immediately after polarity reversal,

the detecting unit is operable to detect the electrical property immediately after polarity reversal, and

the judging unit is operable to judge that the currently occurring discharge is in the outer-tube discharge state, if the electrical property abruptly changes.

13. The lighting system according to claim 12 , wherein

the judging unit is operable to judge that the electrical property changes abruptly, if the electrical property detected a predetermined time period after polarity reversal is equal to or greater than a predetermined value.

14. The lighting system according to claim 12 , wherein

the judging unit is operable to judge that the electrical property abruptly changes, if an average or effective value calculated from the electrical property detected a predetermined time period after polarity reversal is equal to or greater than a predetermined value.

15. The lighting system according to claim 12 , wherein

the judging unit is operable to judge that the currently occurring discharge is in the outer-tube discharge state, if a count of occurrences of abrupt changes immediately after polarity reversal reaches a predetermined count.

16. The lighting system according to claim 1 , wherein.

the lamp lighting device provides, for a predetermined time period from start-up of the metal halide lamp, a mask period during which the judgment unit is inhibited from judging whether the currently occurring discharge is in either of the outer-tube discharge state and the preceding state.

17. The lighting system according to claim 1 , wherein

the outer tube is evacuated to vacuum.

18. The lighting system according to claim 1 , wherein

the outer tube is filled with nitrogen gas.

19. The lighting system according to claim 1 , wherein

the arc tube is filled with xenon gas as buffer gas.

20. The lighting system according to claim 1 , wherein

the arc tube is filled with metal halides which at least include praseodymium halide and sodium halide.

21. The lighting system according to claim 1 , wherein

the arc tube is filled with metal halides which at least include cerium halide and sodium halide.

22. The lighting system according to claim 1 , wherein

the arc tube includes a pair of electrodes, and

L/D≧4 is satisfied, where D denotes an inner diameter of the arc tube in millimeters and L denotes a distance between the electrodes in millimeters.