Fast run-up of metal halide lamp by power modulation at acoustic resonance frequency

1. A method of controlling run-up of a metal halide lamp that has a nominal light output L n during steady state operation and that has a current limit I lim , the method comprising the steps of:

during run-up of the metal halide lamp to steady state operation, evaluating requested power P req and requested current I req to operate the lamp at the nominal light output L n during the run-up;

during the run-up, supplying I lim to operate the lamp so long as I req ≧I lim and supplying P req to operate the lamp when I req <I lim ; and

during the run-up, modulating power P supplied to the lamp.

2. The method of claim 1 , wherein the step of modulating power includes sweeping a sine wave ripple on top of an input voltage waveform, and wherein a frequency range of the sine wave ripple includes an acoustic resonance frequency of the lamp.

3. The method of claim 1 , further comprising the step of, during the run-up, estimating lamp efficacy, and wherein P req is a function of lamp efficacy.

4. The method of claim 1 , wherein the step of modulating the power includes power modulation at an acoustic resonance frequency of the lamp.

5. The method of claim 4 , wherein the acoustic resonance frequency is a frequency of a first azimuthal resonance mode of the lamp.

6. The method of claim 1 , further comprising the steps of determining a function of normalized lamp efficacy η vs. energy E delivered to the lamp, and, during the run-up, sensing lamp current I and determining E and estimating η from the function, and wherein

P req =P n /η, and

I req =I*P req /P

where P n is a nominal power for the lamp during steady state operation.

7. The method of claim 6 , wherein the function is stored in a table in a memory in a ballast that operates the lamp.

8. The method of claim 6 , wherein the function is

for E≧E 0 , η=1−exp(−( E−E 0 )/ E 1 ), and

for E<E 0 , η=0,

where E 0 and E 1 are constants which describe η(E).

9. A method of controlling run-up of a metal halide lamp that has a nominal light output L n during steady state operation and that has a current limit I lim , the method comprising the steps of:

during run-up of the metal halide lamp to steady state operation, continuously sensing lamp current I and voltage V and calculating power P, and continuously evaluating requested power P req and requested current I req to operate the lamp at the nominal light output L n during the run-up;

supplying I lim to operate the lamp so long as I req ≧I lim ;

supplying P req to operate the lamp when I req <I lim ; and

during the run-up, modulating power P supplied to the lamp at an acoustic resonance frequency of the lamp.

10. The method of claim 9 , wherein the acoustic resonance frequency is a frequency of a first azimuthal resonance mode of the lamp.

11. The method of claim 9 , further comprising the step of, during the run-up, estimating lamp efficacy, and wherein P req is a function of lamp efficacy.

12. The method of claim 9 , further comprising the steps of determining a function of normalized lamp efficacy η vs. energy E delivered to the lamp, and, during the run-up, determining E and estimating η from the function, and wherein

P req =P n /η, and

I req =I*P req /P

where P n is a nominal power for the lamp during steady state operation.

13. The method of claim 12 , wherein the function is stored in a table in a memory in ballast that operates the lamp.

14. The method of claim 12 , wherein the function is

for E≧E 0 , η=1−exp(−( E−E 0 )/ E 1 ), and

for E<E 0 , η=0,

where E 0 and E 1 are constants which describe η(E).

15. A ballast for a metal halide lamp that has a nominal light output L n during steady state operation and that has a current limit I lim , said ballast embodying a program that causes the ballast to control run-up of the metal halide lamp to steady state operation, the program causing the ballast to perform the steps of:

during run-up of the metal halide lamp to steady state operation, evaluating requested power P req and requested current I req to operate the lamp at the nominal light output L n during the run-up;

during the run-up, supplying I lim to operate the lamp so long as I req ≧I lim and supplying P req to operate the lamp when I req <I lim ; and

during the run-up, modulating power P supplied to the lamp.

16. The ballast of claim 15 , wherein the step of modulating power includes sweeping a sine wave ripple on top of an input voltage waveform, and wherein a frequency range of the sine wave ripple includes an acoustic resonance frequency of the lamp.

17. The ballast of claim 15 , wherein the step of modulating the power includes power modulation at an acoustic resonance frequency of the lamp.

18. The ballast of claim 17 , wherein the acoustic resonance frequency is a frequency of a first azimuthal resonance mode of the lamp.