Control method and ballast for run-up of metal halide lamp

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, 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 ; and

supplying P req to operate the lamp when I req <I lim .

2. The method of claim 1 , further comprising the step of, during the run-up, continuously determining a light output L of the lamp, and wherein P req is a function of L.

3. The method of claim 2 , wherein

P req =P*L n /L , and

I req =I*P req /P.

4. The method of claim 1 , further comprising the step of determining a nominal voltage V n for the lamp during steady state operation, and wherein P req is a function of V and V n .

5. The method of claim 4 , further comprising the step of determining a control coefficient C and a nominal power P n for the lamp during steady state operation, and wherein

P req =P n *(1 +C *( V n −V )/ V n ), and

I req =I*P req /P.

6. The method of claim 1 , further comprising the steps of determining warm-up energy E w delivered to the lamp when the lamp reaches the nominal light output, and determining, during the run-up, energy E delivered to the lamp, and wherein P req is a function of E and E w .

7. The method of claim 6 , further comprising the steps of determining a power decay coefficient τ for the lamp, and, during the run-up,

if E<E w , then I req =I lim , and

if E≧E w , then P req =P n +( P−P n )*exp(−Δ t /τ), and I req =I*P req /P

wherein Δt is the time elapsed per feedback loop control.

8. 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.

9. 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, determining E and estimating η from the function, and wherein

P req =P n /η, and

I req =I*P req /P.

10. The method of claim 9 , wherein the function is stored in a table in a memory in a ballast that operates the lamp.

11. The method of claim 9 , 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).

12. The method of claim 1 , further comprising the steps of,

determining values for normalized lamp efficacy η and energy E delivered to the lamp when a derivative of lamp voltage V′(E) is at a peak during the run-up,

assigning a value for E 0 ,

determining a value of E 1 from η=1−exp(−(E−E 0 )/E 1 ),

estimating characteristics of the lamp from the determined value of E 1 .

13. 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, 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 ; and

supplying P req to operate the lamp when I req <I lim .