APPARATUS FOR DRIVING A GAS DISCHARGE LAMP
A method of generating a measuring signal indicating arc straightness in a gas discharge lamp (L) comprises the following steps: in a first step, applying a first lamp current (IN) to the lamp; in a second step, adding a brief pulse current (Ip) to the first lamp current (IN), allowing the lamp voltage to regain a steady state, and measuring the resulting average value of the lamp voltage (V 2 ) in this steady state; generating a measuring signal indicating arc straightness on the basis of said average lamp voltage (V 2 ) measured in the second step.
1 . Method of generating a measuring signal indicating arc straightness in a gas discharge lamp (L), the method comprising the following steps:
in a first step, applying a first lamp current (I N ) to the lamp;
in a second step, adding a brief pulse current (I P ) to the first lamp current (I N ), allowing the lamp voltage to regain a steady state, and measuring the resulting average value of the lamp voltage (V 2 ) in this steady state;
generating a measuring signal indicating arc straightness on the basis of said average lamp voltage (V 2 ) measured in the second step.
2 . Method according to claim 1 , wherein in the first step the resulting average value of the lamp voltage (V 1 ) is measured, wherein the ratio (R=V 1 /V 2 ) between the average lamp voltage (V 1 ) measured in the first step and the average lamp voltage (V 2 ) measured in the second step is calculated, and wherein the measuring signal indicating arc straightness is calculated on the basis of said ratio.
3 . Method of operating a gas discharge lamp (L), the method comprising the steps of:
applying lamp current (I N ) to the lamp while performing an arc straightening measure with at least one variable parameter;
setting a first value (X 1 ) for said variable parameter, and measuring arc straightness for this first value (X 1 );
setting a second value (X 2 ) for said variable parameter, and measuring arc straightness for this second value (X 2 ), using the method of claim 1 ;
calculating an optimum setting for said variable parameter on the basis of the measurement results thus obtained.
4 . Method according to claim 3 , wherein the lamp current (I N ) includes a constant current.
5 . Method according to claim 3 , wherein the lamp current (I N ) includes a low-frequency square wave current (I M ), and wherein the brief pulse current (I P ) is applied while the current direction remains constant.
6 . Method according to claim 5 , wherein the arc straightening measure includes applying to the lamp a high-frequency ripple component (I R ) superimposed on the low-frequency square wave current.
7 . Method according to claim 3 , wherein the lamp current includes a low-frequency square wave current, wherein the arc straightening measure includes applying to the lamp a high-frequency current alternating with the low-frequency square wave current, and wherein the brief pulse current (I P ) is applied, coinciding with the low-frequency square wave current, during a portion of time while the current direction remains constant.
8 . Method according to claim 3 , wherein the lamp current includes a low-frequency square wave current, wherein the arc straightening measure includes subjecting the arc to an external magnetic field and varying the duty cycle of the low-frequency square wave current, and wherein the brief pulse current (I P ) is applied a portion of time while the current direction remains constant.
9 . Lamp driver ( 10 ) for driving a gas discharge lamp (L), particularly a Xenon lamp, the driver comprising:
output terminals ( 7 , 8 ) for connecting to lamp electrodes of the lamp (L);
controllable current generating means ( 1 , 2 , 3 ) capable of providing at the output ( 7 , 8 ) a normal lamp current (I N ) and a switchable pulse current (I P ) component;
controllable means for straightening the arc;
a control device ( 5 ) for controlling the current generating means ( 1 , 2 , 3 ) and for controlling the arc straightening means such as to vary one or more parameters of the arc straightening means and such as to switch the pulse current (I P ) component ON (t 1 ) and OFF (t 2 );
a voltage sensor ( 4 ) coupled to the output terminals ( 7 , 8 ) for measuring lamp voltage and providing a lamp voltage measuring signal (S v ) to the control device ( 5 );
wherein the control device ( 5 ) is designed:
to drive the lamp in at least two different parameter settings of at least one parameter of the arc straightening means,
in each parameter setting, to generate a measuring signal (S V ) indicating arc straightness in the lamp (L), using the method of claim 1 ,
to select an optimum setting of the at least one parameter on the basis of the measuring results.
10 . Driver according to claim 9 , wherein the control device ( 5 ) is designed to perform the lamp operating method.