fig1 shows an automobile front beam issued by a front headlamp of a vehicle . for an automobile , the region in which the perceptible and annoying forefront flicker originates is generally up to about 8 metres in front of the vehicle and in the beam region up to about 4 ° below a horizontal plane of the headlamp . fig2 shows a simplified schematic representation of an arc - discharge lamp 1 with a discharge - arc 2 extending between two electrodes 10 . in normal operation , as indicated in the upper part of the diagram , owing to an upward convection in the burner , the discharge arc extends as shown between the two electrodes 10 . when the lamp 1 is subject to an abrupt downward displacement , shown in the centre part of the diagram and indicated by the downward arrow , the discharge arc 2 is briefly ‘ shortened ’ as shown . this shorter discharge - arc is associated with a decrease in lamp voltage , and therefore also with a decrease in luminous flux . similarly , when the lamp 1 is subject to an abrupt upward displacement , shown in the lower part of the diagram and indicated by the upward arrow , the discharge arc 2 is briefly ‘ stretched ’ as shown . this longer discharge - arc is associated with an increase in lamp voltage , and with a corresponding increase in luminous flux . in the diagram , only the effects of an up / down displacement of the lamp are shown . evidently , the lamp could be subject to a mechanical impact resulting in a lateral displacement of the lamp . in that case , the discharge - arc would also be laterally briefly displaced and correspondingly lengthened or stretched . fig3 shows a simplified block diagram of a prior art lamp driver 30 , comprising a converter 5 for converting an input supply signal ( for example from a car battery ) to a level suitable for a lamp 1 , a commutation unit 6 ( generally comprising a h - bridge for commutating the lamp current and an igniter for igniting the lamp ). the driver 30 also comprises a voltage measurement unit 20 for monitoring the lamp voltage . the voltage measurement unit 20 forwards the lamp voltage values 21 to a power correction unit 8 , which interprets the lamp voltage values 21 to determine any required correction to the lamp current in order to maintain a constant lamp power . fig4 shows a plot of experimentally obtained light modulation values ( x - axis , in percent ) against lamp modulation values ( z - axis , in percent ) and phase shift ( y - axis , in degrees ) for a lamp driven using a prior art lamp driver such as that described in fig2 above . as the graph shows , a lamp voltage modulation or fluctuation of about 1 . 0 - 1 . 5 % results in a light output fluctuation of between 0 . 5 % and 0 . 75 %. the interesting aspect of this plot is that the fluctuation in light output clearly exhibits a distinct phase shift relative to the fluctuation or modulation in lamp voltage . an alteration in lamp voltage causes a corresponding increase or decrease in light output , but this is delayed relative to the lamp voltage alteration . this relationship is the basis for determining the power correction in the method according to the invention . using this information , measured lamp voltage values ( in the embodiment using a voltage measurement means ) can be used directly to determine the phase shift required for the lamp power correction . measured light output values ( in the embodiment using a light sensor ) can also be used to directly obtain the required phase shift . similar experimental results can be obtained for acceleration values correlated with lamp voltage fluctuations , so that measured values of acceleration ( in the embodiment using an accelerometer ) can be used to easily derive the required phase shift . the information thus gathered experimentally can be provided to the lamp driver in a suitable form , for example as a look - up table or as a simple algorithm for using the measured values to derive the required phase shift and amplitude for the power correction . fig5 shows a block diagram of a lamp driver 3 according to an embodiment of the invention . again , the lamp driver 3 comprises a converter 5 and a commutation unit 6 . this lamp driver 3 also comprises a voltage measurement means 40 for obtaining lamp voltage values 41 . these are analysed in a lamp voltage modulation detector 41 , which can be a simple envelope detector known to the skilled person , to provide a lamp voltage envelope 43 to an analysis unit 44 , which analyses the lamp voltage envelope 43 to determine a required phase shift and amplitude correction for the lamp power and to generate an appropriate power correction signal 45 for the power correction unit 8 . as mentioned already , the analysis unit 42 can utilise a lut or an algorithm for deriving the phase / amplitude correction on the basis of the relationship described in fig4 above . the lamp driver 3 , in addition to or as an alternative to the lamp voltage analysis , can analyse the light output of the lamp 1 . in such a realisation , the lamp driver 3 comprises a light modulation detector 52 for processing measured lamp light values 51 delivered by a light sensor 50 , which can be placed close to the light source 1 or in the base of a lighting assembly or in any other suitable position . the light modulation detector 52 determines whether any fluctuation in light output is characteristic of a mechanically induced impact , and delivers appropriate power correction signals 53 to the power correction unit 8 . in addition to or as an alternative to the analysis approaches described above , the lamp driver 3 can analyse a proper acceleration of the lamp 1 . in such a realisation , the lamp driver 3 comprises a lamp vibration determination module 62 for processing measured lamp acceleration values 61 delivered by an accelerometer 60 . the lamp vibration determination module 62 determines a frequency of fluctuation in light output as a result of a sudden acceleration of the lamp , and delivers appropriate information 63 to an amplitude and phase adaptation unit 64 , which can use information stored in a lut , for example , to determine a suitable phase shift and amplitude correction for the lamp power . the amplitude and phase adaptation unit 64 accordingly generates an appropriate power correction signal 65 for the power correction unit 8 . in the above description for fig5 , for the sake of simplicity , the lamp driver 3 is shown to include several analysis means . evidently , the lamp driver 3 can be realised to perform only lamp voltage analysis , only light output analysis , only acceleration analysis , or any combination of these techniques . the data processing steps such as lamp voltage analysis , light output analysis , acceleration analysis , phase shift and amplitude correction , etc ., can be carried out by suitable software algorithms running on a microprocessor or microcontroller of the lamp driver 3 . fig6 shows simplified graphs of modulated lamp voltage u , lamp power p , p c and modulated light output l , l c for a lamp driven using a prior art method and a lamp driven using the method according to the invention . in the upper part of the graph , the light output l for a lamp driven using a prior art method is shown . after a mechanical impact , the discharge arc is disturbed and causes the lamp voltage to fluctuate . values of lamp voltage measured at certain points during the lamp period show a fluctuation that can be graphed as the modulated lamp voltage u shown . the prior art lamp driver attempts to maintain a constant power p . as a result , the light output of the lamp fluctuates , and the modulated light output l is shown to follow the modulated lamp voltage u by a time delay or phase shift . when such a lamp is driven by the method according to the invention , the modulated lamp voltage u is analysed to determine a lamp power correction . by applying the lamp power correction to take into account the phase shift φ and an amplitude α , the corrected lamp power p c rapidly leads to a settling of the light output l c . in this way , a mechanically induced impact or a sudden change in velocity that causes the discharge arc to be disturbed will not be followed by a perceptible flicker in the forefront of the vehicle . any flicker in the light output is suppressed so quickly that it may not be apparent to an observer . fig7 shows box - plots of light modulation for a lamp driven by a prior art method ( upper part of diagram ) and for a lamp driven by the method according to the invention ( lower part of diagram ). an abrupt mechanically induced impact will alter the length of the discharge arc , leading to a fluctuation of the lamp voltage . the frequency components of the fluctuation will depend on the detailed ‘ shape ’ of the impact , since an impact or impulse can be expressed as the sum of its fourier components . in the tests carried out , impacts to a headlamp were simulated by subjecting the headlamp to sinusoidal vibrations at different frequencies and a fixed amplitude . depending on the actual nature of the impact , the various frequency components contribute to varying degrees to the voltage and light modulation . for the lamp driven using a prior art method which attempts to maintain a constant current , it can be seen that the higher the frequency , the higher will be the modulation of the light flux . at a frequency of about 5 hz , the light output is already modulated by over 0 . 4 %. at frequencies around 25 hz , however , the light modulation increases to about 1 . 2 %. the degree of modulation within this range of frequencies ( indicated by the broken lines ), with the associated perceptible flicker in the forefront of a vehicle , is easily perceptible to an observer and can be annoying and distracting , and therefore a safety hazard . in contrast , the lamp driven using the method according to the invention can suppress the light output fluctuations to a level below which the flicker is essentially not perceptible . at low frequencies around 5 hz , the fluctuation in light output is very favourably suppressed to about 0 . 2 %. even at higher frequencies around 20 hz , the fluctuation in light output seldom exceeds 0 . 5 %. for a noticeable forefront flicker to arise , the impact to the lamp would have to be considerably stronger , for example two to three times stronger in the 20 - 25 hz range . this demonstrates that the method according to the invention is favourably effective in suppressing forefront flicker , especially in the indicated frequency range 5 - 25 hz to which the human eye is particularly sensitive . the experiments carried out to collect the data showed that the method according to the invention for correcting the lamp power was still effective even for an advanced lamp age in the region of 2000 hours of operation . fig8 shows a schematic representation of a lighting assembly 9 according to the invention . here , a lamp 1 is mounted on a lamp base 90 , in a reflector 91 and behind a projection lens 92 . circuitry for the lamp driver 3 can be incorporated in the base 90 . the lighting assembly can include a light sensor 50 located in front of the lamp 1 or in the lamp base 90 ( both positions are shown for clarity , but a single light sensor 50 is sufficient ). in addition to or as an alternative to the light sensor 50 , the light assembly can include an accelerometer 60 located at a suitable position for detecting an acceleration of the lamp 1 . although the present invention has been disclosed in the form of preferred embodiments and variations thereon , it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention . for example , the driver may include an additional monitoring unit to track the lamp lifetime and make minor adjustments to the lamp power correction algorithm ( s ) used by the driver so that a lamp aging can be taken into account when correcting the lamp power . for the sake of clarity , it is to be understood that the use of “ a ” or “ an ” throughout this application does not exclude a plurality , and “ comprising ” does not exclude other steps or elements . a unit or module can comprise other units or modules .