Hi, I have a question.
I have several Philips Master CityWhite CDO-ET+ lamps, which I burn on a set of Philips SON ballast and matching Philips ignitor, but with all my low wattage CityWhites I notice a vibration or very fast flickering especially in the corner of my eye. I don't have that problem with any other lamps, nor with higher wattage CityWhite lamps, only the 50 and 70 watt CityWhites. Does anyone know why this is and how I can resolve this? Would it make a diffence if I add a capacitor in the set-up?
Thanks in advance.

The flicker is most likely the arc dancing on the electrode.
Could be the cathode has a dark spot, so the light center moves some mm as the polarity is reversing.
Generally there is not much you can do, definitely no capacitor would solve anything (it is an AC circuit, so the current has to return to zero, swap the polarity and reignite the arc twice per mains cycle).
Maybe an electronic ballast will bring an improvement.

Larger lamps generally tend to yield more stable, less temperamental arc, that is just the effect of scale.

thanks a bunch.

Is this what you descibe typical for metal-halide lamps? Because if I burn a SON lamp on the same set it doesn't flicker at all.

thanks a bunch.

Is this what you descibe typical for metal-halide lamps? Because if I burn a SON lamp on the same set it doesn't flicker at all.

There are two main differences that may play a role here, both are related to the arc temperature:
MH has a very hot arc, that needs to be kept in a free space without touching the arctube wall, in order to first prevent the tube from being destroyed and second to prevent excessive energy loss from the arc. The HPS has way colder running arc, whose temperature is within the limits of the material, so may occassionally touch it, so use rather thin tube. The related energy loss isn't rhat extreme because of the lower temperature difference.

The free space around the MH arc compare to the tight constriction of the HPS arc, gives the MH way more "freedom" to wiggle around. So it does, when the position stabilizing forces are not strong enough. And this wiggling, projected by the optics into the beam, causes the flicker. The signature of this flicker is frequency below (usually some wole number fraction of) the mains frequency.
With HPS the thin, tight arctube is constricting the arc so it has no room to wiggle at all.

The dark cathode region uses to be the same size, but because the HPS has a rather long arc, it is proportionally smaller, so affects less of the light. The typical 50W HPS has an arc length of about 20..30mm, about the length of a 250..400W MH...

OK, I get it, thanks

I have one more question; is it possible to reduce the flicker by using an electronis ballast?
Could that make a difference because of the lightly higher frequensy (50-60 hz)?

Yes, assuming the ballast is of the decent square wave low frequency AC type (all MH rated except some cheepeese creations; only some HPS may tolerate the simpler HF AC ballasts, because of the colder running arc, making the arctube to tolerate even the hotter spots, and the arctube length and thinness ensuring sufficient damping of the eventual acoustic resonances there). These ballasts are essentially "class D" amplifiers controlled so they deliver constant current of a rectangular shape to the arc.

The square wave feed has at least three benefits related to flicker:
First the zero cross and reignition happens very quickly before the current "departed" from the full current of the last halfwave. That means there is no time for the arc stream ionization to decay, so the preference for the arc to reignite in the same position is very strong, "discouraging" the arc from wandering around.
Second the square wave yield magnetic field forces and the power dissipation constant over the full period, so no vibrations from either the magnetic forces, nor thermal expansion/contractions, which may create pressure waves, reflecting back from the walls and disturbing the arc back.
Third the frequency uses to be in the 200..400Hz range, so even if some rectification effects cause the arc to burn on different position for each polarity, the resulting light pattern modulation is of so high frequency it isn't perceived as a flicker anymore.

To be clear the HF operation would be even better in these three aspects, but usual MH's do not tolerate it.

Thanks so much!