On my Thorn arrowslim 30w, when I stick a 30w halophosphate 535 in it, it appears to not flicker at all. When I stick a crompton 840 in, it strobes an awful lot more. Even my camera sees this. Why could this be. I have also seen it in some other fixtures.

Maybe phosphor afterglow that smooths out the flicker ?

The more visible flicker on the camera may have other reason: Brighter lamp cause the camera to switch to shorter exposure time, what is then short enough to cause the beat effect (with exposure time longer then the flicker frequency the sensor average the flicker out, so it disapear from the acquired video/picture)

@Medved: "MetalHalideHater" say that the flickering really more prominent with the Crompton 840 lamp than the halophosphate 535 lamp. Not just on the camera.

phosphor persistance.longer persistance less flicker on 50/60hz stuff.

Another option, but guessing here : different filling --> different arc voltage --> different duty cycle when working on sine AC

Thanks for your replies. I will post a pic of the halophosphate etch and try to find out what fill it has.

The gas filling that i knows about in fluorescent lamps is the following:
1. Argon: 4-13W T5s, T8s of 15W, 30W, 50W and 20W (bug zapper lamps only), and T12s (Both regular and the american HO T12s).
2. Krypton + Argon: European T8 retrofits for T12s, British 100W 8ft T12 retrofit for 125W 8ft T12, T5s of 14W and over and CFLs.
3. Neon: GE T17 Powergrooves and american VHO T12s.
The gas filling of the american F32T8, remains a mystery to me, since despite that this is an newer wattage rating for T8s (From the 1990'), and this is an energy saving T8 lamp as a replacement for F40T12, with the same length for both of them (But not a direct retrofit for it), and so theoretically it should have krypton + argon penning (Which have higher starting voltage than argon alone), rapidstarts can ignite the F32T8 quite easily, and this lamp is actually a rapidstart lamp (Lamp that designed to be operated only on rapidstarts and not from preheaters).

I'm in the UK. Based on what you say one of the 30w might have just go argon, whereas the other has krypton aswell.

Yeah the 30W T8 is one of the original T8s with just an argon filling, along with the 15W, 38/40W and 50W. The starting voltage is low and they work with all types of control gear.