11:45 Jan 31, 2011 

"la lampe pilote de la torche" is quite good... And it's already an expression found in a document from Profoto (found on Google).

11:32 Jan 31, 2011 

Thank you Tony, this is very helpful!

11:30 Jan 31, 2011 

Well, your question was unclear, you've now given a lot of very important extra context which makes much more sense of things.

I am happy to accept 'torche' as the technical term for a 'flash head', although I have to take issue with your client's use of 'lampe pilote' for 'modelling lamp' — the term is at best poorly chosen, since it tends to confusion with the idea of a 'pilot lamp', which this is not at all; it is certainly a lamp to help you 'aim' the flash head, but from that kind of 'steering' to 'pilote', I feel it's a bit awkward.

However, now you have explained what your real question is, and the essential background information to go with, I'd say something like « sans (être obligé de) changer la lampe pilote de la torche »

11:11 Jan 31, 2011 

Dear Tony,
I appreciate your help although I disagree with your comment: my customer, Profoto, recommends that "flash head" is translated by "torche" and "modeling lamp" by "lampe pilote", so here is not my problem. My problem is how both can be put together, their relationship. And I may be sensitive but I think it's unnecessary to begin a comment with "absolutely not!".
Thank you.

10:14 Jan 31, 2011 

http://1.bp.blogspot.com/_SMOTQp0_pAE/SmcHnv1RXfI/AAAAAAAAAM...

A 'flash head' refers to a kind of electronic flash unit, in the form of a 'head' that can be fitted onto a stand, that is very commonly used in studio work by profesional photographers (do a Google image search to see plenty of examples).
The disadvantage, however, with this kind of flash, is that you can't get an impression of what the lighting is going to look like in the final image, as the flash is too short and bright for the eye to properly assimilate it; so these units also include an ordinary incandescent lamp, know as the 'modelling lamp' (or light) which can be left on continuously so that the photographer will be better able to judge the final effefct of her/his lighting; you can see this lamp very clearly in the picture to which I've posted the link above.

I'll leave it to my FR colleagues to give you the proper technical term.