What kind of diffuser to buy?

Usually one wouldn't add a diffusor for copy work, as the smaller the source the more brilliant the copy.

But I assume you'll know you reason to add a diffusor... if you wish to fix it to the reflector I'd suggest to look for a sheet of milky white glass so it withstands the immense heat of the bulb.

Alternatively, if you are able to assemble a frame in some distance of the bulb I'd cover it with heat resistant "tough spun" from Rosco.

www.rosco.com/filters/cinegel.cfm?CategoryID=6#24

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cheers, Peter
Germany