Electroforming is when particles of metal and electrodeposited onto a super-precise mandrel which almost perfectly reproduces the shape of the mandrel.
Basically it makes a very precise and smooth surface.
It’s more durable and economical than glass reflectors but still far more expensive than anything you would see in a typical flashlight.
Stuff like the maxabeam and cinema projectors use electroformed reflectors.
And no, rhodium is silver coloured.
@Jerrommel: if you want to compare the “lumens in the beam” just measure the spot size and the lux of each and multiply these values with each other.
Lux = Lumens / m^2 => Lumens_in_beam = Lux x size_of_beam
Concerning reflectors:
Electroforming is a process that concernes the shape of the reflector (the accuracy of the parabola). It effects the throw of a light. The larger the difference between the size of the reflector and the size of the light source, the more accurate the reflector needs to be (if one wants the reflector to be completely lit up by the light source which is needed for maximum possible throw). The base material is usually nickel. It shouldn’t have too large of an effect on reflectance.
Because of their tiny size Xenon and Mercury short-arc bulbs require electroformed reflectors for maximum performance. The Maxabeam is a very good example of this. The tiny spot inside the arc (surface area maybe around 0.125mm^2) can only be focussed this way.
I have never seen a light with a polished reflector. I think it would be noticeably worse than what we are used to, even from cheap LED lights. I gather from the forums that aluminium reflectors are usually more precise than plastic ones (Maglite, Fenix etc.). This can already have an effect when using smaller LEDs like the de-domed XP-G2.
A bad example of an aluminium reflector is the one in my Trustfire X7. Is has a noticeably imperfect shape. A de-domed XP-L lights up maybe 90 percent of it.
Concerning Rhodium:
Rhodium is a special coating used for electroformed reflectors. It is very expensive (type “rhodium price” into Google…) and is considered the most robust coating in terms of corrosion resistance (salt water…) and resistance to strong UV radiation (Mercury bulbs…). It’s downside is the lower reflectance of visible light compared to aluminium and other coatings (70-80 percent). For short-arcs this usually doesn’t make much of a difference though.
These days there are also newer coatings like enhanced rhodium which might be a better compromise in many cases.
Thanks for explaining, guys.
I read something about dielectric coating being even better, but i don’t know what that entails. All i know is that it’s expensive. 
So for this light it will have to be a decently machined aluminium dish that needs to be coated with aluminium vapor, basically like the average flashlight reflector.
Since it will be (in my view) not larger than 5 inches in diameter, i think it can be done good enough.
For DIY you could perhaps form a dish from sheet metal (aluminium, brass, whatever) and put silver on it yourself.
But i suspect silver will not want to stick to aluminium somehow, but that’s just a hunch.
1 Thank
Great idea, I’m in.