Re: what is the precise chemistry in the production of limelight

The only overall chemical reaction that takes place is
2 H2 + O2 --> 2 H2O !!

Let me try to explain: 
Limelight was widely used for stage lighting in the 19th century, because 
it could produce light of a much better and more natural colour to 
represent daylight or moonlight than other forms of lighting then 
available. The light was produced by arranging for a block of lime -- 
calcium oxide -- to be placed in the combustion region of a hydrogen/oxygen 
gas jet flame. The light that is produced is simply the incandescence of 
white hot calcium oxide.

The chemistry is remarkably simple at one level, though all chemistry of 
flames and combustion has very complicated underlying mechanisms. A 
hydrogen/oxygen flame is one of the hottest available. It is much hotter, 
for example, than methane/oxygen (natural gas). A very high temperature 
needs to be reached by an incandescent material to produce a good white 
light, especially the sort of bluish white that realistic daylight or 
moonlight would require. If the temperature is not high enough, a white hot 
object looks yellowish.

So the hydrogen/oxygen reaction is mainly used just to get the block of 
lime white hot at a very high temperature.

Why use lime? Firstly it does not react, neither with hot hydrogen nor with 
hot oxygen. Secondly, it is itself a good white colour. Thirdly, it has a 
very high melting point. Salt melts around 700 deg C; quartz around 1600 
deg C; lime does not melt until close to 2600 deg C.

There is a little extra advantage in that the solid surface of the lime can 
act as a catalyst for some of the steps in the oxygen/hydrogen reaction, 
and make sure that the heat energy is efficiently transferred to the solid 
itself, but that is a rather subtle extra that just adds a little icing to 
the cake of the general principle.