What is that lamp on the top of the TARDIS?

[Dematerialiser]

THE CLASSIC SERIES LAMP
At times the lamp on various Tardis models was anything the production crew could get their hands on.. yes, someone forgot to pack the darn lamp again!

 
(a fresnel lens: 360^o nautical Navigation Beacon - PB Edit)

The lamp that many people use on their builds is something like a ships navigation lamp - basically a glass fresnel lens mounted inside a housing comprised of a cap (usually circular) and three or four legs to a round or square base. This kind of lamp can be best seen on the season 18 (Tom Yardley Jones) Tardis. Hard to find at sizes greater than 5" diameter (the Tardis lamp was a little wider), one place to start looking is marine chandleries - boat supply stores. Or even ebay. Search for 'Anchor Lamp'.

For the curious, a good explanation of what a fresnel lens is and how it works can be found here:

http://en.wikipedia.org/wiki/Fresnel_lens


THE NEW SERIES LAMP
The new series TARDIS lamp is a modified garden candle lantern made by Gardman. It has been available in B&Q and some garden centres but is now apparently discontinued.  However, it still seems to surface around spring time in some DIY stores.

 
(above, the finished Lamp in situ, one of the prop Lamps removed during a spruce up and finally how the original lamp would have looked before the modification - PB Edit)