I'll bet you could have used a white LED and dropping resistor.
It would use less current than the grain of wheat lamp and last
forever.

T.Alan Kraus wrote:
I rule out a LED, because of the 5v
at 120 mA and no current limiting resistor in the circuit. So I start
making calls and finally get the Heidenhain tech in NY. I ask him if he
can repair my sensor. He says it's too old, not supported and I might be
well served with a newer model for $ 600.00 . I ask him if he knows what
the light is on my unit, he says it's a very special lamp, I ask him " a
5v grain of wheat microlamp?" He says it's a very special lamp and no,
they don't have a replacement, and I should buy the new sensor for 600$.
I thank him and hang up. So I find a 5v grain of wheat lamp in my local
electronics supply house, take apart the sub-assembly, remove the old
lamp taking care to measure where the original filaments where placed,
and superglue in my new lamp in the same position.