Reno:
I still have a comment toward this:
You said:
"I was thinking of a separate circuit that, in addition to the usual power, also had back-up power available (either within the fixture, or serving the entire circuit). In the later instance the light would operate as any other light- on at dusk, etc. The difference being that the light would remain 'on' if there were a power failure."
No matter what controls the circuit you describe above, it would not be an acceptable means of REQUIRED egress lighting.
Circuit 'on' with utility power, all is OK
Utility power drops out, HID fixtures extinguish.
Generator (if available) starts & transfer occurs to designated circuit in <10 seconds IF gen is designed and installed as Emergency, HID fixture has voltage. Cool down & restrike require 3.5 to 10 min dependent on bulb, & ballast.
If utility power restored quickly, same conditions exist.
As I said, I did not see a restrike component within the pic in the link, nor is there enough specific info provided to make a firm conclusion.
I just questioned the use oh a metal halide fixture on something referred to as an emergency circuit.
