It depends on what kind of sensors the robots had. If the robots rely primarily on optical sensors for situational awareness, then emergency lighting might matter for them to be able to navigate.
didn't Mose operate independent of lighting?
which electromagnetic wave length would be the cheapest for the robots to operate under??
now we are nitpicking ....
Less nitpicking, more overanalyzing. (How about that, I nitpicked the nitpicking!)
I would think the cheapest would be visible light because there are lots of cheap commercial bulbs available, plus it doubles as being useful for the humans as well.
Okay, I don't need to keep going. But I'm having fun.
With the continued improvements in illumination engineering, LED lamps would be quite feasible and operate effectively on a battery backup system when power is out. This is feasible now, let alone when we have near-sentient robots.