Ok, but back to the original question.
What about testing of tubes?
I've tried to test laptop backlight bulbs with a transistor curve tracer
without much success.
A 5" tube from a battery-powered fluorescent lamp breaks down at about
500VDC. But there seems to be little correlation between breakdown
and goodness of the bulb. Methinks different mechanisms are in play.
mike


You're comparing apples to oranges. A laptop backlight tube is a cold
cathode fluorescent, more like a neon tube. The breakdown voltage is related
to gas fill and pressure, but the "goodness" of the tube is dependent on the
emissive coating on the electrodes and phosphor condition as well. The best,
and only reliable way to tell the condition of a fluorescent tube is to fire
it up on a known good ballast rated to operate that tube. You can tell more
from the brightness of the tube than anything else since they depreciate
gradually with use and will usually get down below 50% before they fail to
light at all.