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Name: James
Status: Other
Age: Other
Location: AL
Country: United States
Date: April 2006

What will cause a neon bulb to begin flickering and go dark if the surrounding ambient light is removed? The bulb will glow if a flashlight is shined on it in the dark. Voltage is applied to the bulb at all times. The circuit is simply a source, series resistor and bulb. I am stumped(and so are my colleagues). Can you offer an explanation?

Hi James
Neon lamps light when the gas inside ionizes and creates a conducting path. The start of ionization can be caused by several factors. A high enough supply voltage is usually what is employed. However, if the supply voltage is not sufficiently high to cause the bulb to light (called the striking voltage), external influences may come into play that will dislodge some electrons and start the ionization cascade. These may be cosmic rays, the natural or embedded radioactivity within the lamp, or photoemission through ambient light (the photoelectric effect). You are seeing the latter.
Hope this helps.

Bob Froehlich

Old age or marginally insuficient voltage. Neon bulbs have been known to be photosensitive for over 50 years now. Of course, in the last 30 or so we have stopped paying attention to the effect.

They are most sensitive when they are not yet glowing, and the voltage is just short of ignition. In that situation, as little as one absorbed photon sucessfully ejecting a photoelectron from the metal electrode can start the lamp. In neon bulbs the extinction (on-->off) voltage is always lower than the ignition (off-->on) voltage, so there's about 5-10v of working margin.

Add to that, older bulbs will be degrading and requiring gradually more voltage until naturally they operate intermittently (flicker).
- losing Neon to sputter-implantation
- eroding easy-emitting metal coating on the electrodes
- maybe leaking in air, which rasies the voltage required

"Comments on little neon bulbs and tubes..."
(There is probably cesium in the electrode surface for easy ingiting, and vacuum phototubes use that too for the photocathode surface.)

"Neon indicator tubes as quantitative detectors in photometry"
The average start-up time can be inversely proportional to the photon flux. So you get an intensity-to-frequency converter!

"Spectral sensitivity of Neon discharge tubes":
Red is weak, bluer is better.

I wonder what is a "flicker photometer"? in a title on page 3 of

Remember that neon bulbs turn off then on again 120 times a second when driven from AC voltage...

Jim Swenson

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