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Operation of Neon Lamps.
Neon lamps typically comprise a small glass envelope filled with Neon (other Noble gases such as Argon may be present) and two specially prepared electrodes.
When an applied voltage (either ac or dc) reaches a value in excess of the strike voltage (also known as ignition or breakdown voltage), then electrons start to be emitted from the surface of the cathode (negative). Prior to this point very little current flows.
After the electrons leave the cathode, they are accelerated across the gap between the two electrodes by the applied electric field. This causes the electrons to collide with the atoms of gas. Two well known effects occur, firstly some of the electrons colliding with the gas atoms elevate some of their own valence electrons to a higher energy state. When these electrons finally return to their original energy state, photons of light are emitted at a wavelength determined by the difference in these energy levels.
The second effect is of ionisation of the gas atoms. This is caused by electrons colliding with the gas atoms with enough energy to knock an electron from its parent atom causing it to become a positively charged ion. Once this has occurred the positively charged ions are accelerated towards, and collide, with the negative electrode (cathode). This has the effect of removing material from the cathode (called "sputtering").
Sputtering is the principle cause of ageing in neon lamps since, as the emissive coating is removed over time, it adheres to the sides of the glass tube causing a reduction of light output. It must be noted that this is in no way a catastrophic failure. Indeed with the type of applications neon lamps are put to, manufacturers don't find it easy to quote accurate lifetimes, although it has been accepted generally to quote typical minimum life for a reduction of 50% of the initial light output.
What is a Neon Lamp - Features - Applications - Operation - Electrical Characteristics - Life
