Method and apparatus for non-destructively measuring internal film resistance in a fluorescent lamp

A method for non-destructively measuring the resistance, R.sub.w, of a conductive film coating on the interior bulb wall of a fluorescent lamp, said method comprising:

• disposing first and second spaced apart capacitive coupling means on the exterior bulb wall of said fluorescent lamp, said first capacitive coupling means being connected through a sensing resistance means to a source of reference potential, and said second capacitive coupling means being connected through circuit means to a first output terminal of a variable frequency AC voltage supply, a second terminal of said AC voltage supply being connected to said source of reference potential, said AC supply being at a selected fixed frequency, and the circuit including said first and second capacitive coupling means and said circuit means being in resonance at the selected frequency of said AC voltage supply;
• and measuring at least the voltage, V.sub.s, across said sensing resistance means, said internal film resistance R.sub.w being determinable from the equation
• R.sub.w = R.sub.s (V.sub.g cos .theta. -V.sub.s)/V.sub.s
• where R.sub.s is the value of said sensing resistance, V.sub.g is the output voltage of said AC supply, and the angle .theta. is the phase relationship between the voltage waveforms V.sub.g and V.sub.s.