Interpretation of the external band technique for cathode fall measurements of fluorescent lamps

Abstract

Cathode fall is a key parameter in assessing the performance of a fluorescent lamp since it is a fundamental part of a lamp's operation and, at the same time, may be responsible indirectly for many undesirable effects, such as shortened life and severe end-darkening. This work addresses the familiar band diagnostic (BD), a technique often used to measure cathode fall. The diagnostic is an easily implemented, non-invasive procedure in which a metal foil is placed around the outer glass wall of a lamp in the electrode region, and the potential difference between the foil and electrode is measured. Unfortunately, there are severe limitations in interpreting the measurement. The goal of this work has been to properly conceptualize this diagnostic to improve understanding, to identify factors limiting interpretation and to identify features from experimental measurements. The result is a lumped parameter model, and a key component is a representation of the wall sheath at the plasma/wall boundary. Conceptually, the BD is identical to the internal floating probe, and therefore the model is applicable to that diagnostic as well. In addition to descriptions of the model and model calculations, comparisons are made between band and probe and between band and experimental measurements.