Visual inspection can reveal sputtering around the electrodes, indicating low filling pressures or excessive current for the electrode size. In mercury tubes, look for darkening conditions caused by poor processing methods, including improper heating, poor evacuation, impure gas or excessive oil backstreaming from the vacuum system. Using a high-voltage tester, you can also compare the tube's voltage to that of a similar, properly manufactured tube. If you find a wide variation in voltage, suspect either the pressure or purity of the gas. Finally, by measuring photometric output with a light meter, the tube's brightness can be compared to other well-made tubes.
Excessive pressure tends to reduce a tube's light output.
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