Measurements of the Hg Fill Level in Low-Pressure Hg Discharge Lamps via Macrophotography

Using a simple macro-photographic method and standard image analysis software we determined the dimensions and subsequently the mass of Hg droplets contained in small, cylindrical, rf-excited discharge lamps like those used in Hg-ion atomic clocks. The mass derived from an analysis of these photographs was compared to that determined from a measurement of heat required to melt condensed Hg in the lamp using the technique of Differential Scanning Calorimetry (DSC). These two mass determination methods were found to agree within ≈ 25 % in an investigated Hg mass fill range that varied from 0.2 to 17.3 mg.