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Gossen Lunasix 3

Gossen Lunasix 3, although built around the early ’70, is a very capable hand-held meter even by today standards. It beats my Canon 20D built-in meter in terms of actual low-light performance by 5 stops. Unfortunately it belongs to one of these designs, that relied on mercury battery voltage staying constant throughout their life and mercury batteries were banned few years ago. This article describes, how to deal with this problem.

There is a battery chemistry available that can replace the now obsolete mercury batteries. Silver oxide cells have equally stable voltage output and are based on the same principle, as mercury batteries, the main difference being use of silver in place of mercury, which makes these batteries more expensive to manufacture. Since the amount of silver in these button batteries is tiny, the manufacturing cost difference isn’t really relevant to the sale price, but it shows why the mercury was used in the first place – cents sum up to dollars fast.

The Gossen Lunasix is a purely passive circuit, i.e. there are only resistors and the galvanometer in it, no diodes, no transistors, no fancy electronic circuitry. Light is detected using a CdS (Cadmium Sulphide) LDR (light dependent resistor) that directly influences amount of current allowed into the galvanometer. For details on such meter design see my article here (although the relevant PDF isn’t finished yet, the part covering mercury cell based meters is ready). Read the rest of this entry »