Light meter accuracy over time

with a very thin layer of gold

on the front.

I think you'll find the transparent front coating is tin. That's the industry standard transparent conductive coating.

 

I know all about the construction of a selenium cell thanks, and contact oxidation is easily fixed. What's not fixable is the decline in output from a cell that's become contaminated through loss of protective varnish, or some other sort of deterioration.

 

Nothing lasts forever, but selenium cells are particularly prone to age-related failure.

I've seen some very fine selenium cells which are no longer accurate, including the one on the Zeiss Contarex Bullseye. And Westons too, and General Electrics. If you keep them in the dark, selenium cells last considerably longer but do tend to get bad over time. I still some very old ones that work pretty well (usually the ones that come in cases like the old Leica clip on meters). The CDs sells are a lot more durable. I don't think any of the ones I have are bad. My old Luna pro is still fine as is my super cheap spot meter from Adorama. Honestly I don't use them much these days though. I think they are cool bits of technology though,

ITO is the favorite transparent conductor, but it might not be that old.

 

In the case of selenium cells, though, it is the difference between the two sides that make a pn junction, and so produce current.

 

I suspect that, like most things, there are a variety of failure modes.

 

But most of the selenium cell meters that I see either work close to right, or not at all.

 

And yes, Noah might have used one.

 

In case you wonder about such things, liquid selenium is a semiconductor, but

liquid silicon is a metal.