On Running out of Electrons
I am no scientist, however having once spent scores of hours researching the subject of the history of Fritts cells, their uses and how the latter evolved from antique burglar alarms into selenium cell powered light meters, I do have some hopefully pertinent observations I'll share.
First, it's consistently been my experience inspecting and testing various meters (or cameras with built in meters) that, examples which have been stored out of the light inside a case have a better survival rate, than those which have not.
When I say "test", that means assessing the accuracy and linearity of a meters response over a wide range of EV by using a calibrated Kyoritsu light as a basis for measuring meter readings: not simply taking one outside on a sunny day and looking for a sunny 16 reading. This is important, because many meters which respond well to light may, nevertheless, lose some linearity of response across a part or whole of their measurement range. Ie. might be off across their entire range, alternatively, accurate across a certain range of EVs, but inaccurate, at others.
Secondly, why (in my experience) meters not exposed to light over long periods tend to remain in better accuracy is a question I do not know the answer to. I can posit a couple of possible causes, but, "possible causes" is all these are, again, I don't definitively know.
One possibility I have not investigated is to what (if any) extent the flow of electrons through the cell circuits, itself, might facilitate corrosion or oxidation of cell contacts. I have no idea on this one, but I've wondered if it might somehow hasten any increase in resistance to current flow. Anybody have any thoughts?
Environmental factors, on the other hand I am reasonably sure will have bearing on cell viability. Eg store your cameras and or meters in a damp basement and their condition will suffer for various reasons including, potentially fungus and mould but also, of course, corrosion. This will not do the meter circuits any good, and I don't think there would be much controversy about this point.
But, what effect will constant exposure to light have on the external protective coating layer of the cell? This layer is critical to cell condition, because it protects the conductive layer underneath.
Perhaps it's worth labouring this point for those readers who are not aware. Whilst the source of electrical power is a layer of grey selenium coated onto a metal base, this photo generative element requires a conductive layer to harness the potential of the cell and deliver it to the wires that take the power to a galvanometer.
This conductive layer is applied on top of the selenium layer and is a layer of silver typically deposited by vacuum sputtering. It is an exceptionally thin layer: so thin, it's transparent. It has to be, because it can't generate electrical current, it can only carry it. Generating power is the function of the selenium layer beneath it, and, unless the conductive layer is transparent, the whole exercise is pointless because, if light can't reach the selenium, (obviously) there will be no output to conduct.
So: the sealing effectiveness of the protective outer layer is critical because once it's breached, atmosphere can reach the conductive layer, decreasing current flow. At this point the cell is a goner, because, whilst the selenium itself is stable, the circuit is compromised. Perhaps exposure to light promotes UV-connected deterioration of the outer coating?
It's reasonably well known, I think, that in some cases, malfunctioning cells can be made good by improving connectivity at their connections to the meter circuit. Well, a cell with a deteriorated conductive layer may manifest similar behaviour. The difference, of course, is that, (short of removing the outer and conductive layers and re-applying new ones), it is not possible to rejuvenate such a cell.
I have noted with interest that some repairers have invested in vacuum deposition chambers for the purpose of Eg repairing prism assemblies. When correctly prepped and processed rangefinder prisms and beam splitting surfaces can be returned to original quality condition, by replicating the processed used to originally make them.
Why have I mentioned this in connection with selenium cell problems? Quite simply, because this equipment might, potentially, also be used to sputter new conductive layers onto defective selenium cells, by substituting a silver electrode in place of Eg gold or aluminium material preferred for optical coatings.
It's not that simple, naturally. Whilst selenium as a trace element is important to human health there are different allotropes of it, some of which are toxic (including that once used for coating cells). As the process of reconditioning cells would inevitably involve taking them back to their base selenium layer and heating them, there are some important environmental and occupational health and safety issues to manage. Nevertheless, I think that it would be technically feasible (if not necessarily economic) to salvage a percentage of defective cells. Sadly, I wouldn't bank on seeing this happen any time soon.