Selenium cells are generators. They generate electric current when exposed to light.
As they age the capacity decreases. This happens much more rapidly if constantly exposed to light.
For example, a camera with a built-in meter and no case, sitting on a mantle for years, will definitely degrade.
The same is true of hand-held meters.
The key is how long the cell has been exposed to light. If the meter is kept in a closed case except when actually
being used, the degradation will be much more gradual, but it still does happen.
The problem with buying a used selenium meter is that you cannot know the storage conditions, and thus the cell condition.
Even if the meter includes a case, you don't know whether or not the meter was stored in the case, out of the light.
The SBC is a great meter.
It uses a Silicon Blue Cell (SBC) rather than selenium, so it's not subject to the degradation described above.
That's why it needs a battery. Silicon is a resistor, not a generator, and thus requires external power.
- Leigh
Yep, the SBC uses a silicon cell; that's why they changed the name from the regular Luna Pro. The original Luna Pro I thought used a gallium arsenide cell (and mercury batteries, now can be used with the usual work arounds) but I found one source saying it was a CdS meter. That would account for Gossen's bragging ads of the time about the new silicon cell SBC, as I don't think a move from gallium to silicon would have made much difference.
The progression of light cell technology basically goes:
Selenium - needs no batteries but deteriorates over time as you say. Also not very sensitive in low light and have a wide acceptance angle, typically something like 60 degrees, which may be good bad or indifferent depending on ones needs.
Cadmium Sulphide, commonly called "CdS" - much more sensitive than selenium with a narrower acceptance angle, but there's a lag after turning them on (sending current through them) - I have an old Ricoh Singlex TLS and this is apparent. When you activate the meter the needle jumps up, then settles back. They have a "memory" effect after exposure to light too, and depending on circumstances can take minutes to completely settle back to the right current reading though a few seconds is usually enough. Much smaller and more sensitive than selenium cells, with a narrower acceptance angle. Battery dependent and the spectral response is not very linear, which has to be taken into account in meter design. Battery dependent.
Gallium and Silicon arsenide cells - largely replaced CdS and selenium. Much better spectral response than CdS, no memory effect or lag, similar narrow acceptance angle. Also battery dependent.
I know Gossen made a big advertising push about the spectral response of the SBC (as you say "Silicon Blue Cell") when it came out, as I recall seeing ads for it in, IIRC, the mid 70s. The older Luna Pro apparently works well enough but the battery work arounds can be a PITA.
The only drawback I find to my SBC is that it is a BIG handful of a meter. It's not a serious problem and it looks impressive to the non-photographers especially those not old enough to have seen a big analog device with complicated dials on it very often.
It fits in a shirt pocket, or most of them, but that pocket will be FULL.
If someone had an original Luna Pro that was working well I'd say to get a battery adapter and use it, but I wouldn't, and didn't, buy one. I'd look for at least the SBC, or the F, which is the same meter with built in flash metering ability. I have the flash attachment for my SBC.
I was really just getting at the point that there's absolutely no reason to avoid "older" meters as such. I prefer how my Luna Pro SBC works to most digital meters.