[tiborsaas]

What does selenium do in the camera? Can it be replaced? How to know it's working?

[jacquesm]

It's a little photovoltaic cell, the voltage output is proportional to the amount of light received.

https://en.wikipedia.org/wiki/Selenium_meter

[Finnucane]

These days it would probably cost less to buy a decent used handheld light meter than to fix an old built-in meter on a camera like the Pen. For years I've used a Gossen Luna Pro with 'spot' attachment.

[100k]

For anyone else interested in a light meter for an old camera, my coworker's friend developed a small light meter and spot meter that are pretty interesting (and affordable): https://www.reveni-labs.com/