Silicon Blue Cell 'filtered' - how?

[Rudybustamante]

So I've been reading up on replacing selenium cells (in cameras and meters) with silicon photodiodes.

Problem is, I keep seeing references to how it's biased heavily towards infrared/visible red.

If I were to perform the modification, I'd like the spectral response to be at least somewhat accurate.
Does anyone know the exact characteristics of the 'blue' filter used over the cells? Any spectral sensitivity experts out there like to hazard a guess?

THIS link (under 'Zenit Repair') states that this kind of cell works in lots of light conditions unfiltered. Should I just stop worrying and jump in?

Thanks for any input

 

[sevo]

You'd have a hard time replacing a selenium cell with a silicone photo diode, blue filtered or not. For one, you'd have to find a place to stow the battery needed by the latter (but not by the former). And unless the galvanometer is unusually tunable, you'd have to add even more active electronics to convert between their different action, or swap out the galvanometer (where you'd probably also have to remake the scale, so that you are left with pretty much nothing except the shell). In general, a conversion to SPD would only be feasible for CdS meters - where it is not needed as you can still get substitute CdS resistors.

SBC have a pretty strong blue/cyan filter (somewhere in between a 80D and a cyan lab filter) as the sensor cover (or as the tint of the potting acrylic). They can be bought off the shelf - better adjusted to visible light than any DIY effort.

If you must convert a selenium cell meter, use amorphous silicone photo cells (solar cells) - these have the same action, and some may even directly swap in current-wise. Like silicone diodes, these are red and IR biased, and if you want to get a colour response that matches film characteristics, you will have to place a 80C or 80D (halogen or tungsten to daylight) filter plus a IR blocking filter on top of the cell.

 

[Rudybustamante]

Ah great just what I needed to know, thank you.

I've noticed lots of the IR cut filters for sale have a blue tint, I assume I'd need more.

Only problem here being the cost, any IR blocking filters I'm findng are front-of-lens specific, and prohibitively expensive, or too too tiny for the silicon cell. I suppose I could block the rest with tape.

It's a real shame there's no way to buy selenium cells in the UK as far as I know. Unless someone's Google-fu is stronger than mine

 

[Landshark]

edmunds scientific(US electronics supplier) may be of help another suggestion is taking a cell from an old camera that uses a selenium cell.

 

[Rudybustamante]

So I did a bit of digging around for UK based sellers of selenium cells (do they sell them at the seashore? ). No luck

Had a look for IR-blocking filters in a reasonable price range for this type of work. No luck.

SO I've had a brainwave. Don't block the IR, block everything else! I've got a couple of films I like to use for infrared work (Rollei Retro 80S, Superpan 200, bits of Ilford SFX), but I find the exposure unreliable.

The idea then, is to use a bit of slide film leader (which I've been saving to make an infrared flash), stick that in front to block all but IR, then calibrate it in sunlight, bracketing etc.

If all goes to plan, this should let me meter IR much more effectively

Opinions?

I wo't be able to start on the project just yet, moving house soon plus a lot of other pressing matters to attend to. But I'm planning on posting progress etc on here in a few weeks.

 

[sevo]

So I did a bit of digging around for UK based sellers of selenium cells (do they sell them at the seashore? ). No luck

The last surviving application for them seems to be in automatic welding visors - I don't know whether any of the distributors of affordable masks sell spare cells, but the masks themselves aren't that expensive either. The cells seem to be much bigger than needed for exposure meters, so you'll have to cut it down and re-seal it.