[dietrichepp]

> Wouldn't that just mean you have to wait longer?

In theory, yes. The actual values may help. I’ve heard exposure times cited around eight minutes. That’s for contact printing using the sun as a source. I don’t know off-hand how much light an enlarger will deposit on paper, but it is a few orders of magnitude less than the sun. For the sake of estimating, let’s say 3 orders of magnitude.

This gives an exposure time of 5 days. Note that you’d want to make a test strip or two—someone who knows what they’re doing might want two test strips to dial in the exposure settings, at which point it takes you 15 days to make the first print (but then only 5 days for each additional print from the same negative).

[balou23]

Thanks, that explains it.

I guess good thing you can "cheat" nowadays with digital enlarging (in the case where you "just" want to have a long lasting picture. I have pictures of my great great grandparents, I don't trust that any of my digital pictures will last 4 generations)

[masfuerte]

You can buy 50,000 lumen lightbulbs nowadays which, from 1 metre away, are in the same ballpark as the sun.

[dietrichepp]

Do they produce enough UV to expose platinotype? Most of the modern lights you see are LEDs that produce mostly visible light.

You can get UV bulbs, but at this point in our scenario we’re modifying enlargers, and the enlarging lens may focus differently in the UV region from the visible region (something you’d have to check and compensate for). Normally you’d focus an enlarger visually, perhaps with a grain focuser—maybe at this point you’d be focusing manually with the grain focuser in the visible region and then making some calculated adjustment for UV. Or you’d be hunting down a lens which was designed to have the same focal length into the relevant part of the UV spectrum.

[seszett]

> Normally you’d focus an enlarger visually, perhaps with a grain focuser—maybe at this point you’d be focusing manually with the grain focuser in the visible region and then making some calculated adjustment for UV

Yes, I have some experience with this (see my other reply in this thread) and it's a real problem but there is an easy fix.

Instead of using a grain-focuser (which would also be dangerous for the eyes anyway, and good quality UV lights don't leak much into the visible) you use plain white paper, cheap paper that is treated with optical brighteners.

These brighteners glow in the UV to make the paper look whiter, which means that under your UV enlarger, they turn the projected UV into a visible image. You can then focus using a regular looking glass to look at the paper. And UV-blocking sunglasses (you can easily test if they actually block UV: the paper will not glow at all if UV-blocking glasses are in the way).

Some lenses are corrected for UV-Vis focus shift, but they are generally extremely expensive.

[davidjade]

This is exactly it. Platinum printing requires UV. All of the platinum printing darkroom setups I have seen all had large and powerful UV sources for contact printing.

[MobiusHorizons]

How hot are they? You don’t want to scorch anything.