Cyanotype-gum printing light box (365nm vs 395nm)

Hello everyone,

I've been contemplating the creation of a UV light box for cyanotype and gum printing. I've received strong recommendations for UV light sources at 365nm, while being discouraged from opting for sources at 395-400nm.

I didn't delve into the discussion at the time, but I'm curious about the differences. Is it mainly about the exposure length required to achieve the same density? Could it also impact the characteristics or quality of the colors?

Also, for prints around 30x40cm, what wattage do you recommend for the light source to ensure a reasonable exposure time? I came across this lamp (https://www.ubuy.fr/en/product/3UWS...flood-lighting-for-aquarium-indoor-or-outdoor) would it be too strong?

Thanks for your insights!

Emilio

I built an exposure box with 32 feet of LED strip lighting that said they were 385-400nm. My exposures are about 13 minutes with half the lights turn on and about 6 inches from the paper. I haven't run any tests with all lit.

Emilio_ said:

Hello everyone,

I've been contemplating the creation of a UV light box for cyanotype and gum printing. I've received strong recommendations for UV light sources at 365nm, while being discouraged from opting for sources at 395-400nm.

I didn't delve into the discussion at the time, but I'm curious about the differences. Is it mainly about the exposure length required to achieve the same density? Could it also impact the characteristics or quality of the colors?

Also, for prints around 30x40cm, what wattage do you recommend for the light source to ensure a reasonable exposure time? I came across this lamp (https://www.ubuy.fr/en/product/3UWS...flood-lighting-for-aquarium-indoor-or-outdoor) would it be too strong?

Thanks for your insights!

Emilio

Click to expand...

For cyanotype and other iron-based processes, 395 nm LEDs are perfectly usable.

Here is the action spectrum for cyanotype, taken from Turner, et al. Photochem. Photobiol. Sci., 2014, 13, 1753



The 365nm LEDs will be about twice as fast as the 395nm LEDs, but many folks successfully use the 395 nm ones.

I use 72 watts of 395 nm LED strips (at about 15 cm from the print frame) in a box big enough for a 16x20 inch print frame. I haven't used the box for cyanotype, but my exposure time for salted-paper is 7 min. I would expect cyanotype to be a bit faster.

For gum printing, the answer will depend on what sensitize you plan on using.

For the traditional dichromate sensitizer (which is more-or-less banned in the EU and not recommended in the rest of the world because of environmental issues) you would need the shorter wavelength LEDs.

The absorption of dichromate in the near UV peaks at 350 nm and is very low by 400 nm (see: https://www.hellma.com/en/laborator...-accuracy/potassium-dichromate-liquid-filter/)

If you are planning to use one of the newer sensitizers the answer will depend on the details.

When I built my UV unit a year ago, 395nm LED strips were less than half the cost of 365nm LEDs. That just meant that I could buy twice as many, so the lower efficiency didn't matter.

My cyanotype exposure times are about 22 minutes, but then cyanotypes are notoriously slow. Gum bichromate is about two minutes. This is with about 10 meters of 5050 LEDs.

fgorga said:

The 365nm LEDs will be about twice as fast as the 395nm LEDs

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But the efficiency of 395nm LEDs is far higher than that of 365nm LEDs, so you may find that the net 'hardening power' you get per Watt (and per Euro/dollar) is far higher with 395nm as long as the process allows it. Both dichromate (for gum printing) and cyanotype will work perfectly fine with 395nm.

fgorga said:

The absorption of dichromate in the near UV peaks at 350 nm and is very low by 400 nm

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But this doesn't mean that dichromate doesn't work at 395nm. It does, and indeed, it does so very well. The net efficiency (see comment below) at 395nm is in fact very high in my experience. I've printed dichromate carbon for a few months with a 395nm LED source and that was by far the fastest and cheapest approach. I'm not back to combined 395+365nm because I've stopped using dichromate and use DAS instead, and DAs does NOT work well with pure 395nm. It needs at least a little 365nm wavelength for highlights to develop properly in carbon transfer.

I don't know what the sensitivity is for materials like SbQ and Printmaker's Friend.

revdoc said:

so the lower efficiency didn't matter.

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'Efficiency' is a confusing term in this context.
On the one hand, it can refer to how much hardening of a polymer (gum, gelatin etc.) or printing out of a salt (cyanotype, Van Dyke etc.) you can get for each photon the medium receives.
On the other hand, it can also refer to how many photons the LED manages to output for each quantity of electrical power supplied to it.
There's of course also economic efficiency, which combines both aspects above, and several more, and of course cost of purchase & running costs.

Emilio_ said:

I came across this lamp (https://www.ubuy.fr/en/product/3UWS...flood-lighting-for-aquarium-indoor-or-outdoor) would it be too strong?

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No, it won't be too strong. With UV light sources for alt. process printing, it's hard to find something that's too powerful...
Moreover, these units are often sold with power ratings they don't really live up to. I wouldn't be surprised if the real-world power consumption of this 100W unit is closer to 30W or so. I've tested a few of these ready-to-go UV floodlights and their power ratings were grossly overstated.
For instance, the unit you link to has 96 individual LED emitters. The kind of emitters used in a fixture like this one are often rated at 350mW-500mW, which puts the unit in the 35-50W region. Depending on the driving circuitry, this can be even significantly lower.

Thanks everyone for your answers!

koraks said:

I've tested a few of these ready-to-go UV floodlights and their power ratings were grossly overstated.

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@koraks do you have a better idea than the one I suggested ? Are these led solutions a good option or should I choose something else (e.g. other kinds of led systems, tubes, etc.)?

Emilio_ said:

Are these led solutions a good option

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Yes, they are, even if you account for the real power rating. They generally offer good value, especially the more affordable ones. You can gang up several units to cover a larger area at high power levels if needed. Start with one unit and add a couple more if you need more light.