Testing contrast with Royal blue, Violet, and UV LEDs

I use Royal Blue LEDs in my LED-head, but I heard that somebody is using UV LEDs to get higher maximum contrast. So I bought violet and UV-A LEDs from ledsupply.com, hacked them into a circuit, and tried them.



I developed the Ilford RC paper in Liquidol. Contrasts in other developers might differ a little. The 31-step Stouffer wedge has 0.1 steps, part # T3110C. I measured the steps with a densitometer, interpolated between steps for the appropriate target-densities, thus computing the exposure-range of each LED. Grade 5 has an exposure-range of 1.9 stops, and reducing exposure-range yields higher grade. The results:

• Royal Blue (450-455 nm): range 2.16 stops
• Violet (425 nm): range 2.13 stops
• UV-A (405 nm): range 2.06 stops

Violet is about the same as royal blue, and is probably within my measurement error.
UV-A is slightly contrastier than royal blue, but (here's the important result) the range-improvement of 0.1 stop is insignificant.

Conclusion: Shorter wavelengths do not significantly boost maximum contrast of Ilford RC paper. Royal Blue (450 nm) is fine.

Well done. The outcome doesn't surprise me, but you just don't know until it's being tested!

And what applies to one specific type of paper might not in the case of another.

The UV-A strip is definitely darker (on the web image), I'm not sure if that means it has more contrast though.

momus said:

The UV-A strip is definitely darker (on the web image), I'm not sure if that means it has more contrast though.

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He checked the contrast range by densitometer.

DREW WILEY said:

And what applies to one specific type of paper might not in the case of another.

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The graph below shows that Foma RC exceeds grade 5 with royal blue LEDs, and Ilford RC comes a little short of grade 5, so my attention was on improving Ilford's max contrast.

I also posted this graph in the Printalyzer Densitometer thread here. I recommend that densitometer.

These results are with diffusion. It's well known that condenser systems produce higher contrast.
But @DREW WILEY is correct: It's conceivable that papers such as Kentmere and Bergger will achieve higher contrast using shorter wavelengths.

How much glass between the emitter and the paper?

I've built 5 different LED lamps so far with Cree Royal Blue. In my tests comparing LED to Aristo V54 coldlight, I found the LED just a touch more contrasty than coldlight with a dicroic cutoff filter that passed only the deepest of blues (violet).

My enlarger lenses are not passing much light shorter than 380 nm. I am not sure about the pass band on the acrylic diffusors, but doubt it passes much UV since I don't get headaches when LEDs are covered with plexi. A really crummy test.

[Results based on Ilford VC]

Loose Gravel said:

How much glass between the emitter and the paper?

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My Cree Royal Blue goes through my EL-Nikkor lens, but the violet and UV-A tests I performed above (see posting #1) had no glass between emitter and paper. Rather, I suspended those LEDs high over the paper and turned them on.

Loose Gravel said:

I've built 5 different LED lamps so far with Cree Royal Blue. In my tests comparing LED to Aristo V54 coldlight, I found the LED just a touch more contrasty than coldlight with a dicroic cutoff filter that passed only the deepest of blues (violet).

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Folks here would be interested in your results. After making five, you probably have plenty to say.
What are you using to drive the LEDs? I found that building the LED head took much less time than designing, building, and programming the controller. I built two controllers; the second used lessons learned from the first, such as the need to use TVS chips to absorb ESD strikes.

Loose Gravel said:

My enlarger lenses are not passing much light shorter than 380 nm. I am not sure about the pass band on the acrylic diffusors, but doubt it passes much UV since I don't get headaches when LEDs are covered with plexi. A really crummy test.

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I've heard that glass attenuates UV, so UV-A at about 400 nm is probably the shortest we can go.

albada said:

I've heard that glass attenuates UV

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Well, yes. Then again, it's not a binary thing. It depends on wavelength, transmission spectrum and thickness. Probably diffusion also comes into play. Long story short, surely you could do something at e.g. 385nm, but the overall efficiency of the system will be lower.
It's mostly of interest to proper UV applications though, such as alt. processes. Look at the few threads about enlarged salt and cyanotype prints. It can be done, with some difficulty. And of course virtually everybody uses plain window glass in their alt process contact printing frames!

Plexiglass attenuates anything below 400nm or so very abruptly, though. I found this out the hard way when trying to make an "unbreakable" alt process contact printing frame using thin Lexan. That didn't really pan out.

DREW WILEY said:

And what applies to one specific type of paper might not in the case of another.

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And graded papers as well as films would show exactly opposite results.

Neither uncoated glass, plain acrylic (plexi), nor tinted acrylic attenuate most UV, just a little. For instance, just place a color photo framed with even pink or amber "museum" anti-UV acrylic in a window receiving sunlight, and after six months compare it to a reference sample either kept in the dark or else displayed under ordinary (non-halogen) tungsten lighting. The window version will be starting to show sign of fading, if not in just six months, inevitably way sooner than the protected reference sample itself.

UV printing processes are a somewhat different subject; and LED's add their own layer of complexity to the discussion. Im just pointing out how glass and plastics are not impervious to UV even when treated or coated to limit it somewhat.

DREW WILEY said:

Neither uncoated glass, plain acrylic (plexi), nor tinted acrylic attenuate most UV, just a little.

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You evidently haven't tried making a UV-process contact print through a sheet of plexiglass.
I have.

I can tell you that the 'little attenuation' you speak of in reality is around 3 stops for a 1mm sheet of Lexan. This might not be adequate for long-term protection of UV-sensitive works, but it's far more attenuation than 'a little' and a heck of a lot more than plain uncoated float glass.

I clearly stipulated my own reference point. Three stops might indeed significant when UV printing, but still a lot of UV gets through glass and acrylic. Lexan is an entirely different material - polycarbonate - not acrylic or Plexiglas at all, inherently tinted to prevent UV degradation, and of significantly less light transmission. So you've got things mixed up and are not comparing apples to apples.

Fair enough Drew, Lexan is PC alright and indeed it's "UV resistant".

This is pretty freaking brilliant work! This forum is a great resource!
Well done!