“White” Panel LED Color Temperature W/VC Filters?

[John Layton]

As a light source for my DIY 11x14 enlarger - I’ve been leaning towards following ic-racer’s suggestion…to employ a single-color (white) LED light panel, and then filter this as necessary to affect contrast changes in VC papers.

With the above in mind…Superbright lists an LED “sheet,” which measures 12X24 inches, which I can trim to size as needed. It consumes 30 watts and 24 volts, and is available in the following color temperatures: 2700K, 3000K, 3500K, 4000K, 5000K, and 6500K.

What I need to know is which color temperature to choose from the above?Knowing that a PH212 on an Omega enlarger rate at just under 3000K, it would seem obvious that this (temperature LED) is what I should choose, but a bit of digging around indicates that LED’s behave a bit differently than do tungsten based lamps, and that their CT ratings might need to be different as well for my application/use with VC filters.

Ideally, I’d be using such a light source with two Rosco filters (sheets between light source and negative) - one being a green #389 and the other a blue #68. My research indicates that Steve Sherman has had great luck with using these two filters for split grade printing.

My gut tells me that I should go with a color temp. closer to either 4K or even 5K - but maybe not? Hmmm…

[Mark Sampson]

I have the old Zone VI cold-light head with the 'blue' lamp meant for graded paper. While usable with the under-the-lens Kodak VC filters, I use a #1 filter as 'normal, and grade spacing can be weird. And Foma prints differently than Ilford. I tried the CP40Y filter above the negative trick, it didn't make much difference. That said, i have had few, if any, unprintable negatives over the decades.
If you're going to print split-grade as your standard, I doubt that the exact color temp will make a difference. Just that your exposures, blue vs. green, would not be the same with a light source of different color temperature. Given what VC papers are designed to see, 3000-3200K, I'd build my light source around that.

[Peter De Smidt]

Personally, I'd go to 6500K, as that will allow the highest contrast. I'd mount them to an aluminum heat sink. I've used these folks in the past:https://www.heatsinkusa.com/


80-84CRI isn't very good. I wish there was a spectral plot. As long as there's enough output in the appropriate green and blue area, though, you should be ok. Problems producing red seem to limit the CRI of panels, and for your use, that shouldn't be a big deal.

[Doremus Scudder]

John,

Since you're planning on using a blue/green split printing technique, what really matters is that the panel you choose emits usable and fairly equal amounts of blue and green light. Take a look at the emission spectra for the different color temperatures and compare blue and green output. Also look for spikes and holes in the emission spectra in those areas. Ideally, you'd want a wide band of both blue and green that matches the spectral transmission of the filters you are using and matches up with the spectral response of the papers.

The actual color temperature is less important if you are not planning on using a regular VC/Multigrade filter set. If, however, you are, and want to keep the grade spacing and the speed matching somewhere close to the performance of the filters with halogen light, then look for a panel with the closest output to a 2800K halogen bulb. Pay special attention to the yellow-green-blue part of the spectrum (red-orange doesn't really matter).

The emission and transmission spectra of the bulbs, LEDs, and filters should be online, so it's just a matter of researching and then matching scale, etc.

Hope this helps,

Doremus

[esearing]

Borrowing from the Ilford 500 VC head - use two light sources with Green & Blue filters that mix the light in a light box - each with its own timer or toggle for split grade printing. Green</mixingbox\>Blue. or set up two LED panels with an upside down V configuration, but you may not get even illumination across the full surface so would need a diffuser.

The Beseler dicro head moved Fiters in front of the light path to adjust blue and green percentages but ultimately into a mixing box. Light <GreenFilter <blueFilter < /mixing box\

[John Layton]

Thanks folks! I've been revisiting the Superbright website...looking over their COB-LED strips, and revisiting the idea of going with either an RGB, or a GB LED head.

(I should probably merge this thread with my other one on my enlarger design/build...as I've been discussing light sources on that thread also).

[Joe O'Hara]

Agreed in principle with Doremus here. The problem is that many kinds of light that "look" more or less white to us are far from natural white light (of whatever color temperature) in terms of their spectrum. LED light is inherently "spiky"-- it does not have a smoothly continuous spectrum like daylight or even "tungsten" light.

Since you only really care about green and blue for variable contrast B/W printing, it makes sense to select sources that produce the relevant wavelengths in abundance (since they will be filtered into the mixture you need with the yellow and magenta filters), regardless of how "white" they look to you. Ideally, you'd want sources that produce light that your filters are good at absorbing, otherwise you may have trouble getting to the full contrast range your paper is capable of. Needless to say, testing and calibration for your own conditions will be essential.

[Ulophot]

John, you're probably way ahead of me regarding specs, since you're building one of these, but when I got a friend to build mine, I borrowed (since bought my own) Stouffer step tablet for contact printing to see how to adjust the RGB for grade 2 without a filter. Once I did that, and it matched what I got with my 2 filter (Ilford), I left it there. In my case, with cheap RGB panel, the blue is at max, with green much lower. It works for me.

[Peter De Smidt]

.... my case, with cheap RGB panel, the blue is at max, with green much lower. ...

Right! It'll be the high contrast light that will tend to be limited. Don't go with a warm color temp.