Theory Check - LED Split Grade Printing

[Geary Lyons]

I have had several questions regarding controlling contrast, split grade printing and a simple LED VC head. I'm looking for some input and consensus.

So, the basics, VC paper uses blue versus green sensitized emulsion to achieve the overall contrast range. The papers were designed to respond to, the then available, tungsten light sources. VC filters and color dichroic filters are designed primarily for tungsten light. That is a light spectrum heavily weight on the red end and lot's of heat, (heat = infra red!).

In every dichroic head, that I have examined, the range of M or Y filtration is determined by how far into the light beam the single value dichroic filter is pushed or pulled. That is, the dichroic filters are not graduated as one might assume from the numbers on the dials.

So my theory, and the basis for my LED head, is that the variation of exposure time at green and blue is the exact same relationship as how much of the tungsten light beam is passed through the Y or M dichroic filter. If one spins the Y-M dials to max, in split grade printing, then the relationship of hard & soft contrast is exposure time based, just as in the simple LED G-B configuration. Varying the ratio of green to blue light exposure time can provide any contrast range between the two extreme produced by the LED spectrum. This theory is confirmed in Lambrecht-Woodhouse Way Beyond Monochrome, (page 79, figure 3).

So why are some folks telling me that they MUST have the ability to vary the G-B light intensity as a control function on the LED head? I can understand if they want to achieve a certain "equivalent contrast grade" overall or localized in the more traditional VC printing concept. But they are insistent that the variable density, (luminosity), of the B-G light is critical in split grade printing as well!

Am I missing something?

You thoughts would be greatly appreciated!

Cheers,
Geary

[Arne Croell]

I think your analysis is right. For pure splitgrade printing, there is no need for adjustable brightness other than maybe wanting to adjust it to be able to use the optimum aperture of the enlarging lens at different magnifications. There is a commercial LED splitgrade head available made by Heiland in Germany (see http://www.heilandelectronic.de/html/deutsch/produkte/kaltlicht_main.htm, in German), and its brightness is not adjustable.

[wfwhitaker]

If all you ever varied was the light intensity, cameras wouldn't need adjustable shutters. Whether you change the intensity of your light source or simply make separate timed exposures for hard and soft shouldn't matter as long as you can calibrate your system to achieve the results you want. I have an RH Designs Analyser 500 which replaces the control unit on my Ilford 500 head and that's exactly how it works. It makes one exposure for green and then one exposure for blue. Changing the relative times of the exposures controls the contrast. There is no change in intensity. It's very simple and it works beautifully. I wonder if it would work to control your LED head.

[ic-racer]

So why are some folks telling me that they MUST have the ability to vary the G-B light intensity as a control function on the LED
Only if you want a single exposure of the same time for both colors. As suggested you can do separate exposures of G & B or rig a timer to start both colors at the same time and let each led color expire as needed.