Split Grade Printing Theory Check

galyons · Jul 22, 2008

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

Jon King · Jul 22, 2008

Geary,

You have it right.

Jon

jeroldharter · Jul 22, 2008

I think some people would want to vary the intensity of the LED output to keep exposure times relatively constant to keep the lens at a particular f-stop that might be sharper than others.

I understand that with LEDs, dimming them is more complicated than with other light sources.

I think that having separate blue, green, and white (for focusing) channels would suffice. I would prefer white to red for focusing because I focus on a dummy sheet of paper anyway and find the white light much easier on my eyes than the red.

Rather than dimming the LED's, it might be easier and cheaper to have a filter drawer for neutral density filters.

Dan Henderson · Jul 22, 2008

I split grade print with green and blue gel filters placed in the filter tray of an Aristo cold light head. I first determine the exposure time needed through the green filter for the highlights, then determine the exposure time needed through the blue filter to set the shadows where I want them, after the green filter exposure has been made. So for me, anyway, split grade printing is at maximum green and blue; no variability in density, just time. Maybe others split grade print at less than grade 5 or more than grade 0 or 00.

Curt · Jul 22, 2008

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.

On one Beseler head I have the filters move up and down into the light to give the mix. However on the Beseler Universal 45 I have the three Dichroic filters, Red, Green, Blue, not Cyan, Magenta, Yellow, are each fixed in a housing in front of the Halogen bulb. A controller makes the filter pack and adjusts the light intensity. Color temperature feedback is part of the mix making it very complicated design wise.

srs5694 · Jul 22, 2008

For split-grade printing, based on my understanding of the theory, I don't think there'd be any difference between dimming the LED and reducing the time it's on. Dimming would be necessary for traditional single-exposure printing, though, and as such, the ability to dim an LED light source would be important if you were to market one or if you expect to ever do single-exposure printing. Also, if the exposure times ever got very low, a dimmer could be handy. For instance, if you wind up with 1-second exposures on some super-fast paper, that's just not enough time to do dodging or burning, and print-to-print timing accuracy might not be good enough.

FWIW, when I was researching LED light sources a while back, I ran across several circuit diagrams for LED dimmers. Unfortunately, I lost them all in a disk crash. I'm sure a Google search will turn them up. They weren't that complex, as I recall.

One final point: According to an article that Pat Gainer wrote, focusing with green light alone is about as good as it gets, because of the way the human eye focuses light. I use green light alone on my Philips PCS130/PCS150, which has halogen bulbs with red, green, and blue filters. According to Gainer, white light is also good, but given the odd spectral characteristics of "white" LED light, I'm not sure I'd trust a white LED for focusing.

srs5694 · Jul 22, 2008

Curt said:
on the Beseler Universal 45 I have the three Dichroic filters, Red, Green, Blue, not Cyan, Magenta, Yellow, are each fixed in a housing in front of the Halogen bulb. A controller makes the filter pack and adjusts the light intensity. Color temperature feedback is part of the mix making it very complicated design wise.

This is the same design as the Philips PCS150 and PCS2000. This is the first I've heard of another manufacturer using this system, although I have heard of people retro-fitting the Philips light source and controller into other enlargers. Do you know if this is the original design, or could this be a retrofit? (A separate control box with "Philips" printed on it would be a dead giveaway! )

RobC · Jul 22, 2008

galyons said:
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?

In another current thread on split grade printing, it has been recommended that (See http://www.lesmcleanphotography.com/articles.php?page=full&article=21) the negative should be of higher contrast for split grade printing otherwise printing at G0 or max green light can produce muddy highlights. i.e. highlights without enough separation. It may be that these people want control over the level of green light so as to increase highlight contrast. But I would of thought that just reducing intensity would just increase required print time. If they require a harder (G1 or G2) low contrast exposure it would require a mix of green and blue.
Not knowing how your head works but assuming you are working on just two exposures, one green and one blue, then you have no control over highlight contrast. Highlight contrast would all have to be done in the negative which kind of negates the need for VC paper. The same applies to the high contrast exposure as not all people use G5 for that. Some use G4 or G3.

So you may require a mix of green and blue for the low contrast exposure and a mix of green and blue for the high contrast exposure. Just using G0 and G5 won't necessarily do it unless the negative is of perfect contrast for the required print.
That obviously creates a major problem in designing an LED light source for split grade printing because the mix of green and blue for any intermediate grade has to be calibrated to the specific paper and developer combination. That appears to be what the the heiland split grade head does although it is not an LED source, but it has to know how the paper will respond to any combination of Y+M or G+B to be able to work out intermediate values for starting point or burning and dodging.

RobC · Jul 22, 2008

Also being able to set time for soft contrast exposure and hard contrast exposure to be equal for a "normal" print would be useful as a starting point.
And being able to adjust times to be relatively equal for different magnifications would be useful. And has been mentioned, being able to set a standard print time to allow burning in and dodging times to be long enough would be useful.

Mick Fagan · Jul 23, 2008

Geary, I agree as well, you have it correct.

Regarding the Lambrecht Woodhouse book, if you read a bit further and carefully, you will note that figures 4, 5, and 6 tell a very interesting story about the accuracy of printing either way first.

That is, highlight or shadow exposure first, will not make a difference in the final product. I actually concur with this, after doing the tests in my own darkroom.

I think the compelling piece of information I gained from that very detailed and concise chapter, is that exposing in f/stops precisely is virtually required to gain the maximum repeatable and logical benefit, of a split grade printing technique.

I myself have been using f/stop printing for over 25 years and have never seen the need to get an f/stop timer.

However with split grade printing, I can see how much easier it could make life in the darkroom. If I was a commercial printer, I would certainly get an f/stop timer.

Regarding the Lambrecht / Woodhouse book. I think it is possibly one of the best written books about the technical side of B&W photography I have had the pleasure to read. The two authors really do compliment each other with their differing but similar backgrounds.

A Masters Degree in Manufacturing Engineering (Lambrecht) and a Masters Degree in Electronic Engineering (Woodhouse). Combining those two disciplines with a personal love of photography by both of them, they ended up with a no nonsense book which covers the technical side of photography in a practical way that the lay person can quite easily understand.

Mick.

Steve Smith · Jul 23, 2008

jeroldharter said:
I understand that with LEDs, dimming them is more complicated than with other light sources.

Dimming LEDs is fairly easy. The most efficient way is to drive them with a rectangular wave of varying duty cycle e.g. ranging from 95% on, 5% off to 5% on, 95% off with 50-50 in the middle. You could set up a single control which increased the brightness of the green at the same time as reducing the brightness of the blue (and vice versa).

A fixed, one off adjustment to the relative levels of the two colours should give you a single control which can be marked off in equivalent grades.

Steve.

jeroldharter · Jul 23, 2008

Steve Smith said:
Dimming LEDs is fairly easy...
Steve.

I suppose it is like large format black and white photography. It is easy if you know what you are doing. I have no background in electricity. I dabbled with the idea of building an LED light source. But with no background I think I figured out which LEDs to buy but was stymied by what to do next. There is no LED Depot.

Steve Smith · Jul 23, 2008

jeroldharter said:
I dabbled with the idea of building an LED light source. But with no background I think I figured out which LEDs to buy but was stymied by what to do next. There is no LED Depot.

I suppose it's the same as everything. It's easy if you know how but not if you don't.

The other method of varying blue/green ratio is just to have two separate exposures one after the other. e.g. 20 seconds of green followed by 30 seconds of blue may put you into grade 3 territory whereas 45 seconds of green and only five of blue may equate to grade 1 (very simplistic, obviously). Similar to the way some people use two exposures using grade 0 and grade 5 filters

I think the LED light levels are going to be lower than comparative incandescent light sources so exposure times should be longer and more manageable.

I have an on/off (mainly off) project ongoing to build a 5x4" LED lightsource enlarger so I have a bit of interest in this. Unfortunately I have about three thousand and seven other things on the go as well!

If you want to give this another go, I would be happy to give advice regarding the electronic control (or anything else I may claim knowledge to!).

Steve.

RH Designs · Jul 23, 2008

The electronics of controlling LEDs, whether you're using a split grade exposure or the single grade equivalent, isn't difficult. As I see it the major difficulties are much more to do with the mechanical design - how to get an even enough light distribution and how to interface the light source to the wide variety of different enlargers that exist in the market at an economical cost.

My experiments have shown that achieving a high enough contrast from the blue LED is also an issue - the "bluest" LEDs I can find (455nm peak) still only managed about a grade 4 contrast equivalent, though his can be improved at the expense of exposure time by adding some magenta (or presumably blue) filtration.

But going back to the OP, Geary is quite correct that for split grade printing, there is no need to control the intensity of the light sources. If the hard light is brighter than the soft light, the required hard exposure time for a given contrast is reduced. It's as simple as that. Using a mix of green and blue light for one or both of the exposures just reduces the maximum range of contrast adjustment available from the system.

Regarding the optimum negative contrast for split grade printing - well, the object of variable contrast paper is to fit the paper contrast to that of the negative and not the other way around, whatever method of paper contrast control is in use. You only need to ensure that the negative contrast falls within the range of paper contrast available. I should add that this analysis omits any considerations of the subtle effects that the interaction of the negative and paper characteristic curves have on the "look" of the resulting print - these are much harder to characterise - but if the negative contrast lies within the available paper contrast range then both highlights and shadows can be rendered on the paper. And this can be achieved by either split exposures or a single exposure through the appropriate filtration.

RobC · Jul 23, 2008

RH Designs said:
The electronics of controlling LEDs, whether you're using a split grade exposure or the single grade equivalent, isn't difficult. As I see it the major difficulties are much more to do with the mechanical design - how to get an even enough light distribution and how to interface the light source to the wide variety of different enlargers that exist in the market at an economical cost.

My experiments have shown that achieving a high enough contrast from the blue LED is also an issue - the "bluest" LEDs I can find (455nm peak) still only managed about a grade 4 contrast equivalent, though his can be improved at the expense of exposure time by adding some magenta (or presumably blue) filtration.

But going back to the OP, Geary is quite correct that for split grade printing, there is no need to control the intensity of the light sources. If the hard light is brighter than the soft light, the required hard exposure time for a given contrast is reduced. It's as simple as that. Using a mix of green and blue light for one or both of the exposures just reduces the maximum range of contrast adjustment available from the system.

Regarding the optimum negative contrast for split grade printing - well, the object of variable contrast paper is to fit the paper contrast to that of the negative and not the other way around, whatever method of paper contrast control is in use. You only need to ensure that the negative contrast falls within the range of paper contrast available. I should add that this analysis omits any considerations of the subtle effects that the interaction of the negative and paper characteristic curves have on the "look" of the resulting print - these are much harder to characterise - but if the negative contrast lies within the available paper contrast range then both highlights and shadows can be rendered on the paper. And this can be achieved by either split exposures or a single exposure through the appropriate filtration.

Well thats all well and good for a straight print. But when you want to print part of the image hard in relation to the rest of the print, then having a negative which fits the middle of the paper contrast, allows printing the chosen areas at upto 3 grades higher contrast. There are people who like to do this instead of having a standard fomulaic look. Its called creative printing. And I know some stuffy people will tell you its not a fine print but not everyone wants to print according to their rules. And rules were made to be broken. i.e. just because VC paper exists doesn't mean you don't have to worry about creating a negative of good contrast leaving you room to play with.

RH Designs · Jul 23, 2008

rob champagne said:
Well thats all well and good for a straight print. But when you want to print part of the image hard in relation to the rest of the print, then having a negative which fits the middle of the paper contrast, allows printing the chosen areas at upto 3 grades higher contrast. There are people who like to do this instead of having a standard fomulaic look. Its called creative printing.

I'm well aware of creative printing, and that's why I said that the analysis ignored the more subtle effects. It's always wise to get negative contrast to around the middle of the paper contrast range to give you some margin for creative dodging/burning etc. However, my point was that if the negative contrast lies within the available range of paper contrast available, then it can be printed, whatever method of adjusting print contrast you use. So there is no need to adjust the negative specifically with split grade printing in mind, that's all. Whether the final result is an aesthetically pleasing print or not relies on the skill and vision of the printer. Whether you use G0 and G5 or some other intermediate grades for split printing, you can still achieve the same result with appropriate dodging and/or burning.

galyons · Jul 23, 2008

Steve,
I think that this is core to the discussion. Technically, one does not "dim" an LED. As you describe, PWM, (Pulse Wave Modulation), actually turns the LED off and on, but does so at a frequency that fools the eyes/brain into thinking that the light is less bright. So the apparent dimming is a result of less time that the LED is actually on.

The driver can modulate the duty cycle to say, as an example, 50% on, 50%. Again this is fine for fooling the eyes, but does it "fool" VC paper? I don't believe so. Isn't the net result the same as just cutting the exposure time by 50%?

Cheers,
Geary

Steve Smith said:
Dimming LEDs is fairly easy. The most efficient way is to drive them with a rectangular wave of varying duty cycle e.g. ranging from 95% on, 5% off to 5% on, 95% off with 50-50 in the middle. You could set up a single control which increased the brightness of the green at the same time as reducing the brightness of the blue (and vice versa).

A fixed, one off adjustment to the relative levels of the two colours should give you a single control which can be marked off in equivalent grades.

Steve.

Will S · Jul 23, 2008

In the workshop I did with Les McClean I'm pretty sure we always used 0 and 5, and never anything else. Time was varied during dodge/burn to achieve local contrast changes where needed.

Will

galyons · Jul 23, 2008

Rob,
IMO, this is contrary to the concept of split grade printing. There are no grades in split grade printing, just the relationship of maximum hard to soft. That is the maximum hard and soft produced by your light source. (Of course this assumes that you have relative range). There is no need to calibrate to an "intermediate grade". Quite frankly there is no calibration.

I find split grade printing combined with the StopClock Pro f-stop timer artistically intuitive. I adjust the hard and soft exposures to get the overall contrast desired and the dodge/burn for localized contrast. To what contrast grade these judgments calibrate is not even in my thought process. That is the core of split grade printing!

Cheers,
Geary

rob champagne said:
Just using G0 and G5 won't necessarily do it unless the negative is of perfect contrast for the required print.

That obviously creates a major problem in designing an LED light source for split grade printing because the mix of green and blue for any intermediate grade has to be calibrated to the specific paper and developer combination. That appears to be what the the heiland split grade head does although it is not an LED source, but it has to know how the paper will respond to any combination of Y+M or G+B to be able to work out intermediate values for starting point or burning and dodging.

srs5694 · Jul 23, 2008

galyons said:
Technically, one does not "dim" an LED. As you describe, PWM, (Pulse Wave Modulation), actually turns the LED off and on, but does so at a frequency that fools the eyes/brain into thinking that the light is less bright. So the apparent dimming is a result of less time that the LED is actually on.

The driver can modulate the duty cycle to say, as an example, 50% on, 50%. Again this is fine for fooling the eyes, but does it "fool" VC paper? I don't believe so. Isn't the net result the same as just cutting the exposure time by 50%?

AFAIK, aside from reciprocity issues, the two are equivalent, no matter the light source. After all, it ultimately just comes down to the number of photons that strike the paper. Certainly I frequently dim my light source and increase exposure time (or vice-versa) to get the exposure time into a range I want for dodging/burning or just to not be sitting at the enlarger twiddling my thumbs for minutes at a time.

If there's some difference in the results (contrast shifts, maybe?) from dimming vs. reducing the exposure time, I'd like to hear what these effects are. I've never heard of nor observed such effects, though. (I've also never looked for them in a systematic way.)

Steve Smith · Jul 23, 2008

galyons said:
Steve,
I think that this is core to the discussion. Technically, one does not "dim" an LED. As you describe, PWM, (Pulse Wave Modulation), actually turns the LED off and on, but does so at a frequency that fools the eyes/brain into thinking that the light is less bright. So the apparent dimming is a result of less time that the LED is actually on.

The driver can modulate the duty cycle to say, as an example, 50% on, 50%. Again this is fine for fooling the eyes, but does it "fool" VC paper? I don't believe so. Isn't the net result the same as just cutting the exposure time by 50%?

Yes, 50% on 50% off is exactly the same as turning it on for ten seconds instead of twenty.

The frequency of operation isn't too important for enlarger use as the eye doesn't need to be fooled.

My idea was to have a single control so that fully anticlockwise it was all green, in the middle it was equal green and blue and fully clockwise it was all blue.

Steve.

RH Designs · Jul 23, 2008

galyons said:
The driver can modulate the duty cycle to say, as an example, 50% on, 50%. Again this is fine for fooling the eyes, but does it "fool" VC paper? I don't believe so. Isn't the net result the same as just cutting the exposure time by 50%?

Yes it is - it reduces the volume of light hitting the paper by rapidly switching the LED on and off, so the total "on" time is reduced although the nominal exposure time is the same. You can also dim a LED by reducing the current flow through it in which case if the exposure time is the same as before, less light will hit the paper. The end result as far as the paper is concerned will be the same if the total volume of light hitting it is the same. I would doubt if reciprocity effects due to short pulses of light rather than continuous exposure would be significant at the sort of pulse lengths likely in this application - and in any case they could be compensated for.

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