LED enlarger head for B&W -- question in passing

[Ulophot]

The LED head that a friend is constructing in a custom 3D-printed housing for my Omega D2 enlarger to replace my clearly aging Zone VI fluorescent, finally reached first trial stage yesterday. Yes!

The light source is an RGB LED panel of 16x16 LEDs. I will use it only for B&W (all I do) and with Ilford contrast filters. So, my first test was to arrive at a preliminary Grade 2 color balance so that the filters would work normally. Once set, it will stay there, unless I find special cases in which altering it will be advantageous.

Realizing that Red would have little or no effect on constrast, I simply dialed it down to zero and adjusted the Blue and Green to render approximately the same contrast with or without the Gr. 2 filter in the light path. (I will borrow a Stouffer step wedge from a friend to hone the settings later this week.)

Here’s my question, just to make sure I’m not missing something. The light coming from the head is intensely blue, though it gives me Gr. 2 at about 2550 Blue, 140 Green. I infer that this is so, simply because I have removed the red component, which, in a tungsten bulb such as 212 would be making the light much more yellow. Theoretically, I could add back red with no effect on the paper other than to change the light color. The actual spectrum of the red LEDs could affect the contrast.

Have I got this right?

[Michael R]

The extent to which the red LED emitted spectrum extends into the sensitivity range of the emulsion (or vice versa, same thing) determines the extent to which exposure from the red LEDs will affect both exposure times and contrast.

A common issue with most consumer-grade red LEDs is the existence of "anomalous" emission spikes in the blue/green spectrum, which is why people sometimes get paper fogging with red LED safelights. You can test for that fairly easily and then determine whether or not using the red LEDs in your head will affect anything.

[Duolab123]

Beyond my knowledge.

[neil poulsen]

I use Ilford Warmtone VC paper, which I really like, with a Beseler 45s color head. My grade "2" contrast is no filtration. To increase contrast, I add magenta filtration and leave both Cyan and Yellow filtration dialed to zero. Correspondingly, to decrease contrast from my no filtration grade, I increase the yellow contrast to filter blue with both magenta and cyan contrast set to zero.

Any paper testing in which I engage is without filtration.

Who cares about a "grade 2" contrast? For me, it's irrelevant. I begin with one contrast (no filtration), and if I need greater contrast for that particular image, I add magenta. If less, I add yellow. If neither, then I begin my dodging and burning.

[koraks]

The light coming from the head is intensely blue, though it gives me Gr. 2 at about 2550 Blue, 140 Green. I infer that this is so, simply because I have removed the red component, which, in a tungsten bulb such as 212 would be making the light much more yellow. Theoretically, I could add back red with no effect on the paper other than to change the light color. The actual spectrum of the red LEDs could affect the contrast.

Have I got this right?

1: You don't need the red for B&W printing. It's plain and simple like that, so don't worry about it.

2: There's something very strange with your setup if you need more than an order of magnitude more blue light than green to hit grade 2. In my experience with LED sources, it's the other way around usually. This has to do with many factors, including spectral sensitivity of the paper (it's inherently more sensitive to blue than to green, because that's how VC works in the first place) and the so-called 'green gap'; the low efficiency of green LEDs compared to blue ones. So while this may work, I would be somewhat concerned if this were my project if the drive electronics are working as they should. Assuming a led panel with roughly the same amount of blue and green LEDs, in terms of nominal power, I would expect grade to to be at something like 10%-25% blue and the rest green (again speaking in terms of power output).

What kind of driver are you using and how is the power level modulated for each channel?

PS: also what neil poulson says above - who cares in the end if grade 2 is grade 2, or 2.2, or 2.5 or maybe 1.8. What matter is that you can somehow use the full contrast range that you need from the paper and that it has to offer. Don't lose too much sleep over the question if your grade 2 is the 'correct' ISO-R.

[Ulophot]

When will I learn to scrupulously proofread each post/email before sending? koraks, that 2250 should have been 250. This demonstrates that a mere 2 in the wrong place at the wrong time between friends can be more than it seems. Like all RGB systems with which I'm familiar, the scale for each hue component in my LED panel ranges from 0-255.

I can't answer your question about driver and modulation; that's why someone else is building this for me.

As for caring about Gr. 2, my approach is simple. The Ilford filters, as you may know, are based on tungsten illumination and designed so that exposure time remains constant for whatever tone they selected (I don't recall, but it's some mid-scale value) while changing between filters from 0-3 1/2. The 00 and 4-5 filters require double the exposure. This change -- made sometime in the mid-'80s, if memory serves -- represented a significant improvement over the original filters, whose factors were all over the place.

I have been using a coldlight with a Gr. 2 filter as my base for the past several years, which is more like a 2 1/2. It used to be standard practice (mine as well) to put a CC40Y in the path first to mimic the tungsten balance. I got lazy and have made do, but now that I have the opportunity to simplify my life and have the filters getting the light for which they were designed, I'm happy to align my set up with this.

Fine printing is, of course, an art, not an application of fixed "recipes." To each his or her own when it comes to what fits a chosen approach. For me it's lining up with Ilford's design. I am hoping that this new source will also solve my previously posted problem with respect to the lacking high-contrast end with Foma Classic; I haven; gotten around to testing that yet. Ilford WTF is my standard paper, and initial tests show it behaving very nicely with my new head.

[Michael R]

Philip, as you note, the grade spacing and speed matching of the filters are based on a specific incandescent light source. The issue you will run into in trying to replicate all of this with your LED head is that as opposed to an incandescent source, your LED head is not putting out a continuous spectrum, but rather some relatively narrow bands/peaks of wavelengths which you see as blue and green. This issue applies to some extent with any light source that differs from the light source for which the filters have been "tuned".

So, even if you get everything dialed in on the LED head to produce the equivalent of whatever you had been accustomed to getting with an Ilford grade 2 filter, all bets are off (to some degree) once you change filters. You might be close, but you might also end up with some different grade spacing and/or paper speeds at different grades.

As others have said what's most important at the outset is to make sure you are able to get the full contrast range you anticipate needing. Then you can "calibrate" (if required) as you go.

[Doremus Scudder]

Philip, as you note, the grade spacing and speed matching of the filters are based on a specific incandescent light source. The issue you will run into in trying to replicate all of this with your LED head is that as opposed to an incandescent source, your LED head is not putting out a continuous spectrum, but rather some relatively narrow bands/peaks of wavelengths which you see as blue and green. This issue applies to some extent with any light source that differs from the light source for which the filters have been "tuned".

So, even if you get everything dialed in on the LED head to produce the equivalent of whatever you had been accustomed to getting with an Ilford grade 2 filter, all bets are off (to some degree) once you change filters. You might be close, but you might also end up with some different grade spacing and/or paper speeds at different grades.

As others have said what's most important at the outset is to make sure you are able to get the full contrast range you anticipate needing. Then you can "calibrate" (if required) as you go.

Philip,

A couple of things:

First, Michael brings up a good point; all bets on grade spacing are likely off when you switch to LEDs. Still, I think it's a good idea to find a "middle value" that allows you the most flexibility in either direction, which is what I believe you are trying to do. So, use your arrived-at grade 2 setting, get that step wedge, and then make quick comparisons with different filters in place. Probably you don't need to use all of them, just, say, #2, #3.5 and #5 on the contrasty side and then #1.5 and #00 on the soft side. The object being to see if the contrast grades space fairly evenly and if you can get maximum contrast and maximum "softness" from your LEDs (it would be helpful to have a comparison for the extremes, so try an exposure with Wratten #47 and #58 filters respectively if you have them to establish those extremes).

Second, while your red LEDs might not be exposing the paper, they may come in handy for focusing and for viewing the image when dodging and burning. If you just use the blue and green components of your head, the image may be rather too dark to work with when you need a lot of contrast (think #5 filter). I'd dial the red up to max and do my tests with them always on.

If you want to see if you have any blue or green spikes in the output of your red LEDs, you can use a CD as a makeshift diffraction grating. Turn on just the red LEDs, hold the CD at an oblique angle to the light and observe the reflection, which should be separated into rainbow colors like a prism does to white light. If you see only red, then you're good to go. If there are some spikes of green or blue, they could, potentially, have an effect on the contrast when exposing. However, unless they are fairly intense/substantial, the effect is likely to be negligible. The test would be to make the same test at extreme settings with and without the red LEDs on and see if there is a difference in contrast (i.e., more or fewer stripes visible on the step wedge).

Keep us posted on your progress.

Best,

Doremus

[Pieter]

Just curious if a color temperature meter would be of any use gauging spikes or dips in LED light output?

[Duolab123]

Philip,

A couple of things:

First, Michael brings up a good point; all bets on grade spacing are likely off when you switch to LEDs. Still, I think it's a good idea to find a "middle value" that allows you the most flexibility in either direction, which is what I believe you are trying to do. So, use your arrived-at grade 2 setting, get that step wedge, and then make quick comparisons with different filters in place. Probably you don't need to use all of them, just, say, #2, #3.5 and #5 on the contrasty side and then #1.5 and #00 on the soft side. The object being to see if the contrast grades space fairly evenly and if you can get maximum contrast and maximum "softness" from your LEDs (it would be helpful to have a comparison for the extremes, so try an exposure with Wratten #47 and #58 filters respectively if you have them to establish those extremes).

Second, while your red LEDs might not be exposing the paper, they may come in handy for focusing and for viewing the image when dodging and burning. If you just use the blue and green components of your head, the image may be rather too dark to work with when you need a lot of contrast (think #5 filter). I'd dial the red up to max and do my tests with them always on.

If you want to see if you have any blue or green spikes in the output of your red LEDs, you can use a CD as a makeshift diffraction grating. Turn on just the red LEDs, hold the CD at an oblique angle to the light and observe the reflection, which should be separated into rainbow colors like a prism does to white light. If you see only red, then you're good to go. If there are some spikes of green or blue, they could, potentially, have an effect on the contrast when exposing. However, unless they are fairly intense/substantial, the effect is likely to be negligible. The test would be to make the same test at extreme settings with and without the red LEDs on and see if there is a difference in contrast (i.e., more or fewer stripes visible on the step wedge).

Keep us posted on your progress.

Best,

Doremus

Compact disc diffraction grating. Brilliant idea.

Wouldn't using a combination of green and/or blue LEDs be easier.???

Best Mike