DIY enlarger LED head calibration

People designing their own enlarger LED head might be interested by this post I wrote in the "smoking solder iron" group about LED head calibration.

(there was a url link here which no longer exists)

Dom

stormpetrel said:

People designing their own enlarger LED head might be interested by this post I wrote in the "smoking solder iron" group about LED head calibration.

(there was a url link here which no longer exists)

Dom

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So that's what digital imaging is all about :confused:

Nothing to do with digital imaging...

With all that data you could have a feature with a probe that you could put on a test print to measure a value. That value could then be the value that won't change when you increase or decrease contrast.

Yep, this will be one of the next upgrade (detailed at the end of the thread).

Nice work!

When I built my LED enlarger head, I decided that I didn't need to know what actual grade it represented. Just more or less contrast.

I had plans to build two PWM power supplies with a control which varied from full green at one end to full blue at the other... but like a lot of my ideas, I haven't got round to it yet!


Steve.

stormpetrel said:

People designing their own enlarger LED head might be interested by this post I wrote in the "smoking solder iron" group about LED head calibration.

(there was a url link here which no longer exists)

Dom

Click to expand...

why would the process be different bedifferent than any ither light source calibration?

Steve Smith said:

When I built my LED enlarger head, I decided that I didn't need to know what actual grade it represented. Just more or less contrast.


Steve.

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for quality printing you want to knowthe actual grade and it isn't difficult to do

RalphLambrecht said:

why would the process be different bedifferent than any ither light source calibration?

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He calibrated for a whole series of different gray values (rather than a single middle gray of the standard Ilford filter set).The possibility exists to pick any value in a test print to remain unchanged during a contrast change. To do this with standard color head calibration one would need a whole series of different tables.

As you control the blue and the green light intensity independently, it is possible to take advantage of this. Ic-racer perfectly got it. You can choose your speedpoint, your grade just by pushing a button without changing your exposure time.

Of course it would be also possible to set the green & blue level at a fixed intensity and use the head in a more classical way (multigrade filter + adjusting the exposure time).

RalphLambrecht said:

for quality printing you want to knowthe actual grade and it isn't difficult to do

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For repeatibility, I would want to know where my control was set or what the blue to green ratio was but I don't need to know how that translates to an actual grade.


Steve.

Steve Smith said:

For repeatibility, I would want to know where my control was set or what the blue to green ratio was but I don't need to know how that translates to an actual grade.

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It is exactly how I used the head at the beginning. I had a slider button which allowed me to adjust the light from full green to full blue. I could change the grade this way and it was repeatable indeed but I had to correct the exposure time everytime I changed of grade. Adjusting the grade this way results in a variable speed point. I presume with a bit of experience people could anticipate the time correction to apply when changing of grade but I preferred to have an easier solution to control the grade with my VC LED head.

bernard_L said:

Nice work!

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Thank you Bernard!


As you already know, the speed point defines the tone which are not affected when changing of grade (at same exposure time). Conventional BW enlarger (dichroic color head, VC head, multigrade color filters) have their speed point in the mid-tones to comply with the ISO standard. As RalphLambrecht mentioned it, it is possible to change the speed point for some of those enlargers with some calibration efforts.

You can live very well with a variable speed point or a speed point placed in the midtones as 99% of people do, but you could also find useful to place your speed point in the highlights for example.

I could have calibrated the enlarger following Ralph’s method but it is more straightforward to “map” the exposure by changing the green intensity and the blue intensity. The process if fully automated now, and it takes 2h to do a full calibration for a new paper (in fact it just takes 15min of my time in the darkroom to develop the test strips, and 5 min in front of the computer to generate the calibration curves for the enlarger).

At this stage I have limited the choice of speed point to few zones (from zone VIII to zone V), but I could do it for any zones.

As Ic-Racer mentioned it, it would be possible to set the speedpoint with a probe placed under the enlarger.
It would be also possible to use such probe to place your zone, and the software would then propose grade + exposure time solutions to achieve your target.
I’m working on a such probe at the moment (colour probe)...

Another advantage of this VC LED Head, is the possibility to change the exposure by increasing or decreasing the light intensity by 1/4th fstop.

APUGuser19 said:

Very scholarly. Your computer does nice work. It certainly wouldn't be a easy thing to do to knock Ralph Lambrecht out of the data books as a source. You're a better man than I am. I'm lazy. I use the Ilford info for a Beseler 67 Dichro. I pick #2, which should put me in the ballpark of Kodak, and go from there. In the end, it's all Eastman Kodak standard, as far as I know. Just makes things easier to understand. #2 is #2. I know there were NAB standards on a tape recorder. Not sure if there was ever anything higher than a Kodak standard on paper grading.

Edit: Pardon me, I'm not being obtuse and critical. I've looked over the work and it's good work. Graph linearity is always subjective, so that has to be taken into account. But I've not moved past my irked and annoyed attitude most any paper made today is a polycontrast. It's not easy hitting the deck square and straight on a carrier in a typhoon. I wish they'd bring back #2 and forget this VC nonsense.

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VC paper is no nonsense but a tremendous opportunity to increase creativity.fixed paper grades always were a rigidhurdle. VC paper is great and if you preferfixed grades, you can always calibrate your VC paper to give you any fixed grade.