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  #11  
Old 24th June 2021, 06:21 PM
MikeHeller MikeHeller is offline
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Quote:
Originally Posted by Martin Aislabie View Post
Mike, how do you manage the spectral output of each of the LEDS, so they match the Y/M/C output of a bulb - or am I over thinking it ?

Martin
I suspect you are over thinking it. I am following just what has been said on FADU and that the use of Ilford MG filters work. I haven't tried printing yet using anything other than a Gr. 2.0 filter and can't say if other filters result in contrast variation as expected with the tungsten bulbs.

Watch this space!!

Mike
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  #12  
Old 24th June 2021, 06:34 PM
MikeHeller MikeHeller is offline
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Originally Posted by Alan Clark View Post
I think the best way to tell check out the brightness of your LED bulb is to make comparison prints.
I had a 75 watt tungsten pearl bulb in my Leitz Valoy 11 enlarger, and bought an 11watt LED bulb (2800K) to replace it. This was supposed to be the equivalent of 75watts tungsten. But it turned out to be brighter. With the tungsten bulb in place I made a print with a known negative. 26 seconds produced a nice print. When I switched to the LED bulb, and kept filters and Fstop the same, 20 seconds exposure produced an identical print.

Alan
Absolutely, but I thought I would initially see what would be demonstrated by using the enlarger light meter. Practice may give different conclusions.

Again, watch this space and I will get around to doing what you have done in due course.

Mike
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  #13  
Old 24th June 2021, 06:36 PM
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Bob Bob is offline
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White LEDS use a phosphor coating stimulated by a blue LED to generate white so their spectral output is different to a tungsten lamp. Tungsten output gets brighter as you go from blue to red and LEDs are somewhat more even with a peak around green falling off on both sides and a smaller peak at the blue end. So, again, it is probably a case of suck it and see with differences probably restricted to the equal exposure function of Ilford contrast filters going out the window, but still perfectly capable of getting all the grades. But that is just hypothesis based on spectral output graphs I have seen - I've not tried it.

[Edit: thinking about it, I suspect the grades obtained may drift as well as I think the filters are subtractive rather than additive (i.e. they pass the compliment colour rather than pass the active colour of the different hard and soft emulsions). As before, suck it and see seems to be the only practical answer...

Last edited by Bob; 24th June 2021 at 06:52 PM.
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Old 24th June 2021, 07:14 PM
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MartyNL MartyNL is offline
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Thanks Bob for the lumens explanation.

Mike, it could be an interesting exercise to see prints side by side revealing not only the difference in exposure time but also indeed if there is a shift in contrast.
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Old 25th June 2021, 11:28 AM
Terry S Terry S is offline
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Quote:
Originally Posted by MikeHeller View Post
I am following just what has been said on FADU and that the use of Ilford MG filters work. I haven't tried printing yet using anything other than a Gr. 2.0 filter and can't say if other filters result in contrast variation as expected with the tungsten bulbs.

Mike
Not so long ago Richard from Jersey did a comparison test of an enlarger, using as a light source both a tungsten and a 'warm' led bulb, across all of MG grades. All of them were achievable. Other than that, I can't remember exact figures given, but based on that I bought a B+Q Dial branded 'warm' led bulb as a replacement, should my current tungsten bulb fail and I am unable to get similar again. I must find time to do the same test that Richard did.

Terry S
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