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  #11  
Old 16th December 2023, 07:06 PM
MattKing MattKing is offline
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Quote:
Originally Posted by Jolyon View Post
By the way does anyone know whether LED bulbs react differently to filament bulbs in a basic condenser enlarger (thinking of the way the paper reacts to the light spectrum, if that makes sense).
The spectrum of light emitted by LEDs differs somewhat from tungsten filaments and, perhaps more importantly, that spectrum tends not to be continuous - there are "spikes". And different LEDs have different spikes.
Fixed grade paper has sensitivity to certain parts of the spectrum - and those parts may or may not match the peaks or values of that spiky LED spectrum. Variable Contrast paper has a more complex spectral sensitivity than fixed grade paper, so its usability with LED sources is also a more complex question.
With my enlarger, I use a diffused, variable contrast light source that is designed to work with a 100 watt halogen source. I have some 50 Watt equivalent substitutes for the 100 watt halogen bulbs that work with variable contrast paper, but the contrast behavior is quite different from the halogen bulbs - the contrast steps are much less evenly spaced, and I can't quite achieve the same maximum contrast.
Different LEDs may or may not behave better, but there is no reliable way to tell other than to buy them and try them.
One further caution: the precise shape and position of the light source is quite important with condenser enlargers. So even if the LED gives you the right light, it may still be a problem if it isn't the right shape.
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  #12  
Old 16th December 2023, 09:21 PM
Jolyon Jolyon is offline
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Thanks, that gives me food for thought. I could set up some testing using the F30 heads and swapping condenser and diffuser between the two heads with the same lens and 35mm negative. That gives me four potential configurations without too many variables changing.
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  #13  
Old 17th December 2023, 09:17 AM
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Bob Bob is offline
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Just to add to Matt's exhaustive observations: one additional thought is that tungsten lamps operate at the warm end of the spectrum (a colour temperature in the 2700-3200K range), so it's possibly better to start experimenting with warm-white LEDs as cold-white ones will have a lot of blue in their light and that may cause difficulties obtaining lower contrasts as blue light increases contrast in multigrade paper. However, as Matt points out, "white" LEDs have a discontinuous spectrum, much like fluorescent lamps do, so it's more complicated than just aiming for a specific colour temperature .

Also, now I think of it, LED lamps with a high CRI (Colour Rendering Index) of over 90 (and preferably over 95 if available) would possibly help, but I'm not sure if normal household style LED lamps are generally sold with that information included.

Unfortunately, I have only experimented (briefly) with green and blue LEDs, so only have general principles to offer...


Good luck!

Last edited by Bob; 17th December 2023 at 09:21 AM.
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  #14  
Old 17th December 2023, 12:31 PM
Terry S Terry S is offline
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Some interesting comments have been made by Matt and Bob, who obviously know much more about the subject than I, as much of it flies way above my head - Whhhhooooossshhh!!!

But this thread immediately reminded me of a set of posts from a while back that may be of interest to Jolyon and others who may not have read it at the time:

http://www.film-and-darkroom-user.or...+wattage+bulbs

Terry S
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  #15  
Old 17th December 2023, 03:49 PM
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Welcome Jolyon and enjoy!
Congratulations on your new enlarger. Have fun!
Frank
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  #16  
Old 17th December 2023, 07:03 PM
MattKing MattKing is offline
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Quote:
Originally Posted by Bob View Post
Just to add to Matt's exhaustive observations: one additional thought is that tungsten lamps operate at the warm end of the spectrum (a colour temperature in the 2700-3200K range), so it's possibly better to start experimenting with warm-white LEDs as cold-white ones will have a lot of blue in their light and that may cause difficulties obtaining lower contrasts as blue light increases contrast in multigrade paper. However, as Matt points out, "white" LEDs have a discontinuous spectrum, much like fluorescent lamps do, so it's more complicated than just aiming for a specific colour temperature .

Also, now I think of it, LED lamps with a high CRI (Colour Rendering Index) of over 90 (and preferably over 95 if available) would possibly help, but I'm not sure if normal household style LED lamps are generally sold with that information included.

Unfortunately, I have only experimented (briefly) with green and blue LEDs, so only have general principles to offer...


Good luck!
It's best to be cautious if you intend to rely on CRI. If you really drill down into the weeds you will discover that CRI is better related to how humans perceive light - not how things like photographic paper respond to light. In essence, you can raise the CRI of a light source while still leaving important spikes and valleys in the spectrum emitted.
There are other, much more complex standards employed in places like the motion picture industry that are more likely to correlate with things like photographic paper sensitivity - or for that matter the not to be referenced here film digitization world - but it is very difficult to obtain information that includes ratings under those standards, unless one visits the much, much more expensive marketplaces that service the commercial motion picture industry.
One really good continuous spectrum source - which was employed with mixed success by Beseler in some enlarger light sources - is electronic flash.
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  #17  
Old 17th December 2023, 11:35 PM
Jolyon Jolyon is offline
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Thank you all for all this input. It really is very useful. I have also been researching a bit on the 'net and found a couple of pieces on spectra of all sorts of lamps, a bit general but at least when someone talks about CRI I now know a bit of what they're talking about. It's true that most of the general information is aimed at how the human eye perceives the light and not how a machine (spectrometer or whatever) might register it.
Interesting about Diall led bulbs (at least for me). Diall stuff is the house label of Castorama (part of the same group as B&Q perhaps) although there is no guarantee that Casto's Diall stuff comes out of the same factories as B&Q's. Ithink I am using a cold led at present which appears to have a more spiky spectrum than a warm led. It's also confusing that"warm"leds have a lower light temperature than "cold" ones.
I am not going to progress with this much until the beginning of march since I will be away from home (and in the UK) doing family stuff. I will probably start a new thread (or take any suggestions to continue in an existing thread if that makes more sense) when I have something worth posting.
I am hoping to escape my duties for a week-end and get down to the meet in Dartmoor but nothing is certain yet. I might not know until the week before if it's going to be possible. I'll take some prints with me just in case.
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  #18  
Old 18th December 2023, 10:03 AM
alexmuir alexmuir is offline
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Hi Jolyon, and welcome to the forum.
I have attached pictures of an LED bulb I have used successfully with Ilford Multigrade paper and their filters. It came from Tesco. I think you may have them in France, or you could look when in the UK. Iím not sure what the equivalent wattage is, but it gives fairly short exposure times for 35mm when installed in a Leitz Valoy II condenser enlarger.
Alex.



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  #19  
Old 18th December 2023, 12:03 PM
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BuzzNL BuzzNL is offline
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I've done some extensive testing with LED bulbs in the past, including measuring and comparing the color spectrum, edge falloff on the baseboard and doing some real-world tests on all grades using Ilford filters.
In short, warm white LEDs worked the best for me. Cold white have much more blue than warm whiter LEDs (and the original tungsten bulb ofcourse), slightly messing up the contrast filter spacing.
Not dramatically and still very usable, but warm white worked better with my filters and papers (I tested Ilford MGIV RC and Fomaspeed Variant).

Even between different warm white bulbs there can be a great difference among them but it's definitely possible to get good results with decent contrast spacing if you use VC paper with an LED bulb.
With all bulbs startup time was no issue and test strips vs final print were at least equally close compared to a normal tungsten bulb.

I also use the LED bulb for graded paper and that gives me good prints too but I have not done any proper contrast testing with that as I only have a limited amount of graded paper, not really worth it to do proper testing in my case.

I've found the cheapest warm white bulb to work best in my setup by the way. I bought it a couple years ago at the Dutch discounter "Action", of which I've also seen stores in France. It's one of those E27 bulbs in a green half-open cardboard box.
That was an 11.5W Warm White bulb, cutting my exposure times roughly in half compared to the original Osram 60W bulb.
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  #20  
Old 18th December 2023, 03:42 PM
JOReynolds JOReynolds is offline
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bienvenue chez FADU
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