using blue/green led arrays as an alternative to cold light

[hrst]

It's the other way round.

All of the emulsions are sensitive to blue and some are sensitive to blue and green.

That is exactly what I said and I know it very well.

 

[Steve Smith]

That is exactly what I said and I know it very well.

Looking back, you did say that but you said the reverse a few lines earlier!


Steve.

 

[RH Designs]

we can deduce that 500-520 nm for green should be spot-on. Do you agree?

Yes, green is not a problem for an LED enlarger, although I do have a couple of RGB LEDs in which the blue is correct at 430nm but the green wavelength is too long at 560nm. 525nm LEDs work very well. Normal 470nm blue LEDs won't achieve much better than about grade 4 on Ilford MGIV without extra filtration. I've not been able to find any easily available 430nm power LEDs as yet.

 

[ic-racer]

Or even better one to three 120volt, correct wattage, correct color temperature halogen bulbs behind either RBG or CMY Dichroic filters that project into a Styrofoam lined tube with two Plexiglas diffusers.

Sounds like the makings of a good thread (with pictures !)

 

[hrst]

Or even better one to three 120volt, correct wattage, correct color temperature halogen bulbs behind either RBG or CMY Dichroic filters that project into a Styrofoam lined tube with two Plexiglas diffusers.

It doesn't make sense to build RGB filtered additive color light system with halogen bulbs anymore. They were used when power LEDs were not available, but you can build exactly the same concept you describe but replace the halogen bulbs with LEDs and:

1) Get the same light output at 1/10 power or even better in case of blue
2) No need to EVER change bulbs
3) Quick on/off time (relevant in some cases, e.g. with very short exposures)
4) In case of Wratten or other dye filters that can fade, or in case of light leak around the filter, non-compromised or only very little compromised performance, versus complete catastrophe with halogen bulbs. Probably no need to change Wratten filters ever. Actually, as said before, you can probably omit the filters altogether and get very good results without filters at all!

You are instructing an obsolete technique that was used when there was nothing better available. Halogens still work today very well, but times change and I'd suggest using the best available technique that has some additional benefits. This is of course true only when we are talking about new designs. Updating old systems can be more time-consuming than just maintaining them as they are.

 

[ic-racer]

Halogens still work today very well, but times change

Where are the 500W to 2000W equivalent output LEDS that run off the mains? I'll pop one in my enlarger and check it out...

 

[hrst]

Where are the 500W to 2000W equivalent output LEDS that run off the mains? I'll pop one in my enlarger and check it out...

I was talking about additive with relatively narrow passbands, so unless you misspelled "one" instead of "three", you are talking about a different beast. You also seemed to miss the important words right after where you stopped the quote.

I would suggest around 20 to 100 watt LED units to replace the 500-2000 W range in an additive system. You need a transformer, though. A side effect is that you can easily build or buy a stabilized power supply to get predictable and even results.

See Ebay for cheap prices, or www.satisled.com for wholesale.

I am not sure if very high power units are available in the exactly desired wavelengths, though.

Let me also remind that the original question was using LEDs. Answering with "halogens are better" is clearly incorrect, even though I admit that there still is some usage for them even in new designs. I would, however, use halogens primarily in illuminating "continuous wavelength" subjects, i.e. photographing natural objects. Printing, on the other hand, can benefit from NON-continuous spectrum. This is exactly the reason why some people have preferred additive systems (with sharp-cut filters) instead of CMY filtered white light sources; but the usually cited problems of additive systems are related to the problems of using incandescent light sources in them, making them big, heavy, high-power, low light output, with quickly wearing filters. These problems are now solved thanks to LEDs.

Times change and LEDs provide excellent printing properties especially in additive systems. An additive halogen system was claimed better than additive LED system which clearly can now be considered untrue in most cases because of the reasons I listed above.

And, if we come back to the original question of multigrade B&W; if the LEDs are selected correctly, all filters can be removed from the system, simplifying the optical path and increasing light output; and contrast grades will work simply with electronic control. I would suggest adding red for paper placement / focusing aid and for future color work even if the OP wouldn't do color right now.