Now that blue light-emitting-diodes are readily available at a reasonable price, I was toying with the idea of making a cold light enlarger head using them. A mixture of blue and green LEDs could make a controllable VC head for large format use.
Is there a commercial head on sale already using LEDs, I wonder?
Anyone else experimented with this idea?

-- Pete Andrews (p.l.andrews@bham.ac.uk), March 18, 2002

Answers

It's a neat idea with some advantages, but I think it would be difficult to actually put into practice. Blue LEDs are still immature products, and I don't think you could find any bright enough. I think the manufacturing tolerances are good enough these days that brightness and wavelength should be very close between individual units, so that's not an issue. However, you would need a couple of hundred to get a bright enough light source for an enlarger. Then you would have the problem of getting a truely diffuse pattern, unless you used a mixing chamber like a dichroic head, but then you would lose more light.

I built an LED safelight (590nM to match an OC filter) using a module from Lumex, it's 24 LEDs in a single package. They're very bright, but at a few feet you can still see the overlapping patterns from the individual lenses. I think they also had high-brightness reds, and maybe greens, but I don't think they had any blue ones yet.

-- Dave Mueller (dmueller@bellatlantic.net), March 18, 2002.

OK, I could be wrong. I checked Lumex's website, they have a blue pack. My yellow pack is 1200mCd and is very bright. They have two wavelengths of green, one is 525nM and is 5000mCd (LXC1282), and a blue at 470nM which is 2000mCd (LXC1287). When I was building my safelight, I emailed a few questions to their tech support and got a quick, complete answer. If you're serious, I'm sure they can help.

-- Dave Mueller (dmueller@bellatlantic.net), March 18, 2002.

I always bad-mouthed blue LEDs as being pretty useless. A couple weeks ago I got some from Digikey (Panasonic- back cover page) and they were fantastic. So bright they were painful to look at. It's an interesting idea- give it a try!

-- Conrad Hoffman (choffman@rpa.net), March 18, 2002.

I don't think diffusing the light from the LEDs is a problem Dave. It's not much worse than filling in the space between the strips of a cold-cathode tube, after all.

I was thinking of using 48 each of the hi-brightness blue and green 5mm LEDs, arranged in a 6 x 8 matrix to cover an area about 120mm x 160mm.
If I can get enough light out of the things, then the system should have a few advantages over both dichroic and fluorescent sources.
1) Instant start, with hardly any temperature dependence.
2) Very lightweight and compact.
3) Very power efficient.
4) The VC filtration will be remotely controllable from a box of electronics on the benchtop, therefore there's no vibration or movement of the head when changing filtration.
5) The brightness pattern of the LED array could be varied to compensate for corner fall-off of the enlarging lens.
6) A final refinement could be to add the facility for separately timed blue and green exposures (a variation of split filtering).

As already mentioned, my main concern, I think, will be getting enough light out of the LEDs. If I need a hundred or more of each colour, then the project starts to look a bit expensive.
I think I should aim for a minimum luminance of around 500 candela/m2 (~EV 12) at the surface of the diffuser. The area works out to be only about .019 square metres, which gives a figure of (500 x .019), ~9.5 candelas total light output needed.
I reckon I can easily get over 500 millicandela out of each LED, and allowing for the diffuser losses, I should be able to achieve my aim value at both wavelengths.
I may also need to add a few red LEDs, just to give the light a whiter appearance for focusing. I'm pretty optimistic after doing the above calculations. Now to look for the best price on the LEDs!

-- Pete Andrews (p.l.andrews@bham.ac.uk), March 19, 2002.

A few years ago someone in the NYC area began to manufacture an enlarger head using fiber optics. I never saw this device, but I heard some great things about it. The guy went out of business, for some reason, but not because of the quality of his product.

In any case, LED's sound like a great idea, but if for some reason you can't apply it to an enlarger due to cost constraints or whatever, you might look into fiber optics. Fiber optic materials have also fallen in price and offer many of the design advantages you've suggested for LED's.

-- Ted Kaufman (writercrmp@aol.com), March 19, 2002.

Ted is right, it was the Salthill enlarger and it was great, shame I could not afford it then. Anyway the light source was separate from the enlarger head and light was transmited via a fiber optic to a sort of plexiglass surface, very neat idea. BTW Zone VI has an LED safelight and it is very bright, but I think for them to cover more a 5x7 area they have used a lot of LED....

-- Jorge Gasteazoro (rossorabbit@hotmail.com), March 19, 2002.

Jorge, you're a fountain of knowledge! That's the enlarger I was referring to all right--Salthill.

I'm amazed no one bought the design when he went out of business, because from everything I have read about it, it was a jewel. Wish I'd gotten one when they were available. Oh well ... another of life's missed opportunities.

-- Ted Kaufman (writercrmp@aol.com), March 19, 2002.

Yes, Pete, there's lots of advantages if you can make it work.

About the fiber enlarger, there is a local company using large (50mm+) fibers for architectural lighting. I think a fiber head would share most of the advantages of LEDs except for the power efficiency. This company used halogen lamps in a weatherproof box, complete with fans. However, they generating a LOT of light. I got a quick tour one day, lots of neat ideas (moving displays, etc). I wish I could have walked out with a "sample" of the fiber. The big advantage for them was the fiber does not transmit any heat, so they were able to win a big contract with the local zoo to light the aquariums in the new reptile exhibits. They like it warm, but I think a 300W halogen would be a little too much. I wonder if 50mm would cover a 35mm frame...

-- Dave Mueller (dmueller@bellatlantic.net), March 20, 2002.

Well, all I know is that the Salthill enlargers were so good most people seem to keep them, I have never seen one on E bay or any other selling venue. Of course now that I am saying this probably there will be a flood of them tomorrow...:-))

-- Jorge Gasteazoro (rossorabbit@hotmail.com), March 20, 2002.

Hmmm. The thing that puts me off about fibre optics is the size and weight of the bundle needed to cover a 5x4 negative. There are lots of other clever light-pipe arrangements around these days though.
I have a tranny adapter 'light lid' for a flatbed scanner, and that gives a very bright, pure white, and perfectly even light. When I took it apart (like you do), it was just two very thin fluorescent tubes strapped to opposite sides of a flat panel of plastic. I still can't figure out how the light is distributed so evenly over the surface of the thing. There's no angle on the plate, it's only about 6mm thick and it seems absolutely parallel sided.
Well, there's another thing to throw in the mix. If I replace the two tubes with two rows of closely spaced LEDs, will it still work, I wonder?

-- Pete Andrews (p.l.andrews@bham.ac.uk), March 21, 2002.

I think it would work Pete, isn't the way they are doing the new slim viewing tables the same way you mention? last one I saw it was just a small fluorescent tube on the side of a white plexiglass....and it works great, I guess you could put the blue LED's on one side and the green on the other...and mix the light with a rheostat....no?

-- Jorge Gasteazoro (rossorabbit@hotmail.com), March 21, 2002.

Just came across this, I have made an led VC enlarger head for my Kaiser. I put it all on the web, www.huws.pwp.blueyonder.co.uk/leden.htm

It only uses 4 leds!

-- Huw Finney (huw@huws.org.uk), May 12, 2002.