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

[vedmak]

I was wondering if anyone tried to use an array of super bright LEDs to print and could you share you experiences, tips, etc?

[guitstik]

Search: http://www.apug.org/forums/forum43/2...-darkroom.html
http://www.apug.org/forums/forum43/2...ht-source.html

The search feature is handy.

[rmann]

Why not white and VC filters - much easier to assemble - do a google search for fish tank lights - there are a lot of people working on LED set ups for fish tanks and a few companies sell DIY kits.

[domaz]

Why not white and VC filters - much easier to assemble - do a google search for fish tank lights - there are a lot of people working on LED set ups for fish tanks and a few companies sell DIY kits.

You will cut output using filters. That said if you used a bunch of five-watt white LEDs it probably won't matter much. Using filters with the typical LED lighting kit is probably going to result in bad printing times though.

[onepuff]

You may well encounter problems using LEDs with VC paper as they are not "full spectrum" lamps. Each chip emits light at a specific wavelength which when pulsed by the controller to vary it's brightness and is mixed with the output from other chip(s) gives light at a given colour temperature. Unlike full spectrum conventional incandescent or TH lamps they do have significant gaps in their spectrum so will not render correct exposure results when using photographic paper. In this respect they present similar limitations to HID lamps. That isn't to say you can't experiment!

[rmann]

You may well encounter problems using LEDs with VC paper as they are not "full spectrum" lamps. Each chip emits light at a specific wavelength which when pulsed by the controller to vary it's brightness and is mixed with the output from other chip(s) gives light at a given colour temperature. Unlike full spectrum conventional incandescent or TH lamps they do have significant gaps in their spectrum so will not render correct exposure results when using photographic paper. In this respect they present similar limitations to HID lamps. That isn't to say you can't experiment!

Most LED manufacturers have web sites where they give the spectrum data - you can select a white LED that will give you both the green and blue you need for VC filters. Getting a set of filters is a whole lot cheaper and easier than building the control & timing circuits you would need to work with both a blue array and a green one - unless you are going to do only split printing. If you search the treads, I think you will find a number of attempts at going with blue / green LEDs. Not all of them end with a working unit.

It probably depends on your skill with circuit design and the amount of time you are going to invest which way you end up going.

[rmann]

Check the fish tank guys - they do very bright as they want to keep coral alive in their tanks without adding extra heat - they want it to be like sunlight. Their DIY kits are not the typical kind that are used for household lighting.

[konakoa]

I've made two homemade LED heads for large format 4x5 film using white LEDs. They work well and compare favorably to a Aristo cold light I have. I've got postings here on APUG about the LED heads I made and a website with how-to instructions.

Green and blue LEDs sound great in theory but don't work out so well in practice. I bought several discrete green and blue LEDs and tested them for VC work. Basically, what I found is green LEDs work great for low contrast and don't have any issues with VC paper. However, there are problems with blue LEDs. To the eye, they're an intense blue color--but the hitch is they're actually a combination of blue and green.

The slight green color hidden within the blue LED spectrum reacts with the low-contrast emulsion in VC paper and limits the VC papers I tried to about a grade 3 to 3 1/2. Hard contrast grades like 4 or 5 just can't be done with blue LEDs alone unless the green bias in the blue LEDs is filtered out somehow.

It's worth mentioning as well that the green and blue sensitive emulsions in VC paper do not have an equal response to green and blue light. Blue prints and builds up density very quickly and green takes much more intensity and time to reach a similar density. If you change the green to blue ratio even slightly, the the density will vary greatly. Be prepared to go through a lot of test strips and work prints.

I did a lot of step wedge testing in the darkroom with several LED colors and even made a spectrometer to study the discrete LEDs to come to these conclusions.

White LEDs with a VC filter set are much, much simpler to work with.

[ic-racer]

Why not white and VC filters - much easier to assemble - do a google search for fish tank lights - there are a lot of people working on LED set ups for fish tanks and a few companies sell DIY kits.

Or even better, a 120v halogen lamp and non-fading dichroic filters

[silveror0]

konakoa,
Can you please enlighten me a bit regarding "white" LEDs? What is the color temp of "white"? The reason I ask is that after reading Ilford's website info on the lightsource for their VC papers, they say that their filters will work OK with any incandescent source. Incandescents are about +/- 2700 deg-K. Incandescents are also going to become harder and harder to find in the near future, or so I hear from retailers who are stocked to the ceilings with CF bulbs as the current trend.