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 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!
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 may well encounter problems using LEDs with VC paper as they are not "full spectrum" lamps.
Why would this cause problems? Please elaborate.
Silver halide are sensitized to two wavelength regions, blue and green, and you need to have those, and you will most probably have those. If you mean that sensitization dye absorption spectra would mismatch with LED spectra, I really doubt that. Neither are that sharp.
Using blue/green led arrays is naturally the easiest way compared to white-light source and dichroic filters. Mechanics are ruled out, it's electrically controlled, all you need to do is mix the light but that will be as easy or easier than in dichroic filter heads.
In fact it may work well enough even without mixing/diffusing but I would make sure it is mixed so that blue and green comes from same (all) directions.
Furthermore, with LED arrays it is possible to adjust separately exposure and also contrast at different locations! I have been thinking of for example 8x8 matrix for 6x6 film. This would work additionally to manual dodge&burn.
vedmak, have a go at it and report back how it works!
There are two problems when using LED light sources for photographic printing.
To do this with LEDs means pulsing the LEDs to vary the brightness and this consequently varies the exposure time required. It is like trying to expose with a lamp which flickers at a varying rate.
it does no harm to experiment.
Also, many white LEDs use a phosphor illuminated by a blue chip to emit white light, they are not a combination of RGB chips.
Discrete single chip LEDs emit light at a single wavelength . . . That the actual emitted wavelength from single colour LEDs is precisely that - one wavelength.
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.
Ilford publishes the spectral response curve for Multigrade IV papers in its data sheet, here.
AFAIK, blue exposure causes both blue- and green-sensitive layers to react in VC papers (in the other words, they have only-blue-sensitive and blue-and-green-sensitive layers).
Or even better, a 120v halogen lamp and non-fading dichroic filters![]()
Unfortunately, the curves for B and G sensitive parts have been combined to create a "total" spectral response curve.
However, we can deduce that 500-520 nm for green should be spot-on. Do you agree?
Typical cheap power RGB led has green at about 520 nm and blue at about 460 nm (an example: http://www.satisled.com/3w-rgb-led-emitter-on-star-1w-for-each-color_p234.html ). However as I have shown above, the bandwidths are not that sharp, thus the G and B in typical RGB leds almost overlap.
AFAIK, blue exposure causes both blue- and green-sensitive layers to react in VC papers (in the other words, they have only-blue-sensitive and blue-and-green-sensitive layers). Thus, if our green is "green enough" not to expose the blue layer, and our blue is "blue enough" to expose the blue layer, it should work and give us all contrast grades from 00 to 5. I said earlier as an educated guess that the green in typical leds is probably spot-on, and this seems to be backed up by the evidence.
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