Well, I have just made one using 3 orange LEDs with 625nm peak, driven by a simple current source (LM317) and will test if this plan works, but the reason why I'm so specific about this is that...
I'm in process of making infrared sensitive plates. I've designed a dye mixture that gives sensitivity in 700-850nm, and this mix should also desensitize below 700nm.
What this means is that I don't need an opaque filter, and I should be able to use a dark orange safelight with no deep red component. Conventional filter plus light bulb emits a lot of IR, so I can't use those. Orange LED is about the only thing I can think of that might work...
I think for most printing emulsions, making a safelight with orange + red LESs may make more sense, but for my application, just orange...
I'm using a red Festoon bulb - part# FS-B22-RD - and I also bought an orange one as well but haven't tried it as the red worked. That's with standard Ilford MGRC papers. I sat a sheet of paper on the enlarger easel with one half covered for twenty minutes. After processing both sides were the same base white. I don't normally take paper out of the box or paper safe until I am going to use it so that seemed good enough for me.
Yes, that would seem to be the case. Interestingly, that same Kentmere Bromide (from the same box) showed no fogging out to 30 minutes (where my testing ended) when the same test was run using my Thomas Duplex DUB low-pressure sodium vapor unit.
Originally Posted by fschifano
Whereas the OptiLED bulbs are supposedly rated for a spectral half-width of about 20nm, I seem to remember reading somewhere that the sodium emission doublet (at 588.9950nm and 589.5924nm) show a combined half-width of something like only 2nm, with the next closest emission at 568.8205nm being only 0.7% the intensity of the stronger doublet line. If true, that's pretty darn pure light and may help explain the difference in fogging times.
In any case, prints I make for eventual scanning and Web display are made under the red LEDs using Ilford MGVI RC glossy, as it scans much better on my older scanner. Keepers for display, however, are printed on the Kentmere Bromide under sodium light, which is so bright and cheerful it's almost stupid...
Ryuji - as pointed out 'red' LEDs often have a 625nm peak value, 'orange' seems to be 600-610nm and 'amber' is 590nm.
'Red' leds with a peak wavelength of 660nm are also available but seem to have a lower light output.
Ryuji, if the mix continues to desensitize below 700nm, why not use yellow or green LEDs ? (uncomfortable on the eyes ?) The 625nm peak LEDs might just have enough spectral bandwidth to fog the emulsion at 700nm. In fact make sure you drive the LEDs at about half their max rated continuous current otherwise the emitted spectrum will spread out more than you want.
Originally Posted by Ryuji
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In fact I'd be inclined to go all out and use RGB LEDs (making sure to use half the rated current and testing rigorously).
Originally Posted by PeterB
Safe, white light!
Btw good LEDs often have a data sheet which states the bandwidth (often at lower than max current) so you can have a better idea how safe they are...
Some LEDs have fogging problems and some don't. I've found a sheet of Rubylith needs to be placed over red LEDs to make them completely safe with all papers - without having to worry about the spectra of any particular LED.
Originally Posted by Ken Nadvornick
I haven't tried Amberlith in conjunction with orange LEDs, but it may work.
For an OC look you may want to use both red and orange LEDs to have a broader but still safe light source.
Of course, if bright enough, any light source will cause fogging.
I haven't actually tested the dye mix with emulsion yet. But based on the known properties of the emulsion and the dye, green will fog the emulsion very easily. Yellow is also close to the sensitivity limit of the emulsion.
Originally Posted by PeterB
Another reason for orange LED of 625nm is that it is probably safe for ortho dyes like simple diethylcyanine. OC safelight will fog those ortho emulsions.
Well, let's see what I find out with orange. If it fails, I may have to make a safelight with 3 switchable colors (yellow, orange and red) and pick one or two colors for each application I guess...
A kit spectrophotometer.
Here is an idea for a tool to help discern the spectral spread of various LEDs. A kit spectrophotometer.
Prior to doing a paper fogging test, you could screen a bunch of LEDs from different manufacturers and at different drive currents using the spectrophotometer. That way you could potentially see which were the most likely to suit your needs prior to doing the paper fogging tests.
I have bought one, and it is quite a novelty. You calibrate it by pointing it at a fluorescent light and lining up the scale to a specific green emission line at 546nm.
I set up 3 orange LEDs of 625nm peak in my darkroom, and did some fogging test. I drove them at 50mA (when abs max rating is 75mA), and it's soldered on lots of copper lands for heat sink. The LEDs were about 50 cm away.
First, I used silver iodobromide emulsion of daylight speed about 800. The safelight fogged emulsion very badly (not usable at all).
Martin from alt-photo list (which is a very good resource) warned me that those orange and red LEDs have undocumented green emission, so I put on a red gel filter (I buy a sheet of 20x24 and cut it out for various uses... such as covering up my iPod, Treo and sometimes MacBook when I work in darkroom). This filter has less than 1% transmission below 600nm, and only about 25% transmission at 625nm. With this filter, fogging is lighter but it still fogs... while regular OC safelight and sodium vapor safelight did not fog this emulsion as badly.
(Of course, I tested with fast emulsions, so this result may not apply to most printing emulsions, which are 100x slower than this emulsion.)
I think the problem is that this LED safelight is quite bright. Even emulsions without spectral sensitization have some red sensitivity if heavily sulfur sensitized (although this is far too slow to be useful for photographic purposes) and this may be playing a role here.
Lesson: even if you use LED for the light source for safelight, make sure to use it at a normal safelight illumination level. (how obvious!)