Green Safelight

I'm in the process of setting up a darkroom to develop glass holographic plates. They are red-sensitive, and you're supposed to use a green safelight. Well, I figured that the best green monochomatic light source would be an LS! I bought a green (I forget the nm)LS from Mark Hannah, mounted it on a heatsink in a box with a LM317 set for max current of 390mA. That way I can drive it with a variable voltage lab PS and never exceed 390mA, so I can dim it. My question to the group is, is it truly monochromatic? I can't have any red light in the room while processing the plates. If I catch a reflection of the green light off a CD (poor mans diffraction grating) I see a rainbow. Does this mean the LS is putting out more than one color? I didn't think it was possible.
Thanks!
Rob

The LED *is* producing a broadband output that reaches all the way into the deep red, but 95+% of that emission is in the green, centered around 525 or 530nm. That's why it looks green to the eye.

The CD (poor man's diffraction grating) is showing you the truth.

Virtually all LEDs - even blue and violet ones - will produce a small amount of their output in a wide band away from the LED's peak emission wavelength.

A typical InGaN green LED's spectrum would look something like this:

The numbers at the bottom are "nm" or "nanometers". Green light would be between about 510nm and 570nm.
The tall "peak" you see is the LED's green light, peaking around 530nm or thereabouts.

So, LEDs are not monochromatic! I must be thinking about lasers, I used to play quite a bit with HeNe lasers and I know they are monochromatic. Looks like I'll have to try the safelight (cranked way down to 50mA or so) and see if it fogs the plates!
Thanks!
Rob

Based on Craig's graph, I'd feel pretty confident saying that the "greenness" of this light is more green (and less of everything else) than an old Wratten safelight filter over a dim incandescent bulb. In other words, the LED is not as monochromatic as a green laser, not as monochromatic as the low-pressure sodium bulbs used for b&w paper, but still pretty dang good compared to anything using organic dyes.

I don't know what these consarned youngsters are using for holo plates these days, but they (the plates) used to be pretty slow to get enough resolution to record all those tricky interference fringes-- they couldn't have got so fast that a properly dimmed green LED would be troublesome, could they?

Perhaps a good old safelight test is in order.

And thanks to Craig for the graph. Very illuminating.

Walt Brand

Re: Green LED Safelight

I don't think you'll fog up the plates with that small a current through your LS, but you never know. /ubbthreads/images/graemlins/icon15.gif

If you fog up plates at 50mA, you'll probably have to get a 530nm dielectric interference filter from Edmunds Scientific (Industrial Optics division), and stick that in front of your LS. Then you ought to be able to crank your LS current up.

HeNe lasers (632.8nm orangish-red) are reasonably monochromatic, though you'll still get a bit of a bluish color out of the plasma and into your beam. That amount is so minor in comparison to the laser beam though, that you usually don't have to factor it into your experiments.

Same with green HeNe lasers - the 543.5nm light is the vast majority of light that comes out, even though a magenta (red and blue) light from the plasma is present in VERY SMALL amounts.

I'd imagine the same would also apply to yellow and orange HeNe lasers, though I have neither seen nor experimented with these colors.

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heya will they ever make 'WHITE
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lasers??

Walt- Yes, the plates are pretty slow, especially because Agfa stopped making them and we use a Russian plate now with 1/10 the sensitivity. My only concern is with long exposures, over 30 seconds, during split beam reflective shots. Single beam short 6 second exposures shouldn't be a problem with the safelight.

Craig- I guess only a test will show if it fogs the emulsion. My plates are coming in next week, I'll expose one and process it it and see if it fogs up!
Thanks,
Rob

Re: Green LED Safelight

Craig, how would a green semiconductor laser graph? Would the "q" of the spike be greater by, say, an order of magnitude?

Walt Brand

Thanks for the fascinating information (as usual), Craig. I was another who for some reason just "assumed" that LEDs were much more monochromatic than they appear to be.

I notice there is no scale on the vertical axis of the chart you presented - any idea what the power ratio represented might be? (I'd understand it best in dB, if that makes any difference in choice of answers ... ) I suspect it to be a fairly large difference, but I'm still surprised. /ubbthreads/images/graemlins/smile.gif

[ QUOTE ]
wualta said:
Craig, how would a green semiconductor laser graph? Would the "q" of the spike be greater by, say, an order of magnitude?

[/ QUOTE ]

Hi Walt,

A green semiconductor laser should show a really sharp spike at 532nm, and a weak spike at 808nm, and pretty much nothing else. If the spectrometer went even farther into the infrared, you might see another weak spike at 1064nm. But by a very large amount, the green spike would outpower the two IR spikes.



[ QUOTE ]
Tomas said:
I notice there is no scale on the vertical axis of the chart you presented - any idea what the power ratio represented might be?

[/ QUOTE ]

Hi Tomas,

That, I don't know. I think that is divided in 10% increments, but I honestly don't know. Looks like a linear scale to me, but I'm not certain about that. /ubbthreads/images/graemlins/tongue.gif
(There. I said it three times.) /ubbthreads/images/graemlins/grin.gif

(There. I said it three times.) /ubbthreads/images/graemlins/grin.gif

Must be true then, eh? /ubbthreads/images/graemlins/smile.gif

Thanks!

Old photographer's trick, lay some coins on the plate during test exposure, develope in total darkness. No coin shadows, everything OK. Fog every where except where coins were means bad safelight. Fog where coins were means bad/old plates or developer problems.

The LED is going to be a LOT more monochromatic than a light bulb with a filter, unless you use a very expense dicro. filter

Re: Green Safelight (Update)

Well, the Green LS fogged the hologram plates, even when cranked WAY down. Looking at it again reflected off a DVD, it has quite a bit of red in the output. I spray painted a 2 1/2" square glass plate with Testors Emerald Green transparent paint and mounted it over the LED and the red reflection in my "poor man's diffraction grating" is gone. Still shows a bit of yellow and blue with the green, but this emulsion isn't that sensitive to yellow. The red emitted from the LED exposed the plates just like white light would! There are reports of people sucessfully using 5MM Green LEDs for safelights, I guess they work because their output is so weak compared to an LS.
More plates arrive Thursday - this time I cut one into 4 pieces, drop a piece in the developer tray and turn up the LS - see if it turns black.
Rob

Re: Green Safelight (Update)

I figured the plates *might* fog up if they are sensitive to red, but there was no real way to tell if they'd fog up at 50mA of LS current or not in your particular setting.

I don't have a green LS or a DVD, so I shined a cyan LS onto a Slayer CD, and still saw the red band. Even a royal blue LS has a small amount of red in it. Probably not enough to fog up photographic or holographic plates, but it's there.

Re: Green Safelight (Update)

Yeah, they were extremely overexposed by the green LS. I got one halfway decent hologram out of 6 tries, developed for only about 15 seconds (turned black real fast) then bleached for longer than normal. It's dim and obviously has poor brightness. I'll try again Thursday night with the green filter over the LS. I still can't understand why an LED isn't monochromatic! These plates (2.5" x 2.5") cost about $3.50 each, I'm practicing on them before I graduate to 4 x 5.
Rob