Simple LED safelight for color RA-4

Rafael Saffirio · Aug 7, 2020

I did a very simple led safe light with some orange LED and a voltage regulator set to 1.8
3 tests:

Single Led as is, the light was like a directed spotlight the paper was fogged in 60 second
Single LED, sanded so the light was more diffuse, no fog at 270 seconds
Two sanded LED, more light but the paper was fogged at 90 to 120 seconds

1 LED Sanded

2 Led Sanded

One LED not sanded

Donald Qualls · Aug 7, 2020

For RA-4 paper, you would surely be ahead to use an LED closer to the paper's sensitivity minimum at 475 nm. That'll be more of a greenish yellow than orange. I've seen individual LEDs similar to those you used that are listed as having their peak at 475 nm -- no idea what they cost, or how wide their emission band is, but if they're the small ones like those, they're probably under a dollar each. Worst case, they might need filtration to narrow their emission peak to get a brighter room with longer safe time.

DREW WILEY · Aug 7, 2020

I use a tiny Jobo Minilux orangish LED device very briefly for aligning big rolls of Fuji RA4 paper onto my roll cutting table. It's still several feet away, and on no more than about 20 sec at a time. Otherwise, total darkness.

wiltw · Aug 7, 2020

The official Kodak safelight for color paper is very, very dim brownish amber, just about requiring a number of minutes for your eyes to acclimate to their low level of light. If you can shut off the white light, and almost immediately be able to see in the 'safelight' it is TOO BRIGHT for color...the brightness might be suitable for B&W paper, but not color!

Ordinarily mine would shine upward to the ceiling, for indirect lighting of the area below. But when it was time to pull out a sheet of paper and put it into the easel, the safelight was turned off...until the exposed paper was safely in the Jobo developing tank.

DREW WILEY · Aug 7, 2020

I found either of the recommended Kodak filters way too dim for any practical purpose, except perhaps telling you the orientation of the room.

pentaxuser · Aug 7, 2020

I didn't think that in LEDs there is, as yet, a bulb that gave safe and reasonable illumination for RA4 that compares with the sodium lamps. Unfortunately I cannot work out how bright to the eye the 270 secs set up is. What is involved in making a LED sanded?

Thanks

pentaxuser

Rafael Saffirio · Aug 7, 2020

pentaxuser said:
Unfortunately I cannot work out how bright to the eye the 270 secs set up is. What is involved in making a LED sanded?

with one Led I have to wait between 5 to 10 minutes to see enough. the sanding part it's simply pass the led over the sandpaper a few times.

I will use this light to roll paper for rolls in the opposite direction to make it flatter and the cut it. For regular printing I don't need it

Lachlan Young · Aug 7, 2020

I think 590nm is the wavelength you need.

mshchem · Aug 7, 2020

Thomas low pressure sodium vapor lamps with color filters. If you wait 10 -20 minutes for your eyes to fully adapt you can see quite well, enough to cut paper etc. Never had issues with fogging . Human eyesight is quite sensitive to the Na emission lines. You need to be in the dark for quite some time before the dark vision really sets in.

cmacd123 · Aug 7, 2020

LEDs do have specified wavelenths, and some have wider light bandwidth than others so you may want to read several data sheets of name brand LEDs (which are likly more consitant) and as Donald Q says, try to pick a unit that provides light at a null in the papers sensitivity.

one clue is that their are sodium safe-lights, perhaps the specifications for those can give you an idea of the preferred wavelength.

koraks · Aug 8, 2020

The problem with a fog test such as this is that you may still have color shifts and changes of contrast even though the whites aren't fogged. An additional test should be done to verify that an actual print is not affected by the safelight, eg by exposing half a sheet to the safelight but not the other half, and also exposing the same entire sheet for a normal print.

Additionally, I'd configure the lm317 not as a voltage regulator but as a current limiter. Set the current to the nominal current that the led needs. Check the data sheet of the led for this. Normal 5mm leds can usually be fed up to 20mA before they start to shift color. Yellow LEDs will usually shift towards orange/red if they're run at higher than their specified permissible current if memory serves.

Rafael Saffirio · Aug 8, 2020

koraks said:
An additional test should be done to verify that an actual print is not affected by the safelight, eg by exposing half a sheet to the safelight but not the other half, and also exposing the same entire sheet for a normal print.

I will do that and post the results

Rafael Saffirio · Aug 9, 2020

Here is the test, the right side was exposed to the safelight for 4 minutes and then to the enlarger, in the white areas you can see it was affected a little, but for my purposes I think it would be ok, it doesn't take me that long to cut a piece of paper from the roll y put it under the table away from the light.

koraks · Aug 10, 2020

Yeah, that's the kind of problem I'd expect. If it's good enough for you, that's fine. I'd be worried a bit about one sheet taking a few seconds more to cut than another, making the pre-exposure inconsistent.

Btw, I do thank you for actually running the test I proposed and showing the result. It's a very good example of a pre-flash technique with RA4. Very useful to have a clear example of this online!

bernard_L · Aug 10, 2020

Donald Qualls said:
sensitivity minimum at 475 nm. That'll be more of a greenish yellow than orange. I

475 nm should be blue, possibly leaning towards teal. Nothing yellow.
Maybe a typo; you meant 575 nm?

Donald Qualls · Aug 10, 2020

bernard_L said:
475 nm should be blue, possibly leaning towards teal. Nothing yellow.
Maybe a typo; you meant 575 nm?

Ugh. Yep, you're correct, it's 575 nm. Which is why sodium safelights work well for color, with a suitable narrow-cut filter -- the 555 nm emission from sodium is quite close to that minimum. These wavelengths are also close to the maximum sensitivity of the human eye, which means they can be quite dim in terms of actual foot-candles, and still be enough to see things like which side of the paper has the glossy emulsion and whether the paper is fully immersed in the developer.

pentaxuser · Aug 10, 2020

Thanks for the test Rafael. I can see what appears to be a distinctive line down the cat where one side is OK but the other is a greyish /blue Is this the problem to which you refer? Unless the scan had exaggerated the cast it suggests that the wavelength of the LED is not ideal

There is a prize for whomsoever can produce an LED that meets the right wavelength. I am sure it would be possible but it is unlikely to be worthwhile given how few print RA4 via an enlarger and fewer still who want a suitable light.

It's a pity

pentaxuser

koraks · Aug 10, 2020

I think most of us who print ra4 in the darkroom have come to grips with working in the (near) dark. I know I have; the idea of a usable safelight is nice, but imo not in the least essential. However I do acknowledge that there are people for whom working in pitch dark is problematic.

MattKing · Aug 10, 2020

I wonder if one could obtain better results with something sufficiently dim that flashes momentarily but regularly - say every one or two seconds.

pentaxuser · Aug 10, 2020

MattKing said:
I wonder if one could obtain better results with something sufficiently dim that flashes momentarily but regularly - say every one or two seconds.

This might work, Matt, but my fear is that these days based on what I see at a certain season of occupants of houses with flashing LEDs in my neighbourhood is that prolonged exposure to small flashing LEDs in a confined space would result in the user metamorphosing into someone with a red hat uttering Ho, Ho, Ho and exhibiting an uncontrollable urge to descend down chimneys.

A price too high to pay, IMO

pentaxuser

barzune · Aug 11, 2020

Donald Qualls said:
Ugh. Yep, you're correct, it's 575 nm. Which is why sodium safelights work well for color, with a suitable narrow-cut filter -- the 555 nm emission from sodium is quite close to that minimum. These wavelengths are also close to the maximum sensitivity of the human eye, which means they can be quite dim in terms of actual foot-candles, and still be enough to see things like which side of the paper has the glossy emulsion and whether the paper is fully immersed in the developer.

I'm not sure if this link will come through, but try :
https://www.mouser.ca/Optoelectroni...mitters/Standard-LEDs-SMD/_/N-b1bb1?P=1yzozqe

There seem to be a selection of 575nm LEDs to investigate.

Donald Qualls · Aug 11, 2020

Yep. With a narrow-emission LED one ought to be able to make an RA-4 darkroom as bright as it would be with a Thomas Duplex and RA-4 filter.

Simple LED safelight for color RA-4

Rafael Saffirio

Donald Qualls

DREW WILEY

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Simple LED safelight for color RA-4

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