Potentially useful LEDs for darkroom/alt applications

[Jim Noel]

Thanks for this link and to Sandy for his input.

[Drew Wiley]

CRI is meaningless with respect to ideal printing wavelengths in relation to UV and other alt processes, even with respect to conventional color printing. It's specific wavelength points that count. High CRI is relevant when displaying prints or making color evaluations. Those Waveform strips and panels are for sake of overhead lighting. The reason they're so expensive is that every one of the blue LED's needs to be fitted with a corrective violet filter to inch the CRI higher. But they what are marketing is actually assembled from components they obtain from others. So if I wanted to replace just one of my 4 ft overhead 5000K 95CRI fluorescent panels with something more modern LED-wise, it would cost me $450, dependent upon a special electronic controller which would probably interfere with certain of my darkroom controllers. But my actual matching lights are 98CRI already, so what's the point? (They cost quite a bit too).

[sanking]

Thanks for this link and to Sandy for his input.

Jim

You are very welcome.

One more general comment. Although there is an efficient heat shield on the back of the units I would still recommend ventilation of the backs of the units with air from a small fan. Radiation decreases with heat build up, and more heat also decreases life of the LEDs.

Sandy

[Michael R]

Obviously CRI is not for UV. My needs would be for white light to replace my non-LED source, and I was curious to see if I could find a good LED solution. Not the usual Ra CRI. A high Re CRI with high numbers for all 14 test colours, and a good continuous spectrum.

It’s not unique technology. As far as I know all really good white light LEDs use violet excitation with R, G and B phosphors (rather than the more common blue excitation with Y and R phosphors) to generate a much better spectrum. These Waveform products have the added benefit of being plug-and-play, so I thought it might be good to mention them here.

CRI is meaningless with respect to ideal printing wavelengths in relation to UV and other alt processes, even with respect to conventional color printing. It's specific wavelength points that count. High CRI is relevant when displaying prints or making color evaluations. Those Waveform strips and panels are for sake of overhead lighting. The reason they're so expensive is that every one of the blue LED's needs to be fitted with a corrective violet filter to inch the CRI higher. But they what are marketing is actually assembled from components they obtain from others. So if I wanted to replace just one of my 4 ft overhead 5000K 95CRI fluorescent panels with something more modern LED-wise, it would cost me $450, dependent upon a special electronic controller which would probably interfere with certain of my darkroom controllers. But my actual matching lights are 98CRI already, so what's the point? (They cost quite a bit too).

[Tin Can]

Sandy

Good point on heat danger

Safeties are needed, with very secure electrical on-off, perhaps a lockout or 3

We certainly do not want to 'forget' and leave the fire on

Jim

You are very welcome.

One more general comment. Although there is an efficient heat shield on the back of the units I would still recommend ventilation of the backs of the units with air from a small fan. Radiation decreases with heat build up, and more heat also decreases life of the LEDs.

Sandy

[j.e.simmons]

What’s the coverage on one of these floods a reasonable distance from the paper? For instance, would one cover 8x1? 2?

[sanking]

What’s the coverage on one of these floods a reasonable distance from the paper? For instance, would one cover 8x1? 2?

John

I tested one of the units last December and posted my findings on the groups.io site on 12/10/2020. I address your question in that post, which I am copying here along with other commends.

Sandy

"UV integrators are useful in printing, and also very nice to have if you need to compare UV output of specific light sources. I have some older Olix light integrators that I use in regular printing, and also own a PPM-1 meter that I use to compare different light sources.

I had the chance during the past several days to work with one of the 365 nm 100-watt UV floodlights, sold by Amazon with the Everbeam brand. I made a few quick comparison tests using the PPM-1 to compare output in various situations.

First, I measured useful radiation on center at 12", 18" and 24". Output decreases with distance more or less in accord with the what one would expect with point source light, i.e, every time you double the distance between the light source and the exposure plane, exposure times increase by 4. So here are the results.

12" Center of flood to exposing plane -- 1000 second gives UV Dose of 6400 -- Would print evenly about 8X10" in size.
18" Center of flood to exposing plane -- 1000 seconds gives UV Dose of 2744-- Would print evenly about 11X14" in size.
24" Center of flood to exposing plane -- 1000 second gives UV Dose of 1700 -- Would print evenly about 16X20" in size.

Other comparisons.

1) Measuring UV dose outside, on a sunny day in open shade, facing north at 12 noon on December 18, at north latitude 34º35 minutes, 1000 seconds gave UV does of 1820.

2) My regular Ryonet LED printing unit, with a mix of 365 nm and 385 nm leds, 1000 seconds gives UV dose of 6675. With this unit my typical exposures require UV Dose of about 5000, or about 750 units/seconds.

The 100 watt 365 nm flood is about 3 lbs. in weight, has an overall size of about 8X10", with the leds in an area in the center about 4.5" X 3" in size. There is good heat shield on the back but you would still need to cool the unit with a fan blowing over the heat shield to keep the unit cool as radiation decreases with heat build up. Combine it with a contact-printing frame and I think you have a pretty efficient UV source for making small and medium size prints. And one could also weigh the possibility of ganging up 4-6 of the floods for wider coverage. The most efficient flood for printing carbon with dichromate sensitized tissue would be the 365 nm leds, but 385 nm would be less expensive and probably almost as efficient. For DAS you would definitely want the 365 nm light.

BTW, have a look at the linked video, about salt printing, produced by the George Eastman Museum. https://www.youtube.com/watch?v=VDfd...ature=youtu.be
In the video you will see that they are using what appears to be a couple of small UV flood lights ganged together for the exposure."

Sandy

[bob carnie]

I am hoping not to hi jack Michaels thread but I have a somewhat related question.

I make up to 30 x 40 pt pd and multiple gums, I currently have a 30 x 40 Nuarc system with a 1000k Metal Halide head, I am finding my exposures too long for my workflow ( I do multiple images each day and would like more power)
I have been talking with Ian Leake about modifying one of his density measuring devices to connect to a power supply that will shut off after a set exposure is met. He believes it is quite simple, I want to just use the base vacuum systems to hold my material I have two units ( one has already been modified so the vacuum turns on independently of the exposure) I would do the same to the other unit. I have a small exposing room where I walk out of the room as work is being exposed and I do need this automatic shutoff system.(important)
I would like to hang a light above each unit. If 1000k is what I have now I am thinking I would like 4000 k - 6000k to speed up my large printing time.

Is there a unit available Used and or New that can do this for me and of course replacement bulbs and parts for the future.
If I designed a bank of these types of lights or Led lights that would work with Pt, gum, cyanotype and salt... do you think the power could equal my needs posted above.. Somehow I think Sandy in the past has talked about (Carbon Arc hanging units) and I think this is something Calvin Grier in Spain may be using.
I have seen Jon Cones system which is quite good, but my space would really work well with a hanging light system above the vacuum table so I can take large film in and out of the unit with ease and not worry about lifting the whole head as I currently do with my
Nuarc 32 1KS unit.

any help

Bob

[bob carnie]

By the way Michael - If I go to a different route I have three heads 1 Ks available to give you , just a 5 hour ride .

Bob

[j.e.simmons]

Thanks, Sandy.