LED Darkroom Lights?

johnnywalker · May 10, 2016

I'm in the process of building a new darkroom, along with the rest of the downstairs area. Can I use LED lights for the overhead lights in the darkroom, or do they continue to shed some light for a bit
after being turned off like fluorescents do?

DWThomas · May 10, 2016

I don't think the duration is nearly as long as with fluorescents, but I note some definite glow immediately after shut-off. This is based on a 60 watt equivalent warm white here in the overhead light in the computer room -- haven't had occasion to try in the dark room.

johnnywalker · May 10, 2016

Thanks Dave. I'll give it a try. If it's a problem I can use the bulbs elsewhere.

David Brown · May 10, 2016

They'll be fine.

bdial · May 10, 2016

I use LED's for safelights in the darkroom but not for the white lights yet. Like Dave I do notice that some I have elsewhere do a fade when powered off, but it's very rapid. I don't think they would be a problem. The red LED's I am using for work very nicely.

Athiril · May 10, 2016

When you turn off LED down lights and look up at them, you can see they take a few seconds to fade. They'll be done before you pull your paper out though.

Mainecoonmaniac · May 10, 2016

LED lights turn off immediately. Here's a geeky fact about them. They turn off and an so fast, LEDs dim by being turned off so fast they they look like they're dim. A technology called Pulse Width Modulation (PWM).

https://learn.sparkfun.com/tutorials/pulse-width-modulation

Athiril · May 10, 2016

There is likely capacitive charge (or phantom current) being held somewhere, if you look at LED down lights in a dim or dark room when you turn the light switch off they take a few seconds to fade.

DWThomas · May 10, 2016

Mainecoonmaniac said:
LED lights turn off immediately. Here's a geeky fact about them. They turn off and an so fast, LEDs dim by being turned off so fast they they look like they're dim. A technology called Pulse Width Modulation (PWM).

https://learn.sparkfun.com/tutorials/pulse-width-modulation

A simple LED semiconductor junction may in fact be instantaneous, but LED lighting units employ tricks like exciting phosphors with UV and the like (which is actually what fluorescent lamps do). I can tell you that as I exit the room I'm sitting in late at night, I can head out the door and flick the switch, then turn and look back and see a definite glow fading (and at my age, I cannot turn my head in a microsecond! ) I don't think the glow is long enough to be a problem as overhead lighting in the darkroom, I'm much less sure if they would not be problematic for enlarger use (which has been discussed on APUG in the past). It's possible there may also be some capacitors used to smooth out line glitches that retain a charge that has to bleed off. Whatever it is, the commercial LED tungsten bulb replacements are decidedly not instantaneous.

L Gebhardt · May 11, 2016

Some LED bulbs have LEDs with a high UV component and phosphors to reemit that as visible light. Those phosphors will glow for a short time after being turned off. I still use those in my darkroom since they have the best CRI ratings and I haven't had an issue.

Frank53 · May 11, 2016

Heiland is selling a led tube that you can switch between red and white (daylight color)
No afterglowing when you switch it off.
Frank

And there is a dimmer built into it.

DWThomas · May 11, 2016

Also -- in regard to Mainecoonmaniac's link, it should be mentioned that pulse width modulation can be used to dim incandescent lamps and heaters also; that it works on LEDs is not necessarily because of their response time. In fact, for PWM to work as a dimmer, something has to integrate the pulses -- it may be the physical device, or it could be our eyes. PWM is varying the average power supplied to the device while minimizing heat dissipation in the controlling semiconductors, that was its original reason for adoption.

Doremus Scudder · May 11, 2016

The thing I would be concerned about with white LEDs in the darkroom is print viewing. LEDs have a discontinuous spectrum and won't show toning effects like a regular incandescent will. I use a mix of regular halogen light with one or two blue daylight 5500K floods in the bank as well to simulate a mix of halogen and daylight. I find this the best for evaluating toning progress.

Best,

Doremus

DWThomas · May 11, 2016

silveror0 said:
Dave, I converted my enlarger to Cree Cool White LEDs (7500K) some time ago and have experienced no afterglow issues (and, btw, I mounted the LEDs on an aluminum heatsink just as a conservative measure (belt-and-suspenders approach) so I'd not have to do further re-work later.

Interesting -- since you mention a heat sink -- were these literally LEDs in some sort of array -- or the lightbulb replacement devices? As I suggested above, I might expect different behavior from the simple component. I'm interested because I actually picked up an extra lamphouse for my Omega B8 in case I decide to roll some "new technology" into it!

My guess is the bulb unit I have in this ceiling fixture probably dwindles downward five or six stops in a fraction of a second, so even it might not be a problem for enlarger use, but I've not so far worked up enough curiosity to try and measure something.

DWThomas · May 11, 2016

Thanks! Some good info there.

Mainecoonmaniac · May 11, 2016

Since LEDs are semiconductors, they will cook if they get too hot. That's why the heatsinks.

Mainecoonmaniac · May 11, 2016

silveror0 said:
Tom, that's why the heat transfer paste under each LED is important; it transfers the LED's heat to the heatsink. Only a dab of paste is needed.

Yep. But hopefully, your LED is glued on a heat dissipating piece of metal.

L Gebhardt · May 12, 2016

Also, some of the PWM control circuits

DWThomas said:
Also -- in regard to Mainecoonmaniac's link, it should be mentioned that pulse width modulation can be used to dim incandescent lamps and heaters also; that it works on LEDs is not necessarily because of their response time. In fact, for PWM to work as a dimmer, something has to integrate the pulses -- it may be the physical device, or it could be our eyes. PWM is varying the average power supplied to the device while minimizing heat dissipation in the controlling semiconductors, that was its original reason for adoption.

There are also constant current drivers that respond to PWM pulses but adjust the voltage to provide a constant current proportional to the PWM input. This gives a flicker free response with LEDs.

LED Darkroom Lights?

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