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  1. #1
    GRHazelton's Avatar
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    LEDs and light persistence??

    I just bought two LED night lights, photocell controlled white light. For whatever reason I pulled one from the AC outlet in the dark while it was lit. To my surprise the glow GRADUALLY diminished, taking perhaps 5 seconds to completely disappear! Both of these night lights did the same thing. Another night light, blue, non-photocell controlled, doesn't do this. Would the photocell controller have any effect on this?

    Has anyone else encountered this? I'd had the impression that LEDs pretty much were just on or off, with no "cool down." If this is common it could vitiate making a "cold light" enlarger source.
    This email is a natural hand-made product. The slight variations in spelling and grammar enhance its individual character and in no way are to be considered flaws.

  2. #2
    DWThomas's Avatar
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    There could be some sort of filter capacitor that stores a bit of charge and fades slowly. I have some 60 and 75w equivalent LED lightbulbs that definitely are nowhere near instant off (which suggests maybe not too good fro enlarger use).

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    The electroluminescence of a colored LED (emission of photons by the charge carriers) should cease very quickly at switch off (nano seconds).
    However white leds employ a phosphor to partially convert a discrete color band to white light. These use the principle of fluorescence, which, as I understand it, has an exponential decay after the light source is switched off. ( For example an oscilloscope's persistence or an old type fluoro light )

  4. #4
    Steve Smith's Avatar
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    Quote Originally Posted by DWThomas View Post
    There could be some sort of filter capacitor that stores a bit of charge and fades slowly. I have some 60 and 75w equivalent LED lightbulbs that definitely are nowhere near instant off (which suggests maybe not too good fro enlarger use).
    My enlarger fas an LED light source. It will be instantly off is the switch is after the capacitor.

    EDIT: I just realised that you are probably referring to an LED light which replaces a standard bulb. In which case, the power supply is integral and there's not much you can do about it.


    Steve.
    "People who say things won't work are a dime a dozen. People who figure out how to make things work are worth a fortune" - Dave Rat.

  5. #5
    GRHazelton's Avatar
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    Thanks for the quick responses! When one dies I might dissect it to see what's in there. Both the phosphorescence and filter cap theories sound reasonable. Somewhere I saw a thread about making a BW safelight with LEDs. It would seem that if the LED has a very narrow bandwidth it could make an ideal safelight, nice and bright, like the old Thomas or whatever discharge units. They were always too costly for me!
    This email is a natural hand-made product. The slight variations in spelling and grammar enhance its individual character and in no way are to be considered flaws.

  6. #6
    Steve Smith's Avatar
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    Quote Originally Posted by GRHazelton View Post
    Both the phosphorescence and filter cap theories sound reasonable.
    Much more likely to be the capacitor. I have never known an LED to continue glowing without any current flowing through it (and I have had many years experience with LEDs).

    Just for interest, here is the website of a company producing domestic and architectural LED lighting. The company I work for makes the flexible LED circuits and light guides for them: http://designledproducts.com/


    Steve.
    "People who say things won't work are a dime a dozen. People who figure out how to make things work are worth a fortune" - Dave Rat.

  7. #7

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    Quote Originally Posted by Steve Smith View Post
    Much more likely to be the capacitor. I have never known an LED to continue glowing without any current flowing through it (and I have had many years experience with LEDs).

    Just for interest, here is the website of a company producing domestic and architectural LED lighting. The company I work for makes the flexible LED circuits and light guides for them: http://designledproducts.com/


    Steve.
    I am interested in this because I just replaced all the old fluorescent lamps in my basement with
    "LE 10 Watt 2 Foot T8 LED Tube, 20 Watt Fluorescent Tube Equivalent, Daylight White"

    I don't open the film in this area but I do bw enlargements occasionally as the lights are in the sink area

    They have quite a long afterglow, and I agree it may be partially due to the filter caps.
    The best way to see the after glow is to leave the eyes closed until just after the light is switched off.

    These ones have a decaying visible glow for around 2 seconds. I will try again tonight in the dark.

    So on my electronics bench, I powered up a component super bright white led of 700mcd, 25mA, and aimed a fast photodiode at it.
    Just with a mechanical switch, the oscilloscope indicates a fast fall of < 1 millisec of light emission to something less than 5% brightness, then a slow decay of that 5% to zero in about 800 millisec.
    These are approximate, i will have to set up a switching transistor to get more accurate results.
    But there is definitely an afterglow and filter capacitors are upstream of the switch in this test.

  8. #8
    Roger Cole's Avatar
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    I have an LED lamp house on my Omega D2. Works great and I love it. Others have talked about the reasons this might vary in a night light.


    Sent from my iPhone using Tapatalk and 100% recycled electrons - because I care.

  9. #9
    DWThomas's Avatar
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    Re: my 1st post, yes, I was e-speaking of self-contained LED bulb replacements or plug-in night lights and such. My understanding is that white LEDs are actually UV emitters with a white phosphor surround, sort of a solid state fluorescent lamp. As such, their turn-off characteristic could be a property of the phosphor more than the LED. If you used separate RGB LEDs for a variable contrast/color enlarger head, the phosphor would be out of the system.

    As one who remembers the first feeble little dim and inefficient LEDs that came out, the latest generation boggles what's left of my mind.

  10. #10
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    When we moved into our now house, the ground floor was lit with 23 X 50 watt halogen bulbs, which is 1150 watts. I replaced these with 23 LED 7 watt bulbs (50 watt equivalent). Total wattage is now 161 watts.
    Last edited by cliveh; 02-01-2015 at 02:37 PM. Click to view previous post history.

    “The contemplation of things as they are, without error or confusion, without substitution or imposture, is in itself a nobler thing than a whole harvest of invention”

    Francis Bacon

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