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  1. #21
    Steve Smith's Avatar
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    Quote Originally Posted by galyons View Post
    Steve,
    I think that this is core to the discussion. Technically, one does not "dim" an LED. As you describe, PWM, (Pulse Wave Modulation), actually turns the LED off and on, but does so at a frequency that fools the eyes/brain into thinking that the light is less bright. So the apparent dimming is a result of less time that the LED is actually on.

    The driver can modulate the duty cycle to say, as an example, 50% on, 50%. Again this is fine for fooling the eyes, but does it "fool" VC paper? I don't believe so. Isn't the net result the same as just cutting the exposure time by 50%?
    Yes, 50% on 50% off is exactly the same as turning it on for ten seconds instead of twenty.

    The frequency of operation isn't too important for enlarger use as the eye doesn't need to be fooled.

    My idea was to have a single control so that fully anticlockwise it was all green, in the middle it was equal green and blue and fully clockwise it was all blue.

    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.

  2. #22
    RH Designs's Avatar
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    Quote Originally Posted by galyons View Post
    The driver can modulate the duty cycle to say, as an example, 50% on, 50%. Again this is fine for fooling the eyes, but does it "fool" VC paper? I don't believe so. Isn't the net result the same as just cutting the exposure time by 50%?
    Yes it is - it reduces the volume of light hitting the paper by rapidly switching the LED on and off, so the total "on" time is reduced although the nominal exposure time is the same. You can also dim a LED by reducing the current flow through it in which case if the exposure time is the same as before, less light will hit the paper. The end result as far as the paper is concerned will be the same if the total volume of light hitting it is the same. I would doubt if reciprocity effects due to short pulses of light rather than continuous exposure would be significant at the sort of pulse lengths likely in this application - and in any case they could be compensated for.
    Regards,
    Richard.

    RH Designs - My Photography

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