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    Re: led lights
    From: Herbert Prinz
    Date: 2003 Nov 26, 11:58 -0500

    Ken James wrote:
    
    > There has been some question about intensity/brightness of pulsed LEDs. In
    > that regard, it is a well established fact that the phenomena known as
    > "flicker brightness enhancement" is real.
    > What it boils down to is that for the human eye, a periodic train of light
    > pulses of correct pulse duration and duty cycle (on-off %) can look
    > 'brighter' than the same amount of light spread out uniformly over the same
    > time frame. The explanation seems to be that the eye does not have enough
    > time to tell the brain the light is off before it comes back on, and the
    > peak value gets taken for the average value, more or less.
    
    Hello Ken,
    
    I find your post very interesting, but I have not seen the thread that you are
    refering to. I hope I am not repeating arguments that have been discussed. If
    so, I apologize.
    
    The obvious question that comes to mind first is: "If the brain does not have
    enough time to tell that the light is off, why does it have enough time to tell
    it's on?" In other words, the argument could be reversed to explain why a
    flickering LED should look darker than a continuous one, and therefore does not
    really explain anything as long as you don't provide details on the duty
    cycles.  From your statement "...up to 80% more...", I gather that the light has
    to be slightly longer on than off. So the nerves (or whatever) actually have
    more time to regenerate from the previous "on" signal than from the previous
    "off" signal. This does not seem to make your explanation very plausible.
    
    However, when you say that the phenomenon of "flicker brightness enhancement" is
    real, I assume you mean that statistically valid tests have been performed to
    verify this. Am I right? I am therefore not questioning the
    neurologigal/psychological fact, just your explanation. At any rate, I am not
    convinced that the exploitation of this phenomenon makes any physical sense.
    
    1) The trick works only as long as there is only one light source, or as long as
    all light sources are driven by the same driver. Otherwise the intensity of
    light would be averaged before it reaches the eye. This would seem to limit the
    applicability. Individual cabin lights that can mutually interfere would all
    have to be driven by a common chopped source. Is this how you are proposing the
    installation?
    
    2) As you say yourself, the test person may well have the _impression_ that the
    light is brighter, but that does by no means imply that he sees better. The
    maximum optical resolution is a function of the amount of available light (as
    every amateur astronomer knows who wants an ever bigger scope). Violating this
    principle is akin to inventing the perpetuum mobile. So, what is the advantage
    of having a cabin light "LOOK" brighter? You won't be able to read better in
    this kind of light. (And at the same time possibly retain the negative side
    effect of impaired night vision corresponding to the full peak brightness. Any
    tests on this?)
    
    On the other hand,  I do see the advantage in the case of nav lights, if the
    principle works. Are there tests that show that the phenomenon also exists over
    longer distances, i.e. when the light source appears more like a dot, rather
    than filling the room?
    
    Herbert Prinz
    
    
    

       
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