piclist 2002\12\27\113151a >
Thread: Pushing LEDs to the limit
picon face BY : Spehro Pefhany email (remove spam text)

At 09:30 AM 12/27/02 -0600, you wrote:
> >In general, where the flicker rate is not perceptible, the highest
>apparent brightness will be with DC drive, give or take a bit depending
>on the semiconductor material and absolute drive levels. It will *not*
>make a big difference  except negatively at the extreme high end.
> >It's because of the way the human eye works- it is an "average" reading
>device, not a peak reading device.
>I'd have to politely disagree in one case, Sphero.  Flashing light for a
>bicycle (which I built into my bike helmet for that ultra-nerdy look). The
>goal is to make the most annoying and perceptible light with the least
>amount of power.  I run a high brightness LED at about 5 or 6 times it's
>rated power for 100 mS, and repeat these flashes every 2 S.  That's only
>5% duty cycle, but the 100mS pulse is long  enough that these flashes are
>visible for blocks.  I can see the reflection of the flash off a stopsign
>a full block away on a dark night.

I did qualify it above "where the flicker rate is not perceptible",
meaning over 50-100Hz. There's a transition range from 10Hz up to 100Hz
where the flicker is very perceptible to somewhat perceptible (especially
if there is vibration or other movement) but I don't think the apparent
brightness is much better at the top end of it. At the bottom end,
you're obviously getting a much better visible brightness.

So, I don't think we have any disagreement, polite or otherwise. ;-)

What is the off-time between 100ms flashes? 100ms?
It's a white LED?

        1       2           5-6                          1

        --      --          --                          --
       |  |    |  |   \    |  |              \         |  |     |
  -----    ----    -- | ---    --------------| --------    -----
           <-->        \                      \
       |<------------------------ 2s ----------------->|

>So far I have run this light on the same set of 4AA batteries for over a
>year.  My night riding usually occurs in the wintertime, don't really have
>much data on how many hours I have used this thing but it is quite a lot.
>Runs on a 12C508 with a small power transistor as the pass element and 4
>blinking LEDS, right-left-front and back.

That *does* sound nerdy-looking. ;-)

>I guess this applies at low frequencies (< 1 hz) but at higher frequencies
>(> 10 hz) the eye will begin to average the pulses like you said.

There's a factor with the peripheral vision being "faster" response time
that might be an important factor with a bicycle lamp- our eyes are
designed to detect movement of predators or prey off at the sides of our
high-resolution vision, so light changes, fast ones, get our attention.

Just what you want from the guy driving the 18-wheeler at night, when that
mountain of chrome fills up your bicycle mirror.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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