There are ways for the flicker to be more evident. Don't laugh, but
chewing something hard like a pretzel can bring out the flicker.
Basically you can get beat patterns between the vibration of your eye
and the light flicker.
There is a common problem with DLP projectors that use color wheels. You
will see reviewers shaking their heads and eat crunchy food in order to
see "rainbows" on the screen.
A similar problem occurs with matrixed LED displays mounted on machinery
that has vibration. Very common in industrial controls since they like
LEDs for readability.
When I designed the 2nd generation LED display drivers, I bumped the
refresh rate to 500Hz min. That was about 2x the frequency where I ran
out of convoluted experiments to detect flicker.
On an analog scope, you can display a flat line and have it wiggle by
eating something crunchy. I don't have an analog scope on the bench at
the moment, otherwise I would figure out the right circumstances to make
that happen.
The test pattern for flicker detection is to arrange LEDs where a group
of them form a recognizable pattern. Take a plus sign as an example. Put
the LEDs in an array. Illuminate the LEDs that are not in the symbol out
of phase with those in the symbol. Vary the refresh rate. When the eye
can see a pattern, the refresh rate is too low.
On 9/18/2012 12:06 PM, John Lofgren wrote:
<snip>
I would hence believe that a 50 Hz flicker must be pretty close to the edge of
what can be perceived, so I'm having trouble believing that a flicker at more
than twice that rate would be perceptible at all by anyone.
<snip>
Oh, but it is. A couple of years ago I bought one of the Chinese 30 LED spot light bulbs
for about $8 on ebay. I thought I'd give it a try for a workbench light. When I plugged
it in at work (60 Hz power, here) the two guys standing behind me yelled
"gaahhh" at the same time I did. The flicker was horrendous. The earlier
comment about peripheral vision also applies, though. It's worse in the periphery than
in direct view.
The "power supply" is nothing more than a bridge rectifier, two current
limiting resistors, and a filter capacitor. The capacitor obviously wasn't big enough,
though, because it flcikered plenty.
-John
-----Original Message-----
From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf
Of Dennis Ferguson
Sent: Tuesday, September 18, 2012 1:52 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Hi Power LED Light power supply...
On 18 Sep, 2012, at 12:42 , Chris Albertson wrote:
On Tue, Sep 18, 2012 at 5:21 AM, Bob Camp <li...@rtty.us> wrote:
Hi
I suspect those same 120Hz sensitive people would not be able to watch TV or
a movie :)....
In the old CRT type TV sets, the phosphor has some persistence.
Movies are modulated with a square waves, the frame blinks off and
goes dark then blinks on. But the LED's brightness is fast enough to
track the sine wave and would be bright only for an instant with quick
pulses of light.
Just to add to this...
Ontario, Canada originally ran its power grid at 25 Hz. When they
switched the grid to 60 Hz in the 1930's some of the industrial power
users, particularly in northern Ontario where private (usually
hydroelectric) power generators were common, never got around to changing
their plants over. Mine and paper mills using 25 Hz power were common as
recently as the 1980's, and might still be there for all I know.
Standard incandescent light bulbs don't have a lot of persistence when
run on 25 Hz power (I assume there might have been a time when you could
buy incandescent bulbs designed for 25 Hz, but not in my lifetime). They
don't go entirely off, but they get significantly dimmer in the visible
spectrum in the dips as the output red-shifts towards the infrared; they
follow the sine pretty well. In my teens, when visiting a place using
25 Hz power for lighting, I could initially see an incredibly annoying
flicker when I first got there but after a minute or two this would fade
and I'd no longer notice it. Some other people would also see the flicker
but others, including my parents, couldn't see it at all so there seemed
to be variation (maybe age-related, maybe not) among individual abilities
to see this.
Dennis Ferguson
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