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dicer

dicer

Automotive
Feb 15, 2007
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I didn't want to change the topic on my first thread.
I'm curious, about some of the LED tail lights used on some vehicles. Are there any that are laser diodes? And how bad are they for our eyes? I know there are some that cause comet trails when moving eyes rapidly side to side. Wouldn't coherent light cause that?
 
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Do you have a link or reference to "comet trails"? I think I get them from some light source in my flat, but can't work out which or when. Thought I was going mad/blind or drinking the wrong stuff.

- Steve
 
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I think that what you are observing is a different (but annoying) phnomenon.
LED read lamps are multiplexed and PWM controlled to differentiate between read- and brake intensity.
Unforunately, the designers (at least in the beginning) opted for a too low pulse frequency (below 100 Hz). This is not a problem with incandescent lamps due to the inherent brightness integration, but with LEDs it is highly disturbing, specially in the periphery of your field of vision.
LED pulsing should be at frequecies above 200...300 Hz for this effect to disappear.

From a car with LED lamps passing across your field of vision, you'll even see a kind og pearl-chain effect.

Benta.
 
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I don't get the obvious strobe effect, but a seemingly continuous ghost that follows (and catches) moving objects, like my hand reaching out to my beer glass.

- Steve
 
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Agree, ivymike, 75 Hz is good for a screen -- as long as it's sitting still. Have someone move it across your field of vision, though, and it's kind of psychedelic.

I've had a kind of OCD thing since I was a little kid and saw my first LED display on the move. I had a calculator with such a low strobe rate that if I passed it from right to left in a pretty straight line, I could see each individual segment. It used to fascinate me, and even today I will often test new things by passing them back and forth in front of my eyes. (Yes, I'm weird). A couple hundred Hz seems to do the trick, though.



Good on ya,

Goober Dave

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75 Hz is enough for a CRT, that's correct.
But that is because the after-glow of the phosphor helps.
With LEDs it pure on/off and it is disturbing also above 100 Hz.

Benta.
 
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No mass consumer light such as a tail light would be a laser; there's too much risk of getting a lawsuit.

Comet trails are most likely due to saturation/persistence of your retina, since you're probably only seeing them when ambient lighting is low.

Frame rate is highly dependent on intrascene motion; gamers tend to want frame rates in the 100-Hz range, or even higher, because the brain can detect the stop-start motion going from frame to frame. Interesting, intentional blurring of the image tends to quell that phenomenon, and lower frame rates can be used under those circumstances.

TTFN
faq731-376
7ofakss
 
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Phenomenon of comet trails from bright light sources being swept rapidly across your vision, and visibility of moving, pulsed LEDs is due to "persistence of vision", i.e. your eyes are a chemically based light detector, with a typical chemical half-life "tail" that persists after the initial stimulus. More at wikipedia if you are interested, but there are better writeups elsewhere.

Steve, I recall reading somewhere that as we age, we tend to become more easily fatigued and lose eye coordination (ability to keep the eye focused in a single spot), which exacerbates the POV problem noted above, especially if you are reading in a flickering light source (fluourescents, especially as they age, can be bad this way). I have started wearing reading glasses, they help quite a bit.

There's also this "syndrome" - try rose colored glasses? :

 
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If I recall correctly, there is also a substantial performance difference between the foveal part of the retina, which is conprised of mostly cone cells and the periphial areas comprised mostly of rods. Not only are the rods more light sensitive, they are also faster. This makes sense from an evolutionary point of view... Predator detection in periphial areas would be optimized for instant response to movement; e.g. light changes. Central vision predator dection would seem to be more optimized around careful color and pattern analysis. Note that the time response difference is more likely in the brain post-processing algorithm than in the chemical response times.
 
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Finally, a post suited to me ;-)

A lot more taillights out there are direct drive rather than PWM'ed... companies like cheap, which means directly driving them from the battery with a resistor or two and parallel blanks of LEDs. On older third brakelights you'd often see a strip or two of dead LEDs.

Now, my animated tailights use a PWM-like method, but that's after-market equipment :-D

Dan - Owner
Footwell%20Animation%20Tiny.gif
 
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Recent (for maybe 5 years or more) Cadillacs have been particularly annoying with pearl-drop effects as they cross your field of view. Maybe they haven't reached AUS yet, but they're kind of a plague here in The Colonies.

I'm hopeful they'll wise up and increase the multiplex frequency to help these old eyes a bit.



Mike Halloran
Pembroke Pines, FL, USA
 
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Mike.

I think Aus was a colony until well after American independence.

It's almost my lifetime since we got Cadillacs here and I don't see that changing any time soon.

There is a case here for exceptionally bright lights for use in the remote outback where the risk of collision with livestock and large wild animals is very real.



Regards
Pat
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