how to know about the led stop blinking

[harjeemann]

dear guys,

         i am using labview to calculate the frame rate of my camera for this i am having a led and camera facing to this led and i increse the frequency of this led and at 30 hz the led seems not to be blinking. my problem is how i can make lab view to see to this problem and decide when led is not glowing. a reply is very much appreciated and i will like to thhanks in advance

cheers

[GuenterMueller]

I don't like the idea to use a blinking LED to calculate the camera's frame rate. It can be hard (or even impossible) to check with a camera that a LED is blinking if you don't know about the rate at which it flashes.
I would take a timestamp each time an image was acquired and calculate(!) the frame rate.
This makes you independent from any "LED on/off alignment".
Regards, Guenter

[Ray.R]

I agree with Guenter..

Some cameras have a sample rate of 30Hz, so if the LED blinks at a higher rate, you won't detect it.  Furthermore, depending on your sight threshold, you may not see a flickering light at rates higher than 30Hz (well... usually ~40Hz, but I read about people with lower threshold). 

[harjeemann]

how can i take a time stamp for the same, that can be a good idea but i also want to implement that the led idea if you can throw some light on the idea u given to me and some solution for LED idea.

cheers

[Ben]

Not that it helps with the original question but...

Doesn't Nyquist say that a camara operating at 30 Hz can only measure blink rates of 15Hz or below?

Ben

[Ben]

Arguing with myself now

I beleive that is true only if the Led intensity is being modualted with a sine wave of 15 Hz. If the Led is being turned on and off using a square wave, then the highest freq component of the intensity modulation signal must be 15 Hz or less.

Ben

[Ray.R]

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@Ben wrote:

Doesn't Nyquist say that a camara operating at 30 Hz can only measure blink rates of 15Hz or below?

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That's usually when you want to sample an analog signal reliably, it order to reproduce it (evaluate freq, etc) later on... 

In this case, you would have two "digital" signals, so the camera should be able to "see" blinking up to 30 Hz.  (Not taking into account transition time etc.. , in an ideal (but not real) world).

We're starting to sound too much like engineers...

In laymen's terms..  yeah.. it should see most blinks..  😄 😄

Message Edited by JoeLabView on 09-06-2007 01:05 PM

[Ben]

Something seems wrong ther Ray.

What if both the camera and led were running at exactly 30 Hz BUT the led was always on while the image was not being acquired and off while its was.

It would never see the LED on.

If it was shifted another 180 degrees, it would look like it was always on.

Ben

[johnsold]

I agree with Ben. You have to have one frame off and one frame on to have any blinking, hence a maximum of 15 Hz blink rate.

Nyquist refers to the highest frequency component which can be detected and reproduced after sampling. The 15 Hz square wave of the blinking LED would have many components at higher harmonics which the 30 Hz sampling could not detect. You could tell that something was happening at 15 Hz but you could not determine the wave shape. Actually, I think the requirement is a strict less than condition, not <=. If you had a sinusoidal signal at 15 Hz and sampled at 30 Hz and your samples happened to be at the zerocrossings, you would see a constant of 0.

Lynn

Message Edited by johnsold on 09-06-2007 01:24 PM

[Ray.R]

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@harjeemann wrote:

i am using labview to calculate the frame rate of my camera for this i am having a led and camera facing to this led and i increse the frequency of this led and at 30 hz the led seems not to be blinking.

---

OOPs!!  My mistake..

Yes!!  If you want to detect "blinking", then it's 1/2, so 15Hz max....

For some reason, in my mind was the ability to "see" the LED being ON.. 

Thanks for the explanation Lynn..  You're completely right... 

I'm going away to get some coffee.. 🙂  Thanks for keeping my brain active, guys!  🙂

😄

--- I'm going insane, aren't I??  😞  ---

Message Edited by JoeLabView on 09-06-2007 01:31 PM