Capturing Lit LED's?

[Prideaux]

Does anybody have any experience or suggestions for viewing lit LED's?  I am talking about standard 5mm red LED's, running 20ma of current (fairly bright).  I can dim the LED's down and that helps some.

I am currently using a basler A631fc CCD color camera (~2mp), and have played with every setting possible and no matter what I get a nasty image from all the lights blooming.  It's almost as if the LED's are oversaturing the camera.

I have heard there is a difference for this application between CMOS and CCD, has anyone ever had any experience in this area? 

I am trying to figure out where to go from here, be it filters, different camera, ?????

Thanks in advance for any assistance.

[AnalogKid2DigitalMan]

I would presume that the exposure(gain) and shutter control(time) would affect the image brightness. What values have you tried?

And- have you read the manual, link on this page:

http://www.graftek.com/pages/A631FC.htm

Message Edited by AnalogKid2DigitalMan on 11-16-2006 02:10 PM

[tomwp]

Just a couple of basic things to try:
1. If you are just trying to measure a single LED make sure it is on-axis (in the center of the image).  That way any internal reflections within the optics will remain on axis and not destroy your image.
2. Stop down the shutter as far as possible - if you dont have one, then make one - a small hole over the end of the lens will do.
3. An alternative to an external shutter is crossed polarisers  - buy one sheet of linear polariser, cut it into two and place the two sections crossed on top of the lens, rotate them towards parallel to allow more light through. (be warned this will add a purple tint to your image as polarisers are optimised for the green).

Is the blooming dependent on position of the LED within the image or the rotation of the camera with respect to the LED?

What are you trying to achieve here?  Are you trying categorise the LEDs by luminance?

[Prideaux]

I will have to buy a sheet of polariser, good idea.

What I am trying to do is some inspection on the LED digits we make that has anywhere from 80 to 500 LED's that make up the 7 segment '8'.  I am wanting to check the presence of the LED's to ensure that all LED's are there, as well as do a color check against a known good template to verify the hue.  <if I can even do any of this>

That was a good thought about rotating the camera, I hadn't thought of that.  I tried it and the blooming seemed about the same, as you mentioned the only way to really dampen it is to crank that shutter down to a pinhole.

Thanks for the suggestions, I will keep playing.

[Vladimir_Drzik]

Hi,

Can you put a ground glass over the LEDs? That should make the situation a lot better

Vladimir

[Prideaux]

Interesting idea Vladimir, thanks!

I will give that a shot, should work well.

THANKS.

[tomwp]

If you are trying to measure the colour point of the LEDs be carefull driving them at a lower current, they might change colour slightly.  If you are not worried about that, try driving them at a low duty but high frequency, thus effectively reducing the brightness.

So are you saying there are 300 LEDs per 8 segment display, must be massive a massive display!

[Prideaux]

Yeah they are big 7 segments, the 10" tall digits have about 100 LED's making up the '8' while the 24" tall digits have 480 LED's.  Thanks for the warning on lowing the current too, I hadn't thought of that affecting hue.