Author Topic: CMOS Sensor colors  (Read 922 times)

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Offline robo23Topic starter

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CMOS Sensor colors
« on: August 10, 2012, 10:56:34 AM »
I am working on a project that would use computer vision to locate an object.  What I hoped to do was have a green led and a red led be recognized in an image, and then I could draw some conclusions about where the object is.

Ok, here's my problem:
Using a webcam, the image does not show the color properly. In the image, the leds show more as white with color around the edges than a solid color.  I have tried a couple different webcams with the same results.  However, when I take a picture with my phone, the color seems much better.  I have also tried both clear and translucent covers over the leds in attempt to disperse the light more evenly.  It still just shows as white.  Actually, with the translucent cover, it showed uniformly white.

In my final application, I am intending to process this with a camera module and a microprocessor, I'm just using the webcams to get my software routines right.  I can understand that different sensors would have different capabilities.  Without simply buying a bunch of them in the hope that one would show better than another, is there a standard that I should be looking for?

Does anyone have any suggestions on the best way to proceed?

Offline waltr

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Re: CMOS Sensor colors
« Reply #1 on: August 10, 2012, 05:50:48 PM »
Most web cam type cameras are very sensitive in the Red to Near IR wave lengths. So it doesn't surprise me that they look 'white' white.

It could be that the Red LED is just too bright and saturating the pixels (related to the red sensitivity), and bleeding into the blue & green pixels therefore looking white. Try running the LED with a lot less current to dim it and see how it looks.
Or instead of directing the LED straight at the web cam have the LED illuminate a white card and see how that looks.

Also, look at the raw pixel value read from the web cam. None of the pixels value should saturate (255 dec for an 8-bit ADC). If they are then the LED is too bright.

Which web cam are you using?
I don't see any reason the web cam will not work.

Offline robo23Topic starter

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Re: CMOS Sensor colors
« Reply #2 on: August 10, 2012, 06:22:04 PM »
First of all, thanks for responding.  Those are good suggestions, I'll try that.

As far as the brands, I have tested two webcams, a Logitech Quickcam 4000, and a Roswill Pro Web.  Both are similar in color values.  However, the Logitech has a much better frame rate.  I was surprised at how poorly the Roswill performed on speed (640x480 @5 fps), because it is considerably newer than the Logitech (640x480 @ 15 fps).  I had just assumed that newer would probably out perform.

Offline Soeren

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Re: CMOS Sensor colors
« Reply #3 on: August 10, 2012, 10:07:06 PM »

What you see is called burn-out, where light parts saturates the pixels completely. The LED will normally be substantially brighter than its surroundings (ambient light) which it's adjusted for and there's mainly two routes to try... Either light up the surroundings a lot (experiment) to shift an automatic white balance and aperture, or with a good quality cam software, set it manually. You may be able to adjust white balance and/or "aperture" value.
Have an LED of each color on when you adjust the parameters - easier than guessing :)

In the unlucky case of not being able to set anything (Logitechs software is very adjustable), whether on a webcam or on what you're gonna use later and assuming you need the footage for other stuff than just telling the color of an LED, you could experiment with different sized apertures mechaniclly shifted in front of the lens (don't get false light from the sides).

With the right software, it may be possible to make every other frame a "darkened" one.

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives


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