Thursday, January 10, 2013

Noise and CMOS sensors

Old school” machine vision people are often dismissive of cameras with CMOS sensors. “Too noisy,” they say, and indeed that was once true. But “CMOS Sensors Increase Inspection Speed and Accuracy,” published in the December 2012 Photonics Spectra sets out to explain why the future will be CMOS.

Read the article for details, but the bottom line is this: CMOS technology has been steadily improving, to the point where it looks like it will be used in all new machine vision cameras. The Photonics Specttra article also includes a handy little table summarizing “The merits of CMOS sensors, at a glance.”

Well worth a look.

But, top of the list of merits is “Good full well capacity”. Now I ask you, is that always a desirable characteristic?

If you plow through the very technical “Balancing sensor parameters optimizes imaging device performance,” published in Laser Focus World, December 1st, 2012, you’ll gain a better appreciation of the complexities of sensor noise. For there is not one source, but several. And driving down one tends to increase the others.

It all boils down to what you want the sensor to do. Scientific, low-light applications place very different demands on the sensor than do most machine vision applications where you can flood the target with photons.

And the takeaway for us machine vision craftsmen? It’s this: noise is a complicated issue, but it pays to get a better appreciation of the nature of the sources. That way, you’ll know which camera parameters matter most to your application. And yes, a CMOS sensor may be in your future.

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