Thursday, April 7, 2011

Does sensor technology matter?

Many of the new cameras coming on to the market, and especially those with high resolutions, like the 10Mp UI-1495LE from IDS, make use of CMOS rather than CCD sensors. In addition to their pixel count, these cameras have several things going for them. They tend to be less expensive than CCD-based cameras, (I think the IDS camera referenced above sells for under $1,000,) and they generally have a higher dynamic range than their CCD equivalents. (This makes it possible to get good images in situations with extremes of contrast.) In addition, CMOS sensors can be coerced into delivering a high frame rate simply by reducing the image size. But they also have at least one drawback. Prudent engineers should be sure to understand this before using CMOS-based cameras in their machine vision applications.

The issue is the rolling shutter. Now not all CMOS cameras use rolling shutter technology, but it’s important to check the specs for this, especially if you intend using the camera in an application where the target is in motion. The reason for this is that the rolling shutter exposes pixels row by row. That means the image of an object that moves past the camera horizontally during the exposure period will be distorted. (There will be distortion if the part moves vertically too, but it won’t be so evident.)

The preferred alternative would be to look for a sensor with a global shutter. In a global shutter all the pixels are exposed simultaneously, so there’s no distortion of the image.

If you’d like to see how these two exposure techniques differ in practice, hope on over to “Sensor Artifacts and CMOS Rolling Shutter” by Barry Green and published on the dvxuser website. There you’ll find some great animations that really illustrate the difference.

While you’re there, make sure to read about the other sensor artifacts, smear, wobble and partial exposure because they can all play havoc with your vision application too.

Lastly, no discussion of CMOS versus CCD sensors would be complete without talking about image quality. The conventional wisdom is that CMOS sensors produce lower quality images than do CCD’s. My view is that while this may have been true in the past, I’m not sure it’s really an issue any more. Now I say this, not by analyzing performance specs but based on what my eyes tell me. I recently purchased a beautiful Nikon camera for home use, and it came with a CMOS sensor. Admittedly, it’s a large format sensor, but it produces gorgeous images. (Notice how I’m crediting the hardware and not the photographer?!) So my feeling is that if CMOS is good enough for the “pro-sumer” camera buyer, then it’s probably good enough for 95% of machine vision applications.

So does sensor technology matter? Yes, but because of artifacts like smear, wobble and partial exposure rather than “image quality.” As always, caveat emptor.

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