Sunday, August 28, 2011

Advice on selecting a camera – sensor considerations

Number of pixels is, as we discussed previously, the number one criteria in selecting a camera, but there are some others to consider. Here a few bullet points you might want to take on board:
  • Frame rate. This matters because it drives the communications standard you pick. Slow applications can generally make do with FireWire. These days, for the midrange I like to use GigE, while for high speed imaging (and especially with linescan cameras,) I always go with CameraLink. In the future I may have to add HSLink to this list, but for now CameraLink handles pretty much anything I throw at it.
  • Pixel size. Small pixels give better fine resolution because the positional uncertainty is reduced, but they capture fewer photons per unit time than do larger pixels.
  • Spectral sensitivity. Maximize the number of photons you capture by matching the wavelength of the lighting to the peak sensitivity of the sensor. Interestingly, most sensors today seem to peak in the green yet we continue to use red LED illumination in most applications. Perhaps it’s time to go green?
  • Noise. Frankly I put this well down the list of parameters to consider because with good lighting it’s possible to minimize the effects of noise in most industrial applications. I will note though that if you are struggling for light, cranking up the gain or exposure will accentuate any noise in the image (which manifests as gray levels that vary from image to image.) Now if you’re doing astronomy or scientific imaging noise may well be a more important consideration.

Lastly, we should probably mention the great CCD versus CMOS debate. Old school vision guys will tell you that CCD’s always give better images, but I’m not sure that’s as true today as it was a decade ago. However, there are a couple of differentiators to be aware of.

First, CMOS sensors come with either a rolling or global shutter. This can have a huge impact if you’re imaging something in motion, so make sure you understand this point properly. Second, CMOS sensors have better windowing capability, meaning that if you can accept a reduced image size you can turn the frame rate up to some pretty astounding levels – it’s all driven by bits per second coming down the pipe from the camera.

So recapping, the imaging task will dictate the resolution of the sensor required. Once the number of pixels has been established it’s time to decide which of the other parameters matter. That will result in a specification, and from that point it’s time to get out the credit card and go shopping.

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