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How can we know the camera sensor size from the calibration matrix?

I already have the calibration matrix. But the focal length is in pixels form.

kenneth
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2 Answers2

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The "focal length" in the calibration matrix is not actually expressed in pixels. It is in fact a ratio that relates the sensor size in the real world to the size of a pixel in the image. It relates the focal length of the lens to distances in the image using a scaling factor in each dimension. More details in this article.

Zaphod
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  • Okay, so how can we get the scaling factor ? – kenneth Jun 06 '13 at 10:45
  • is the scaling factor same thing as the pixel size of each edge? – kenneth Jun 06 '13 at 10:51
  • Do you know the focal length in mm? If yes then the scaling factor `m_x = \alpha_x / f_x`. – Zaphod Jun 06 '13 at 11:00
  • all i want to know is how can we get the alpha_x ? – kenneth Jun 06 '13 at 11:02
  • alpha_x is the first entry in the intrinsic calibration matrix. – Zaphod Jun 06 '13 at 11:10
  • opps....actually this is not what i mean. what i mean is if we have a camera and with the datasheet, is the m_x something equal to each pixel size? what i mean pixel size is (number of pixel)/(sensor size) for one edge. – kenneth Jun 06 '13 at 11:11
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    m_x is the scaling factor that converts the focal length from the number of meters (f_x) to the number of pixels (alpha_x). This implies that the physical size of every pixel on the sensor is 1/m_x. – Zaphod Jun 06 '13 at 13:05
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As Zaphod says, you can get a decent approximation combining the calibration matrix with a known focal length for the lens in mm.

A more precise method: open the box, read the serial type number of the CCD, look it up on the interwebs. There are only a handful of CCD manufacturers, and they all publish their spec sheets on the web. Here is an example.

Francesco Callari
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