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Pixel Form

While the pixels shown in Figure 1 appear to be square and to "cover" the continuous image, it is important to know the geometry for a given camera/digitizer system. In Figure 18 we define possible parameters associated with a camera and digitizer and the effect they have upon the pixel.

Figure 18: Pixel form parameters

The parameters Xo and Yo are the spacing between the pixel centers and represent the sampling distances from equation . The parameters Xa and Ya are the dimensions of that portion of the camera's surface that is sensitive to light. As mentioned in Section 2.3, different video digitizers (frame grabbers) can have different values for Xo while they have a common value for Yo.

Square pixels

As mentioned in Section 5, square sampling implies that Xo = Yo or alternatively Xo / Yo = 1. It is not uncommon, however, to find frame grabbers where Xo / Yo = 1.1 or Xo / Yo = 4/3. (This latter format matches the format of commercial television. See Table 3) The risk associated with non-square pixels is that isotropic objects scanned with non-square pixels might appear isotropic on a camera-compatible monitor but analysis of the objects (such as length-to-width ratio) will yield non-isotropic results. This is illustrated in Figure 19.

Figure 19: Effect of non-square pixels

The ratio Xo / Yo can be determined for any specific camera/digitizer system by using a calibration test chart with known distances in the horizontal and vertical direction. These are straightforward to make with modern laser printers. The test chart can then be scanned and the sampling distances Xo and Yo determined.

Fill factor

In modern CCD cameras it is possible that a portion of the camera surface is not sensitive to light and is instead used for the CCD electronics or to prevent blooming. Blooming occurs when a CCD well is filled (see Table 11) and additional photoelectrons spill over into adjacent CCD wells. Anti-blooming regions between the active CCD sites can be used to prevent this. This means, of course, that a fraction of the incoming photons are lost as they strike the non-sensitive portion of the CCD chip. The fraction of the surface that is sensitive to light is termed the fill factor and is given by:

The larger the fill factor the more light will be captured by the chip up to the maximum of 100%. This helps improve the SNR. As a tradeoff, however, larger values of the fill factor mean more spatial smoothing due to the aperture effect described in Section 5.1.1. This is illustrated in Figure 16.

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