Some detectors, or their associated read-out systems, are limited in the number of counts they can accumulate in one pixel. The number recorded reaches a maximum number which cannot be further increased, i.e. the pixels can become saturated. This means that these pixels retain the same maximum value on longer exposure whilst other, non-saturated, pixels continue to accumulate counts. The intensity in saturated pixels will hence be underestimated compared to the others and any intensities estimated from profiles including such pixels will be biased towards low values. It is essential that pixels that are saturated are flagged and recognized by the processing software. There are several ways to deal with the problem of saturation.
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(1) Reject all reflections that contain saturated pixels. These will tend to be at low resolution. If more than a very few are rejected, this can be a truly disastrous choice, especially if the data are to be used for molecular replacement. In addition, missing the largest terms degrades the continuity and information content of all electron-density maps derived therefrom. This point is relevant to several applications (Section 9.1.13![[link]](/graphics/greenarr.gif) ).
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(2) Reject only those pixels that are saturated, and fit average standard profiles estimated from the non-saturated spots. This gives a poorer estimate than if the pixels were not saturated, but for applications such as molecular replacement or direct methods where the high-intensity data are essential, it is certainly better than option (1).
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(3) Reduce the exposure time to ensure that there are no overloaded pixels. This is a trade-off, because if there is a large contrast between the intensity of the weakest and the strongest terms in the pattern, then the weaker terms will have a low and possibly unacceptable signal-to-noise ratio under this regime.
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(4) Use more than one pass through the rotation range, with different exposure times. The longest exposures should be sufficient to ensure that the intensities of the data at the high-resolution limit of the pattern are statistically significant. The shortest should ensure that the number of saturated pixels in the `low-resolution' pass is minimized. If the contrast between the low- and high-resolution passes is too great, differing by a factor of much more than about ten, then additional passes with intermediate exposure times should be used to allow satisfactory scaling of the data from these images. The CTDD for each pass with shorter exposure should be increased only so as to cover the resolution to which reflections were saturated on the previous pass. The rotation range on individual images can then be increased accordingly, in the wide φ-slicing option. On bright synchrotron beamlines, if the second pass requires exceedingly fast rotation of the spindle-axis motor and rapid opening and closure of the beam shutter beyond the limit of reliability, it may be better to attenuate the beam, for example with a series of aluminium foils. As discussed in Section 9.1.7.1, if high-resolution data are collected in several passes with different exposures and resolution limits, it may not be necessary to cover all of the theoretically required rotation range in the highest-resolution pass. The curvature of the Ewald sphere results in the high-resolution data being completed with a smaller total rotation range than the low. It is vital that the lowest-resolution pass covers the total rotation range required for complete data.
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Clearly the optimum solution is to have a detector with a sufficient dynamic range to cover pixels of both weak and strong reflections. The dynamic range has already been increased with recent imaging plates and CCDs. Enhanced dynamic range may prove to be the most important advance of solid-state pixel detectors.
An additional advantage of the fine-slicing approach is that it leads to fewer overloads. Each reflection profile is divided between several separate images and as a result the effective dynamic range of the detector is increased.
