Help! There is a comet in my computer! 30
5.3 Image saturation
The recorded image is not suitable for measurement of the comet parameters if is either
overexposed or underexposed or both (Figure 18). This means that some pixels in the
image have the minimum grey value (0) or the maximum grey value (255); we say that
these pixels are saturated. Consequently, an image that contains saturated pixels is also
saturated. Images containing pixels with grey value 255 (white) are overexposed, and
those containing pixels with grey value 0 (black) underexposed.
The problem with saturated pixels is that they do not contain enough information for a
precise comet measurement. Let us look at examples in Figure 18. Image A has a good
contrast – it contains high and low grey values. However, a large area of the background
has grey value 0 - in the histogram of image grey values, the frequency of pixels with grey
value 0 is high – the image is underexposed. We can also visualise the saturated pixels by
using a special LUT (LUT1 in Figure 18), where all grey values of the recorded image are
shown as the original grey values in the displayed image, but the pixels with grey value 0
or 255 are displayed in colour (in this case, grey value 0 is green and grey value 255 red).
While we know from the information contained in the image that the saturated regions of
the background have a lower light intensity than the region of the comet, we do not know
about the variation of the signal in the background itself (see the graph to the right of
image histogram in Figure 18). If we now position the background ROI above the comet
and measure the comet parameters, these parameters are wrong, because the subtraction of
the background signal during calculation of the comet profile is incorrect. The background
signal in Image A was not recorded appropriately, and such an image should not be
measured.
Likewise, images B and C in Figure 18 should not be measured. Image B is overexposed,
so all we know about some pixels in the head is that they are very bright, but not also how
bright they are. Image C illustrates that an image can be both underexposed (no detailed
signal from the background) and overexposed (no detailed signal from the comet head) at
the same time.
We should only measure images that contain full information about the variation of light
intensity in the comet head, in the comet tail and in the background. If we keep in mind the
problem of contrast explained in the previous subchapter, we should only measure
images that have a good contrast and are not saturated. Among examples shown in
Figures 17 and 18, image A in Figure 17 is most appropriate for comet measurement – it
gives us a precise measurement of the comet parameters. Images B and C in Figure 17
give us a less precise measurement of the comet parameters, because we can only crudely
distinguish between shades of grey in the image (few grey values are used to record
information). However, measurements from these images can be regarded as rough
estimates of the true value of the comet parameters. Comet parameters measured from any
of the images shown in Figure 18 are wrong (or uncontrollably inaccurate at best), because
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