Help! There is a comet in my computer! 28
The two basic properties of an image, related to the range of grey values that are used to
store information, are image contrast and brightness. Image contrast is the difference
between the highest (brightest) and the lowest (darkest) grey value in an image. Image
brightness describes whether the range of grey values used in the image is in the darker
part of the greyscale (at low grey values) or in the brighter part of the greyscale (at high
grey values).
Let us compare the three images of the same comet shown in Figure 17. Image A has a
high contrast, using grey values from 14 to 198, so that the total contrast range is 185.
Image B has a lower contrast – it contains grey values from 6 to 85 and its contrast range is
only 80. Image C has the same contrast range as image B, namely 80. However, in image
C, the range of used grey values is in the brighter part of the greyscale (82 to 161), hence
image C had a higher brightness than image B.
We must not forget that the grey values on the image are our measurement tool and that
they must linearly relate to the intensity of light emitted from the comet (we postulated this
general condition of precise comet measurement in Figure 4). The graphs on the right of
the histograms of image grey values in Figure 17 show that in all three images this
relationship is linear. However, there is a major difference between image A and images B
and C. In image A, the intensity of the signal from the microscope is described with 185
different grey values. On the other hand, in images B and C, there are only 80 grey values.
Therefore, the measurement of the comet parameters from image A would be more precise
than the measurement from images B or C. Measurement from all three images would be,
within limitations of different precision, valid.
To illustrate the problem of measurement precision, let us imagine that we are running an
experiment in which we need to measure the height of a person. The precision of our result
will largely depend on the instrument we will use for measurement. If we use a ruler that
only shows metre marks, we will be in serious trouble – we can only say that this person is
between 1 and 2 metres high. If we take a ruler that has also decimetre marks, we can do
our job a sort of acceptably. If we use a ruler with centimetre marks, we are really getting
the best out of our experiment.
If we return to the comet world, we are measuring the same comet in all three images in
Figure 17. If we measure image A, we will have a “ruler” with 185 grey values to describe
the range of light intensities emitted from the background and the comet. In images B and
C, our ruler only has 80 different grey values.
We can conclude that we have to use as many grey values as possible to describe
different light intensities emitted from the comet and (!) the background. However,
there is an important limitation, as we will see in the next chapter – no pixels should be
completely black (grey value 0) or completely white (grey value 255).
Of the three images shown in Figure 17, image A has the highest informational content and
is most suitable for comet measurements.
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