Help! There is a comet in my computer! 32
Some comet software packages feature an option to set the maximum number of allowed
saturated pixels. A very small number of saturated pixels does not severely affect the
measurement. In addition, some light-sensitive detector elements in the camera may not
function properly; they may not be able to detect the signal or may misfire when there is no
light. Even a new camera may have some dysfunctional pixels. In any case, we should
avoid systematically overexposing or underexposing images.
5.4 How can I evaluate the quality of my image?
A histogram of image grey values is an essential tool for evaluation of image quality and
optimisation of the settings of image acquisition. Nevertheless, not all commercially
available comet software packages have the option to display the histogram. In some cases,
the histogram is included in the user interface for adjustment of the camera settings (see
examples in Figure 25). If we want to display histograms of image grey values for our
comet images but our comet software cannot do it, we can use another image processing
software that features the histogram function. Here we will look at functions available in
Adobe Photoshop.
In our comet program, we save the image in a lossless format, such as TIFF (the TIFF
format preserves all information contained in the grabbed image without any distortion).
We need to make sure that the image that we export (save) is the original grabbed image
recorded by the camera, and not just a displayed version of the grabbed image. We are only
concerned about the quality of the recorded images because these are measured; the quality
of the displayed images is not important (for further comments see chapter Which image
does my software measure?, page 41). Then we open our saved image in Photoshop (from
the main menu File / Open). To see the histogram of image grey values, we select from the
main menu Image / Adjust(ments) / Levels (Figure 19A). Because an image of the whole
field of view usually contains a lot of background regions and just a few comet regions, it
is difficult to see the frequency distribution of the signal that we are most interested in,
namely the brighter regions of comets. (In Figures 16, 17 and 18, this was not a problem,
because only the small part of the whole image that contains the comet was analysed.) In
some image processing software packages, it is possible to scale up the y-axis of the
histogram, which solves the problem.
For a more detailed analysis of image grey values in Photoshop, we can segment the image
on the basis of grey values. We select from the main menu Image / Adjust(ments) /
Threshold (Figure 19B-E). When we check the “Preview” checkbox, the original greyscale
image is converted to a binary (black and white) image. If we now drag the slider under the
histogram from right (grey value 255) to left (grey value 0), we see different details of the
image emerging on the binary image. At each position of the slider, we see all regions of
the image that have the grey value higher than the grey value selected with the slider as
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