Help! There is a comet in my computer! 23
Images of orange comets on a black background (Figure 13C) are very popular. Notably,
most systems for comet measurement use monochrome cameras (called “black-and white”
cameras in ordinary life) that only detect the intensity of light, but not colour (wavelength).
Monochrome cameras can thus only record greyscale images, not colour images. So how
does the computer construct the orange image of a comet when all it received from the
camera were grey values (information about light intensity) and no information at all about
the colour of the comets?
To know how to display an orange image on the screen, the computer looks into a special
table, which is, not surprisingly, called a look-up table (LUT). A LUT is a table similar to
a translation dictionary; each grey value (0-255) is paired up with its colour translation.
The LUT for the orange images of comets says that black (grey value 0) is translated to
black, white (grey value 255) is translated to bright yellow, dark grey (grey value 72) to
very dark orange, and so on for all 256 grey values.
An image analysis software may contain many different LUTs. Some translate a greyscale
image to a colour image, some to another (different) greyscale image. Using different
LUTs, we can display a grabbed image of a comet in many different ways (Figure 14).
When we want to see what the original grabbed image actually looks like, the computer
uses the LUT where each of the grey values on the grabbed image is translated to the same
grey value on the displayed image (Figure 14B), so that the displayed image is an exact
copy of the grabbed image saved in the computer memory.
LUTs are in many ways a wonderful thing. LUTs can bring out the details in an image that
we would otherwise not be able to see. However, it is clear from Figure 14 that the type of
LUT that we use to display an image of a comet affects the way we perceive a comet. For
example, with some LUTs, the comet tail looks more prominent than with others, hence
using different LUTs during measurement of one comet experiment can affect our
decisions about the size of the head and the tail if we do interactive and not automatic
measurements.
If your comet software uses LUTs to display images, there should be a menu from which
you can select the LUT you want to use.
4.2 Live image and grabbed image
When we place a comet slide under a microscope and switch on the camera to record
images, we see the image of the slide displayed on the screen and the comets on the screen
move as we move the microscope stage. This kind of communication between the camera
and the computer is called “live mode”, and we say that we see a live image. The camera is
grabbing subsequent images at very short time intervals and sending the image files to the
computer. The computer saves each image file in temporary memory, quickly decodes it,
displays it on the screen and deletes it. Then it displays the next one. Hence we have the
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