Comet Labs WN591 Bedienungsanleitung PDF herunterladen (Seite 12)

Help! There is a comet in my computer! 9

3 Measurement of the comet parameters

Now we recorded an image, and finally we come to image analysis. Many software

packages can display the information they receive from a camera on the screen and save

image files, but this is not image analysis. The (relatively vague) definition states that

image analysis is a tool for extraction of quantitative (!) information contained in an

image. And this is what we intend to do – we will use the information contained in the

image to measure the intensity of light emitted from the comet, and hence the amount of

DNA.

Although the general principles of image analysis used in different comet software

packages are the same, there is some variation in the exact procedures, so you need to

check the user guide of your own comet software for details.

3.1 Grey values

Our measurement is based on the relationships between different parameters. So far we

learned that the relationship between the amount of DNA and the intensity of emitted

orange light is linear. The intensity of light (and consequently the amount of DNA) is now

coded as numbers in our image file. But what kind of numbers do we have in the file? How

are they related to light intensity?

Not so long ago, there would be only one answer to the first question. Lately things got a

bit more complicated. For clarity of presentation, let us pretend to go back in time a little

and deal with additional problems later (see chapter Why a 10-bit camera beats an 8-bit

camera, page 38). For the present purposes, the answer is: Light intensities are represented

as whole (integer) numbers from 0 to 255 (together 256 values), where 0 means a black

pixel on the image and 255 a white one (Figure 4). Because they are represented as shades

of grey in the image, these numbers are called grey values. The answer to the second

question is: The relationship between light intensity that a camera element detects and

the grey value of the pixel on the image is linear. Hence, the relationship between the

amount of DNA present in one small dot in the comet and the corresponding pixel

grey value is linear (Figure 4).

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Comet Labs WN591 Bedienungsanleitung Seite 12