Comet Labs WN591 Bedienungsanleitung PDF herunterladen (Seite 30)

Help! There is a comet in my computer! 27

5.1 Histogram of image grey values

It is, however, not important at all for our measurement whether the image “looks” fine to

us. The only important thing is what the computer can distinguish in it. Because our eyes

are a lousy tool for estimation of the number of shades of grey that an image contains, we

will evaluate the quality properties of an image that are important for measurement of the

comet assay by inspection of a histogram of image grey values. The histogram shows the

frequency distribution of pixels along the whole range of grey values (0-255; Figure 16).

The majority of the pixels in our comet image belong to the background, hence there is a

high peak of pixel frequencies in the low grey value range. Because there is some variation

in the background signal across the image, the background grey-value peak is not very

narrow. The pixels of the comet have brighter shades of grey than the background and are

represented by a long trail of lower pixel frequencies to the right of the background peak.

Figure 16. The histogram of image grey values. Because most of the image is very dark

(background), the frequency of pixels with low grey values is high. To see also the distribution of

brighter pixels (the pixels of the comet), the lower part of the y-axis has been scaled up and hence

the top of the peak at low grey values is cut off. Note that in this image there are no pixels with very

low or very high grey values.

5.2 Image contrast and brightness

The goal of image acquisition in the comet assay is to describe the brightness of the comet

with as many different grey values as possible. The more grey values we use, the more

accurately we can tell apart different intensities of light emitted from different parts of the

comet, the more information the image contains and the more precisely we can measure

the comet parameters. However, because the calculation of the comet parameters depends

upon subtraction of the background signal, we also have to record variation in the

background signal, not only in the comet.

255

Grey values

Number of pixels

comet (bright pixels)

background (dark pixels)

255

Grey values

Number of pixels

255

Grey values

Number of pixels

255

Grey values

255

Grey values

Number of pixels

comet (bright pixels)comet (bright pixels)

background (dark pixels)background (dark pixels)

1 2 ... 25 26 27 28 29 30 31 32 33 34 35 ... 48 49

Kommentare zu diesen Handbüchern

Keine Kommentare

Comet Labs WN591 Bedienungsanleitung Seite 30

Kommentare zu diesen Handbüchern