Comet Labs WN591 Bedienungsanleitung PDF herunterladen (Seite 14)

Help! There is a comet in my computer! 11

Figure 5. Segmentation of the comet image. During segmentation, the regions of interest (ROI) for

measurement of the comet parameters (the head and the tail) are defined. The user can

interactively draw rectangles that define the head and the tail ROI (A; box segmentation). In some

software packages, the head and the tail are detected (semi-)automatically and outlined (B; close-

fitting segmentation).

3.3 Geometric calibration

Let us measure one comet parameter for the simple case shown in Figure 5A – the length

of the tail. All we need to do is think with computer logic: we count the number of pixels

in the tail ROI, along the x-axis. The tail length is 14 pixels – and now we have another

problem. We usually express lengths in micrometers, not in pixels. To be able to do this,

we need to know how many micrometers there are per each pixel. The computer also

needs to know this. So we will now perform a geometric calibration of the image analysis

system.

What we need to do is record an image of a calibrated microscope slide with an engraved

“ruler” in micrometer units, and then measure how many pixels there are per 100 µm (or

another known length on the ruler; Figure 6). We only need to perform the geometric

calibration once – the scaling factor (the number of micrometers per pixel) is saved in the

memory of the computer. Geometric calibration has to be done for each objective

separately – do not just calculate the scaling factor of your 100× objective from the

measured scaling factor of your 10× objective. The objective magnifications are not exact.

In our system, the scaling factor for the 10× objective is 1.06 µm per pixel, and for the

100× objective 0.104 µm per pixel (and disagreements can be much larger). Correct

geometric calibration is essential for measurement of those parameters that are expressed

in absolute units (µm, µm

-2

). We will see later that also the Olive tail moment has absolute

units.

head tail

head tailhead tailhead tail

head tail

head tailhead tail

1 2 ... 9 10 11 12 13 14 15 16 17 18 19 ... 48 49

Keine Kommentare

Comet Labs WN591 Bedienungsanleitung Seite 14