The edge effect is a phenomenon that can occur if there is not enough image surrounding the counting frame and the edge of the image is encountered before enough of the particle can be viewed. The counting frame rules imply that events outside of the main rectangle of the counting frame are important. For example, if a particle touches a green line, and then snakes around, and touches a red line or red line extension, then the particle is not counted. In order to apply the counting frame rules properly it is necessary to be able to view more than just the counting frame rectangle. The counting frame is typically much smaller than the entire view. The amount of ‘edge’ used depends on the particles being counted. More edge must be provided for counting squiggly shapes than for counting nearly round particles.
Figure 1 is an illustration of some purple star shaped particles. They are bounded by the image edge, which is the black rectangle. On the image is a counting frame. The upper particle has an extension that runs off the image. It is not possible to tell if the extension reaches the upper red extension of the counting frame. If the extension is short, such as the green loop indicates, then the object is counted. If it is as long, as the red loop indicates, then the particle is not counted. On the right side of the counting frame, two extensions touch the counting frame. Should 1 or 2 particles be counted? If the extensions are part of the same particle as indicated by the orange lines, then one particle should be counted. If the extensions are from different particles as suggested by the yellow drawings, then two particles are counted. The lower particle is not counted no matter what is outside of the field-of-view.
This is an example of the edge effect. There is an edge to the field-of-view that cuts off important decision-making information. The solution is to use a counting frame that is small enough so that the edge effect is not an issue. Another way to avoid problems with the edge effect is to move the counting towards the lower left corner of the image.
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