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2 Microscopy and Imaging Systems
53
Fig. 2. Two point sources at
a separation corresponding to
the resolution limit. The resulting intensity profIle dearly
shows two peaks with a contrast
of 0.2
It is the numerical aperture of the lens that determines the resolution
limit, not the magnification. Magnification will only adapt the image size
to the image sensor [eye, film, CCD (charged coupled device) sensor].
Contrast
Up to now we assumed the ideal situation of two bright objects on a completely black background. In reality, there will be some background fluorescence due to several reasons that will be discussed later. Figure 3 shows
the situation oftwo signals lying on top ofthe background intensity 1 0 • The
effective contrast of the dip has been reduced to 0.1. It will be more difficult to distinguish the two signals.
The presence of fluorescence background will decrease the effective
contrast of two adjacent signals, making it harder to distinguish them.
0.8
i
Fig. 3. Contrast drops to 0.1
in the presence of a background
level of 40%