Biophotonics_Concepts_to_Applications

Thelimit of resolutionrefers to the smallest separation between two closely
spaced Airy disks for which a microscope objective can distinguish the individual
Airy patterns. For example, Fig.8.8shows two Airy disks and their intensity
distributions that are separated by a short distance. If the separation d between the
two disks is greater than their radii, then the two corresponding image points are
resolvable. The limiting separation for which the two Airy disks can be resolved
into separate entities is often called theRayleigh criterion. That is, two Airy disks
usually cannot be resolved when the center-to-center distance between the
zeroth-order maxima is less than the Airy resolution given in Eqs. (8.6)or(8.7).
If several objective lenses have the same focal length but different NAs, then the
Airy disks in the images become smaller as the NA becomes larger. This is illus-
trated in Fig.8.9where the NA increases from left to right. As the Airy disk
projected in the image becomes smaller, then more details within the specimen can
be seen. That is, smaller Airy disks allow the viewer to better distinguish two
closely spaced points in the specimen.

Airy disk 1 Airy disk 2

Intensity x axis

Separation between

Airy disk centers

d

Fig. 8.8 The separation of
two closely spaced Airy disks
determines the diffraction
limit

Fig. 8.9 As the NA increases, the Airy disks get smaller for objectives of the same focal length.a
Small NA large Airy disk.bIntermediate NA medium Airy disk.cLarge NA small Airy disk

246 8 Microscopy