Biophotonics_Concepts_to_Applications

indicate a better ability to distinguish two closely spaced points. Resolution also
depends on the wavelength of the illumination used to examine a specimen.Shorter
wavelengths can resolve details better than longer wavelengths. Two commonly
used expressions for thewide-field resolutionR are

R¼ 0 : 61 k=NAobj ð 8 : 6 Þ

and

R¼

1 : 22 k

NAobjþNAcond

ð 8 : 7 Þ

whereλis the wavelength of the illuminating light and NAobjand NAcondare the
numerical apertures of the objective lens and the condenser lens, respectively. In
order to get good imaging results, the NA of the condenser lens should match the
NA of the objective lens.

Example 8.5What are some ways to increase the resolution of a

microscope?

Solution: Smaller values of resolution indicate a better ability to distinguish

two closely spaced points. Thus Eqs. (8.6) and (8.7) indicate that a better

resolution can be obtained by using a shorter wavelength or by increasing the

numerical aperture.

Objective lens

Focusing lens

Point source from a pinhole

Airy disk

x axis

Intensity (z axis)

Zeroth-order maximum

x = 0

x 0 (location of

first side lobe)

Light source

84% of the

luminous

energy

First side lobe

Fig. 8.7 Example of an Airy disk diffraction pattern and the point-spread function

8.2 Resolution and Diffraction Limit 245