Biophotonics_Concepts_to_Applications

transverse resolutionΔx. This parameter commonly is specified as the focal
diameter of the incident sample beam and depends only on the wavelength and the
characteristics of the objective lens optics. Thus the transverse resolution is given as

Dx¼

4 k 0

p

f

D

¼

2 k 0

p

1

NA

ð 10 : 12 Þ

where f is the focal length of the objective lens, D is either the diameter of the beam
on the lens or the diameter of the lens (whichever is smaller), and NA is the
numerical aperture of the objective defined by Eq. (8.1).
The full width at half-maximum power of the confocal axial response function
gives the axial range (depth range) within which the transverse resolution is fairly
constant. This range is defined as thedepth of focusΔzfof the OCT system and is
given as

Dzf¼

2

p

k 0

NA^2

ð 10 : 13 Þ

Equations (10.13) and (10.12) show that as the numerical aperture of a con-
ventional OCT system increases, the transverse resolution improves as it becomes
smaller but the depth of focus deteriorates. Recent developments in the use of an
adaptive optics method called depth-encoded synthetic aperture OCT have resulted
in improvements in the depth of focus [ 11 ].
Thelateralfield of viewFOVlatis given by

FOVlat¼2fhmax ð 10 : 14 Þ

where f designates the focal length of the objective lens andθmaxis the maximum
one-sided scan angle of a rotating scanning mirror.

Example 10.5Consider an OCT system in which the objective lens has a

numerical aperture NA = 0.26. (a) What is the lateral resolution at a wave-

length of 1310 nm? (b) What is the depth of focus for this setup?

Solution: (a) Using Eq. (10.12) yields the following lateral resolution

Dx¼

2

p

k 0

NA

¼ 0 : 64

1310 nm

0 : 26

¼ 3 : 22 lm

(b) Using Eq. (10.13) yields the following depth of focus

Dzf¼

2

p

k 0

NA^2

¼

2

p

ð 1 : 31 lm)

ð 0 : 26 Þ^2

¼ 12 : 3 lm

300 10 Optical Imaging Procedures