Handbook of Biomedical Optics

8 Handbook of Biomedical Optics

Since the wavelength in the image space, 

0

is equal to /n

i

,

where  is the wavelength in air or in vacuum, Equation 1.22

becomes, using Equation 1.20,

Δx

n

≥ =

λ

θ

λ

i im i

/ NAsin( )

.

2

(1.23)

Similarly, we can calculate the depth-of-focus, z, using zp

z

≥ h,

where p

z

is the momentum dierence between rays FB and FA

along the z-direction, as shown in Figure 1.9. Details of the deri-

vation have been provided (Poon 2007), and we state the nal

result as follows:

Δz

n

≈

2

i

i

2

λ

NA

. (1.24)

To summarize the above results, Figure 1.10 shows an image-

forming instrument with 

0b

and 

im

denoting the ray of maxi-

mum