25. Fourier Optics
Fraunhofer diffraction and Fourier transform
( ) ( ) ( ξ η ) ξ η
λ η π
λ U ξ j z x y d d
z j e y e
x U
y z x
j k jkz
− +
= ∫ ∫
∞∞
−
+
2
exp ,
,
) 2 (
2 2
( )
{ }
f x z f y zy z x
j k jkz
Y X
z U j
e e
λ
η
λλ ξ
/ , /) 2 (
,
2 2
=
= +
= F
Spatial frequency :
xx ,
yy
f f
z z
λ λ
= =
Example of Optical Fourier Transform
2-D Fourier transform pair
{ }
{ }
{ }
{ }
{ }
2
1
( , ) 1 ( , ) exp ( )
(2 )
( , ) exp 2 ( )
( , )
( , ) ( , )exp ( )
( , )
x y x y x y
x y x y x y
x y
x y x y
f x y g k k j xk yk dk dk
g f f j xf yf df df
g f f
g f f f x y j xf yf dxdy
f x y π
π
+∞
−∞
+∞
−∞
−
+∞
−∞
= − +
− +
= +
=
∫ ∫
∫ ∫
∫ ∫
=
= F
F
Fourier Transform
{ } { }
Inverse Fourier Transform
1
( , )exp 2 ( )
( , )exp 2 ( )
X Y
X Y X Y X Y
G g g x y j f x f y dx dy
g G G f f j f x f y df df
π
π
∞
−∞
− ∞
−∞
= = − +
= = +
∫ ∫
∫ ∫
F
F
x
y
1/fx
1/fy
(
y)
exp j 2 π xf
x+ yf
Fourier and Inverse Fourier Transform
α
α β β
Fourier Components
(
x,
y)
g f f
exp− j2π(
xfx + yfy)
Some Properties of FT
( ) ( ) { }
( ) ( ) { }
( ) ( )
) , ( )
, ( )
, (
. definition for the
' ' use books some
but sign,
'-' use We
*
, ,
) , (
) , ( )
, ( )
, ( ,
,
) , ( ) , ( ,
,
*
*
* 2
*
y x p
y x g y
x c
d d y x
p g
y x c
f f G y
x g
y x g d
d y x
g g
f f H f
f G h
g d
d y
x h g
gp gp
Y X Y X Y
X
−
−
⊗
=
+
=
−
=
−
−
⊗
=
− −
∗
=
⊗
=
− −
∗
∫ ∫
∫ ∫
∫ ∫
∞
∞
−
∞
∞
−
∞
∞
−
η ξ η
ξ η
ξ
η ξ η
ξ η
ξ
η ξ η ξ
η ξ
∓
∓ n
rrelatio (Cross-)Co
theorem ation
Autocorrel
theorem n
Convolutio
F F
F F
Real and Spatial-Frequency Spaces
y2
x2
( ξ , η )
y1
x1
Optical System
Input Output
f
Yf
XFourier Transform
Inverse FT FT
IFT
Fourier Transform with Lenses
Input placed against lens
Input placed in front of lens
Input placed behind lens
back focal plane
Input Placed Against Lens
( )
x y At( )
x yUl , = A ,
Pupil function ; ( )
=
otherwise
0
aperture lens
the inside
1 , y
x P
( ) ( ) ( ) ( )
− +
= 2 2
'
exp 2 ,
,
, x y
f j k y
x P y x U y
x
Ul l
( ) ( )
( ) ( ) (
xu y)
dxdyj f y
f x j k y
x f U
j f u j k u
Uf
∫ ∫
l + − +
+
= ∞
∞
−
λ υ π λ
υ
υ exp 2
exp 2 2 ,
exp
, ' 2 2
2 2
( )
( )
( ) ( ) (xu y ) dxdy
j f y
x P y x f U
j f u j k u
U f
∫ ∫
l − +
+
= ∞
∞
−
λ υ π λ
υ
υ exp 2 , , exp 2
,
2 2
Fraunhofer diffraction pattern!
From the Fresnel diffraction formula (z = f):
Input Placed in Front of Lens (I)
(
X,
Y)
0(
X,
Y) exp [ (
X2 Y2) ]
l
f f F f f j d f f
F = − πλ +
( ) ( )
+
= f f
F u f
j f u j k u
U f l
λ υ λ
λ
υ
υ exp 2 ,
,
2 2
( ) ( )
( ) (
ξ ηυ)
ξ ηλ η π
λ ξ
υ
υ u d d
j f f t
j f u d f
j k A
u
U f A
− +
+
−
=
∫ ∫
∞∞
−
exp 2 ,
2 1 exp ,
2 2
Input Placed in Front of Lens (II)
( ) ( ) ( ξ ηυ ) ξ η
λ η π
λ ξ
υ u d d
j f f t
j u A
U
f A
− +
= ∫ ∫
∞∞
−
exp 2 ,
, If d =f, then
Exact Fourier Transform!
Input Placed Behind Lens
( ) ξ η ξ η ( ξ η ) ( ) ξ , η
exp 2 ,
,
2 20
t
Ad j k d
f d
P f d U Af
− +
=
( ) ( )
d f d
j d u j k A
u U
fλ
υ
υ
+
=
2 2
exp 2 ,
( ) (
ξ υη)
ξ ηλ η π
ξ η
ξ u d d
j d d
f d
P f tA
− +
×
∫ ∫
∞ ∞
−
exp 2 ,
,
Example of Optical Fourier
Transform
4-f System and Optical Filtering
Example of Spatial Filter
Image Formation
( ) u υ h ( u υ ξ η ) ( ) U ξ η d ξ d η
U
i, ∫ ∫
∞, ; ,
0,
∞
−
=
( u υ ξ η ) K δ ( u M ξ υ M η )
h , ; , ≈ ± , ±
(hopefully)
Impulse Response of a Positive Lens
( ) =
2 (
2+
2) (
2+
2)
1 2
2
1
exp 2
exp 2 , 1
;
, υ ξ η
η λ ξ
υ z
j k z u
j k z
u z h
( ) ( )
+
+ −
× ∫ ∫
∞∞
−
2
1
21 1
exp 2 ,
2 1
y f x
z z
j k y
x P
dxdy z y
x z z
u jk z
+
+
+
−
×
2 1
2 1
exp ξ η υ
The Lens Law
1 0 1
1
2 1
=
− + z f
z
Lens Law (Imaging Equation)( ) ∫ ∫
∞( )
∞
−
≈ P x y
z u z
h 1 ,
,
; ,
2 1
λ
2η ξ υ
( ) ( )
[ u M x M y ] dxdy
j z
− − + −
× ξ υ η
λ π
2
exp 2
1 2
z
M = − z
Magnification* Impulse response is the Fraunhofer diffraction pattern of lens aperture.
Coherent/Incoherent Imaging
Incoherent : 2 2 2
g g
i
h I h U
I = ⊗ = ⊗
Coherent : 2
g
i
h U
I = ⊗
Incoherent :
F { } I
i= [ H
★H ] [ G
g★
G
g]
Coherent
:
F { } I
i= HG
g★
HG
gcorrelations
Amplitude Transfer Function
For coherent systems,
(
X Y) (
X Y) (
g X Y)
i
f f H f f G f f
G , = , ,
( ) ( ) ( )
− +
= ∫ ∫
∞∞
−
dxdy y
z ux j
y x z P
f A f
H
i i
Y
X
υ
λ π λ
exp 2 ,
, F
( A λ z
i) ( P − λ z
if
X− λ z
if
Y)
= ,
( ) ( ) u , υ h ~ u , υ U ( ) u , υ
U
i= ⊗
gAmplitude transfer function
Optical Transfer Function (OTF)
For incoherent systems,
(
X Y) (
X Y) (
g X Y)
i
f , f f , f g f , f
g = H
( )
∫ ∫ ( )
∫ ∫
∞
∞
−
∞
∞
−
∗
− −
+ +
=
dpdq q
p H
dpdq q
p H
q p
H f
f
Y X
Y X
Y X
f f
f f
,
2, ,
,
2 2 2 2H
OTF : the normalized autocorrelation function of the amplitude transfer function!
( f ,
Xf
Y)
H
Modulation transfer function (MTF) :