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5. Aberration Theory5. Aberration TheoryLast lecture
•Matrix methods in paraxial optics •matrix for a two-lens system, principal planes
This lectureWavefront aberrationsChromatic AberrationThird-order (Seidel) aberration theory
Spherical aberrationsComaAstigmatismCurvature of FieldDistortion
AberrationsAberrations
• A mathematical treatment can bedeveloped by expanding the sine and tangent termsused in the paraxial approximation
ChromaticChromatic MonochromaticMonochromatic
Unclear Unclear imageimage
Deformation Deformation of imageof image
SphericalSpherical
ComaComa
astigmatismastigmatism
DistortionDistortion
CurvatureCurvature
n (n (λλ))
Third-order (Seidel) aberrationsThird-order (Seidel) aberrations
Paraxial approximation
Aberrations: ChromaticAberrations: Chromatic• Because the focal length of a lens depends on the
refractive index (n), and this in turn depends on the wavelength, n = n(λ), light of different colors emanating from an object will come to a focus at different points.
• A white object will therefore not give rise to a white image. It will be distorted and have rainbow edges
Aberrations: SphericalAberrations: Spherical• This effect is related to rays which make large angles
relative to the optical axis of the system• Mathematically, can be shown to arise from the fact that
a lens has a spherical surface and not a parabolic one• Rays making significantly large angles with respect to
the optic axis are brought to different foci
Aberrations: ComaAberrations: Coma• An off-axis effect which appears when a bundle of incident rays
all make the same angle with respect to the optical axis (sourceat ∞)
• Rays are brought to a focus at different points on the focal plane• Found in lenses with large spherical aberrations• An off-axis object produces a comet-shaped image
ff
Astigmatism and curvature of fieldAstigmatism and curvature of field
Yields elliptically distorted imagesYields elliptically distorted images
Aberrations: Pincushion and Barrel Distortions
Aberrations: Pincushion and Aberrations: Pincushion and Barrel DistortionsBarrel Distortions
• This effect results from the difference in lateral magnification of the lens.
• If f differs for different parts of the lens,
o
i
o
iT y
yssM =−= will differ also
objectobject
Pincushion imagePincushion image Barrel imageBarrel image
ffii>0>0 ffii<0<0
M on axis less than off M on axis less than off axis (positive lens)axis (positive lens)
M on axis greater than M on axis greater than off axis (negative lens)off axis (negative lens)
Ray and wave aberrationsRay and wave aberrations
LA
TALA : longitudinal ray aberrationTA : transverse (lateral) ray aberration
Actual wavefrontWave aberration
Ideal wavefrontParaxial Image plane
Paraxial Image point
Longitudinal Ray Aberration Longitudinal Ray Aberration
Modern Optics, R. Guenther, p. 199.
n = 1.0nL = 2.0
R
sinsin sin sin2
ii i t i tn n θθ θ θ= ⇒ =
Longitudinal Aberration – cont’d Longitudinal Aberration – cont’d
Modern Optics, R. Guenther, p. 199.
n = 1.0nL = 2.0
Rsinθt
( )
( ) sinsin sin
t i i t
ti t
RR z
γ π θ π θ θ θθγ θ θ
′′ = − − − = −
′′ = − =+ Δ
( ) ( ): , sin sin
sini i t t
i t i t
For paraxial rays Rρ θ θ θ θθ θ θ θ
≅ ≅
− ≅ −
ΔZ = ΔZ (θi )
Modern Optics, R. Guenther, p. 199.
n = 1.0
nL = 2.0
( ρ )
Δz/R
Longitudinal Aberration – cont’d Longitudinal Aberration – cont’d
ΔZ
Third-order (Seidel) aberrationsThird-order (Seidel) aberrations
Paraxial approximation
2 3( 1)(1 ) 1 ( )2!
n n nx nx x O x−+ = + + +
Third-order aberrations :On-axis imaging by a spherical interface
Third-order aberrations :On-axis imaging by a spherical interface
( ) ( ) ( ) ( )1 2 1 2opda Q PQI POI n n n s n s′ ′= − = + − +
To the paraxial (first-order) ray approximation, PQI = POI according to Fermat’s principle.
Beyond a first approximation, PQI ≠ POI , thus the aberration at Q is
Let’s start the aberration calculation for a simple case.
P
Q
I
O
h
Rℓ
α
β
φP
Q
CO
s+R
( )( )
( )
( ) ( )
( ) ( )
2 2 2
2 222 2 2 2 2
2
2 2 2
22 2
cos sin
sin cos 1 2
2 cos:
2 cos
Rs R s R
Rs R R R
s R
s R R R s R RSubstituting and rearranging we obtain
s R R R s R
β αφ φ α β
β αφ φ β β α
β β φ
φ
+= = = +
+ +
+ ++ = = ⇒ + = + + +
+
+ = + + = + −
= + + − +
l
Refraction at a spherical interface – cont’d Refraction at a spherical interface – cont’d
R s’-Rℓ '
αβ
φφ
Refraction at a spherical interface – cont’d Refraction at a spherical interface – cont’d
IC
O
( )
( )( )
( )
( ) ( ) ( )
22 2
2 222 2 2 2
2
22 2 2 2 2
22 2
cos sin
sin cos 1
2 2
cos 2 cos
Rs R s R
s Rs R
R R R R s R
s R R s R R s R
α βφ φ α β
α βφ φ β α
α α β α
α φ φ
′= = = + +′ ′− −
+ ′+ = = ⇒ − = +′ −
′ ′= + + + = + + −
′ ′ ′ ′= − ⇒ = + − + −
l’
( )
( )
1/ 22 2 42
2 4
21/ 2
2
cos :
cos 1 sin 1 12 8
1 12 8
1
Writing the term in terms of h we obtain
h h hR R R
where we have used the binomial expansionx xx
Substituting into our expressions for and and rearranging
h R sRs
φ
φ φ⎡ ⎤⎛ ⎞= − = − ≅ − −⎢ ⎥⎜ ⎟
⎝ ⎠⎢ ⎥⎣ ⎦
+ ≅ + −
′
+= +
( )
( ) ( )
( ) ( ) ( )
1/ 24
2 3 2
1/ 22 4
2 3 2
6
22 4 4
2 3 2 2 4
4
14
12 8 8
1
h R sR s
h R s h R sRs R s
Use the same binomial expansion and neglecting terms oforder h and higher we obtain
h R s h R s h R sRs R s R s
⎧ ⎫⎡ ⎤+⎪ ⎪+⎨ ⎬⎢ ⎥⎪ ⎪⎣ ⎦⎩ ⎭
⎧ ⎫′ ′⎡ ⎤− −⎪ ⎪′ = + +⎨ ⎬⎢ ⎥′ ′⎪ ⎪⎣ ⎦⎩ ⎭
⎧ ⎫+ + +⎪ ⎪= + + −⎨ ⎬⎪ ⎪⎩ ⎭
′ = +( ) ( ) ( )22 4 4
2 3 2 2 42 8 8h R s h R s h R s
Rs R s R s
⎧ ⎫′ ′ ′− − −⎪ ⎪+ −⎨ ⎬′ ′ ′⎪ ⎪⎩ ⎭
Refraction at a spherical interface – cont’d Refraction at a spherical interface – cont’d
2 2 4 4 4 4 4
2 2 2 3 4 3 2 2
2 2 4 4 4 4 4
2 2 2 3 4 3 2 2
12 2 8 8 8 4 8
12 2 8 8 8 4 8
h h h h h h hss Rs R s R s s Rs R s
h h h h h h hss Rs R s R s s Rs R s
⎧ ⎫= + + + + − − −⎨ ⎬
⎩ ⎭⎧ ⎫
′ ′= + − + − − + −⎨ ⎬′ ′ ′ ′ ′ ′ ′⎩ ⎭
Refraction at a spherical interface – cont’d Refraction at a spherical interface – cont’d
1 2 2 1
'n n n ns s R
−+ =
Imaging formula(first-order approx.)
( ) ( ) ( )1 2 1 2
2 241 2
4
1 1 1 18 ' '
a Q n n n s n s
n nhs s R s s R
ch
′ ′= + − +
⎡ ⎤⎛ ⎞ ⎛ ⎞= − + + −⎢ ⎥⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠⎢ ⎥⎣ ⎦
=Aberration for axial object points (on-axis imaging): This aberration will be referred to as spherical aberration.
The other aberrations will appear at off-axis imaging!
Third-Order Aberration :Off-axis imaging by a spherical interface
ThirdThird--Order Aberration :Order Aberration :OffOff--axis imaging by a spherical interface axis imaging by a spherical interface
Now, let’s calculate the third-order aberrations in a general case.
Q
O
B
ρ ’r
θ
h’
( ) ( ); , , 'a Q a Q r hθ=
O
( ) ( )( ) ( )( ) ( ) ( )
opd
opd
a Q PQP PBP
a O POP PBP
a Q a Q a O
′ ′ ′= −
′ ′ ′= −
′ ′= −
Third-Order Aberration Theory Third-Order Aberration Theory After some very complicated analysis the third-order aberration equation is obtained:
( ) 40 40
31 31
2 2 22 22
2 22 20
33 11
cos
cos
cos
a Q C r
C h r
C h r
C h r
C h r
θ
θ
θ
=
′+
′+
′+
′+
Spherical Aberration
Coma
Astigmatism
Curvature of Field
Distortion
Q
O
B
ρ ’r
θ
On-axis imaging 에서의 a(Q) = ch4와일치
Spherical Aberration Spherical Aberration
Optics, E. Hecht, p. 222.
Transverse SphericalAberration
LongitudinalSphericalAberration
( ) 4 40 4 0a Q C r c h= =
Spherical Aberration Spherical Aberration
Modern Optics, R. Guenther, p. 196.
Least SA
Most SA
Spherical Aberration Spherical Aberration
For a thin lens with surfaces with radii of curvature R1 and R2, refractive index nL, object distance s, image distance s', the difference between the paraxial image distance s'p and image distance s'h is given by
( ) ( ) ( ) ( )
2 1
2 1
2 32 2
3
1 1 2 4 1 3 2 18 1 1 1
L LL L L
h p L L L L
R R s spR R s s
h n nn p n n ps s f n n n n
σ
σ σ
′+ −= =
′− +
⎡ ⎤+− = + + + + − +⎢ ⎥′ ′ − − −⎣ ⎦
( )22 12
L
L
np
nσ
−= −
+Spherical aberration is minimized when :
Spherical Aberration Spherical Aberration
Optics, E. Hecht, p. 222.
σ = -1 σ = +1
For an object at infinite ( p = -1, nL = 1.50 ), σ ~ 0.7
worse better
2 1
2 1
R RR Rs sps s
σ +=
−′ −
=′ +
( )22 12
L
L
np
nσ
−= −
+Spherical aberration is minimized when :
σ
Spherical Aberration Spherical Aberration
σ
2 1
2 1
R RR Rs sps s
σ +=
−′ −
=′ +
Coma Coma ( ) 3
1 31 ' cos ( ' 0, cos 0)a Q C h r hθ θ= ≠ ≠
Coma Coma
Optics, E. Hecht, p. 224.
Coma Coma
Modern Optics, R. Guenther, p. 205.
LeastComa
MostComa
Astigmatism Astigmatism ( ) 2 2 2
2 22 ' cos ( ' 0, cos 0)a Q C h r hθ θ= ≠ ≠
Optics, E. Hecht, p. 224.
Astigmatism Astigmatism
Tangential plane(Meridional plane)
Sagittalplane
Astigmatism Astigmatism
Modern Optics, R. Guenther, p. 207.
Least Astig.
Most Astig.
Distortion Distortion ( ) 3
3 11 ' cos ( ' 0, cos 0)a Q C h r hθ θ= ≠ ≠
Field Curvature Field Curvature
Optics, E. Hecht, p. 228.
( ) 2 22 20 ' ( ' 0)a Q C h r h= ≠ Astigmatism when θ = 0.
Chromatic Aberration Chromatic Aberration
Optics, E. Hecht, p. 232.
Achromatic Doublet Achromatic Doublet
Optics, E. Hecht, p. 233.
Achromatic Doublets Achromatic Doublets ( ) ( )
( ) ( )
( )
1 1 1 11 11 12
2 2 2 22 21 22
1 2 1 21 2 1 2
1 2 1 1 2
1 1 11 1
1 1 11 1
587.6
:
1 1 1
0 :
1
D D DD
D D DD
D
P n n Kf r r
P n n Kf r r
nm center of visible spectrum
For a lens separation of L
L P P P L P Pf f f f f
For a cemented doublet L
P P P n K n
λ
⎛ ⎞= = − − = −⎜ ⎟
⎝ ⎠⎛ ⎞
= = − − = −⎜ ⎟⎝ ⎠
= =
= + − ⇒ = + −
=
= + = − + ( ) 21 K−
1 11 2
1
1 2
:
0
, " " :
0
, .
Chromatic aberration is eliminated when
n nP K K
In general for materials with normal dispersionn
That means that to eliminate chromatic aberrationK and K must have opposite signsThe partial deriva
λ λ λ
λ
∂ ∂∂= + =
∂ ∂ ∂
∂<
∂
:
., 486.1 656.3 .
F C
F C
F C
tive of refractive index withwavelength is approximated
n nn
for an achromat in the visible region of the spectrumIn the above equation nm and nm
λ λ λ
λ λ
−∂≅
∂ −
= =
Achromatic Doublets Achromatic Doublets
( )
( )
1 11 1 11 1
1 1
2 22 2 22 2
2 2
1 2
1:
11
11
D
F C
F CD D D
F C D F C
F CD D D
F C D F C
nDefining the dispersive constant V Vn n
we can write
n nn n PK Kn V
n nn n PK Kn V
V and V are functions only of the material
λ λ λ λ λ
λ λ λ λ λ
−=
−
⎛ ⎞⎛ ⎞−∂ −= =⎜ ⎟⎜ ⎟∂ − − −⎝ ⎠⎝ ⎠
⎛ ⎞⎛ ⎞−∂ −= =⎜ ⎟⎜ ⎟∂ − − −⎝ ⎠⎝ ⎠
( ) ( )1 2
2 1 1 21 2
.
0 0D DD D
F C F C
properties of the two lenses
P P V P V PV Vλ λ λ λ
+ = ⇒ + =− −
Achromatic condition for doublets
Achromatic Doublets Achromatic Doublets
( ) ( )
1 21 2 1 2
2 1 2 1
1 21 2
1 2
1 2
:
1 1
D D D D D D D
D D
D D
We can solve for the power of each lens in terms of the desiredpower of the doublet
V VP P P P P P PV V V V
P PK Kn n
From the values of K and K the radii of curvature for the two surfacesof the lenses can
= + ⇒ = − =− −
= =− −
1211 21 12 22
2 12112
be determined. If lens1 is bi - convex with equal curvaturefor each surface :
rr r r r rK r
= − = =−
Achromatic Doublets Achromatic Doublets