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© SCHOTT AG
As this illustration indicates, the lens employs a full seven elements of extra low dispersion glass, including three of large diameter at the front for maximum aberration control.This optical formula provides superior image quality that will be obvious particularly at the edges of images made with a full-frame DSLR.
ED Glass
Color correction in optical systemsor why optical design needs fluoro-phosphate glasses
Dr. Ralf Jedamzik, Application Manager, SCHOTT Advanced Optics
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
http://www.photocrati.com/nikon-70-200mm-f2-8g-af-s-ed-vr-ii-lens-review-field-test-report/
SCHOTT Advanced Optics Color correction in optical systems
© SCHOTT AG
Optical glasses are mainly categorized according to their refractive index and Abbe number
2
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
SCHOTT Advanced Optics Color correction in optical systems
© SCHOTT AG
3
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
n = 1.487
air n1
glassn2
1
2
)sin(
)sin(
n
n
n = 2.02
The world of optical glass
The refractive index n is a measure for the deflection of light in transition to a different medium
SCHOTT Advanced Optics Color correction in optical systems
© SCHOTT AG
The Abbe number is a measure for the change of refractive index with the wavelength (dispersion)
4
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
CF
dd
nn
n
1
The higher the Abbe number the lower the dispersion
CF nn dn
wavelength in µm
Refractive index
1.53
1.49
1.51
0.3 0.4 0.5 0.6 0.7 0.8
SCHOTT Advanced Optics Color correction in optical systems
© SCHOTT AG
Refraction of different glasses as seen with a prism
5
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
Flint glasses:
high refractive index +
high dispersion
Crown glasses:
low refractive index +
low dispersion
N-FK58 XLD
SCHOTT Advanced Optics Color correction in optical systems
© SCHOTT AG
Chromatic aberration: color fringes in high resolution lens systems (example tele zoom lens)
6
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
Chromatic aberration show stopper forhigh resolution optics
SCHOTT Advanced Optics Color correction in optical systems
© SCHOTT AG
Chromatic aberration of a single lens: „blue refraction (�B) is stronger than red refraction (�R)“
7
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
�G
�R �B‒
SCHOTT Advanced Optics Color correction in optical systems
© SCHOTT AG
The size of the chromatic aberration of a single lens is the quotient of the focal length and the Abbe number
8
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
∆� = �� − �� =�
�
� =�� − 1
�� − ��D=
�
�= � − 1 ∗
�
��−
�
��
The longitudinal chromatic aberration error is proportional to the focal length
and decreases with increasing Abbe number.
Large Abbe number => low chromatic aberration!
SCHOTT Advanced Optics Color correction in optical systems
© SCHOTT AG
Correction of chromatic aberration with two lenses:
9
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
Focal length of two lenses with short distance:
Achromatic condition (�R = �B):
= Abbe number
Abbe number is always > 0,�1 or �2 < 0
The achromat
1
��+1
��=1
�
1
�� ∗ ��+
1
�� ∗ ��= 0
classical: Fraunhofer
BK7 and F2
crown glass flint glass
white light
achromat achromat image
SCHOTT Advanced Optics Color correction in optical systems
© SCHOTT AG
Positive lens: crown glass
Negative lens: flint glass
At fixed focal length of the system
(example 100 mm), the focal length
of each single lens is larger if the
Abbe number difference
is large.
Large focal length of single lenses
= less lens bending = less
monochromatic image aberrations
Achromat: large Abbe number difference between crown and flint glass needed!
10
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
-200
-150
-100
-50
0
50
100
10 20 30 40 50
18.18
33.33
46.1557.14
66.67
‒ 22.22
‒ 50.00
‒ 85.71
‒133.33
‒ 200.00
Abbe # Difference crown-flint
Focall
ength
, cro
wn,
flin
t
f (crown)
f (flint)
SCHOTT Advanced Optics Color correction in optical systems
© SCHOTT AG
The achromat is corrected for two wavelengths: but an error remains, the secondary spectrum!
Color error diagram
Example:
Achromat with 100 mm
focal length (SCHOTT N-BK7®, F2)
has an color error of 0.5 mm
The single SCHOTT N-BK7® lens has
a color error of 15.8 mm
11
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
Achromat
Secondary
spectrum
Single lens
Pos.
e.g. VIS
2
1
�
SCHOTT Advanced Optics Color correction in optical systems
© SCHOTT AG
The reason for the secondary spectrum is the different bending of the dispersion curves of „crown“ and „flint“ glasses
12
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
The secondary spectrum
is small if the bending of
the dispersion curve of
the „crown“ and „flint“
glass is the same:
glasses with anomalous
partial dispersion
Calculated from datasheet Sellmeier coefficients.
SCHOTT Advanced Optics Color correction in optical systems
© SCHOTT AG
Fg nn
CF nn
CF
Fg
Fgnn
nnP
,
The partial dispersion is a measure for the bendingof the dispersion curve
13
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
Principle dispersion
Relative Deflection of Rays in 1 m Distance [mm]
SF66 Dispersion Angle of Incidence 65 deg
r-Line
C-Line
d-Line
e-Line
F-Line
g-Line
Relative partial dispersion
Partial dispersion
N-SF66
SCHOTT Advanced Optics Color correction in optical systems
© SCHOTT AG
In the diagram relative partial dispersion versus Abbe number, many glasses are located on a line called „normal line“
14
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
The line is given by
the glasses K7 and
F2 (be careful, other
glass vendors have
different definitions)
)001682,06438,0(, d
CF
Fg
Fgnn
nnP
Abbe number d
Normal line
SCHOTT Advanced Optics Color correction in optical systems
© SCHOTT AG
The slope of the normal line is directly proportional to the secondary color error!
15
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
An achromat built with two glasses only
on the normal line has always
the same secondary color error.
The longer the focal length of the
lens the more critical the color error!
Glasses with anomalous partial
dispersion are located away from
the normal line!
SCHOTT Advanced Optics Color correction in optical systems
© SCHOTT AG
The smaller the slope of the two partners in the PgF diagram, the smaller the secondary spectrum and the better the color correction! Without PK/FK glasses no color correction possible!
16
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
PK/FK glasses and short flint glasses (KZFS glasses) have a very pronounced anomalous partial dispersion
Low slopes are possible with this combination
SCHOTT Advanced Optics Color correction in optical systems
© SCHOTT AG
Ideal: position of CaF2, but expensive and sensitive pro-cessing. Alternative: Fluoro-phosphate glasses on CaF2 position
17
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
CaF2
SCHOTT Advanced Optics Color correction in optical systems
© SCHOTT AG
On the way to CaF2! Extremely low dispersion glasses (XLD) Target: better processability!XLD glass N-FK58 successful production run!
18
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
optical position: = 1.45600, = 90.80• extremely low dispersion • excellent processing properties• offers outstanding apochromatic correction capabilities in combination
with SCHOTT KZFS glasses (e.g. N-KZFS4/5/8/11)• supplements the low dispersion glass portfolio of N-PK52A and N-FK51A
CaF2
N-FK58
ddn
SCHOTT Advanced Optics Color correction in optical systems
© SCHOTT AG
19
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
SCHOTT has improved its melting capabilities for the production of low dispersion glasses. During a recent melting campaign for N-PK52A and N-FK51A, development of a new extremely low dispersion (XLD) glass N-FK58 was accomplished by a successful production run
„We are not selling glass, we are selling properties!“
Most anomalous dispersion glasses are
available in step 0.5!
Highly accurate and economic metrology is
an important prerequisite for the success!
SCHOTT Advanced Optics Color correction in optical systems
© SCHOTT AG
• nd = 1.45600, vd = 90.80• extremely low dispersion • excellent processing properties• outstanding apochromatic
correction capabilities in combination with SCHOTT KZFS glasses (e.g. N-KZFS4/5/8/11)
• supplements the low dispersion glass portfolio of N-PK52A and N-FK51A
The datasheet of XLD glass N-FK58 is currently generated and will be available soon.
N-FK58 XLD: A new extremely low dispersion glass with excellent processing properties
20
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
SCHOTT Advanced Optics Color correction in optical systems
© SCHOTT AG
N-FK58 XLD: A new extremely low dispersion (XLD) glass with excellent processing properties
21
N-FK58
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
SCHOTT Advanced Optics Color correction in optical systems
© SCHOTT AG
N-FK58 XLD: A new extremely low dispersion (XLD) glass with high internal transmittance!
22
200 300 400 500 600 7000,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1,0in
tern
al t
ran
sm
itta
nce
wavelength [nm]
N-FK58, 25 mm competitor 1 competitor 2 competitor 3 competitor 4
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
SCHOTT Advanced Optics Color correction in optical systems
© SCHOTT AG
Supporting glasses: SCHOTT‘s N-KZFS4 shows the largest deviation from the normal line compared to the competition
23
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
-0,012
-0,01
-0,008
-0,006
-0,004
-0,002
0N-KZFS4 N-KZFS5 N-KZFS8
P
gF
SCHOTT
competitor 1
competitor 2
competitor 3
SCHOTT Advanced Optics Color correction in optical systems
© SCHOTT AG
SCHOTT’s N-PK52A: High transmission up to 4 µm
24
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
1.0
Wavelength (nm)
Spectr
alt
ransm
ittance
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.9
2500 3000 3500 4000 4500 5000 5500 6000
41-201400156-05 N-PK52A 24.09.2013 16:50 1.99 mm 41-201400156-06 N-PK52A 25.09.2013 23:00 1.99 mm 41-201400156-07 N-PK52A 29.09.2013 02:35 1.99 mm
SCHOTT Advanced Optics Color correction in optical systems
© SCHOTT AG
High end applications need glasses with anomalous partial dispersion
25
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
Fluoro-phosphate glasses
are used as LD, ED, ELD
or SLD lenses in many
applications.
SCHOTT offers all
glasses that are needed
for high quality designs!
As this illustration indicates, the lens employs a full seven elements of extra low dispersion glass, including three of large diameter at the front for maximum aberration control.This optical formula provides superior image quality that will be obvious particularly at the edges of images made with a full-frame DSLR.
ED Glass
http://www.photocrati.com/nikon-70-200mm-f2-8g-af-s-ed-vr-ii-lens-review-field-test-report/