ED Glass
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. https://www.doczj.com/doc/fd14858962.html,/nikon-70-200mm-f2-8g-af-s-ed-vr-ii-lens-review-field-test-report/
Color correction in optical systems or 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 ? SCHOTT AG
SCHOTT Advanced Optics
Color correction in optical systems
2
Optical glasses are mainly categorized according to their refractive index and Abbe number
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014 ? SCHOTT AG
SCHOTT Advanced Optics
Color correction in optical systems
3
The refractive index n is a measure for the deflection of light in transition to a different medium
air n1
?
glass n2
The world of optical glass
?
n = 1.487
n = 2.02
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014 ? SCHOTT AG
sin(? ) ? n2 sin(? ) n1
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Color correction in optical systems
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The Abbe number is a measure for the change of refractive index with the wavelength (dispersion)
Refractive index
1.53
nF ? nC
nd
1.51
1.49
0.3
0.4
0.5
0.6
The higher the Abbe number the lower the dispersion
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014 ? SCHOTT AG
?d ? nd ?1 nF ? nC
0.7
0.8
wavelength in μm
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Color correction in optical systems
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Refraction of different glasses as seen with a prism
Flint glasses: high refractive index + high dispersion
Crown glasses: low refractive index + low dispersion
N-FK58 XLD
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014 ? SCHOTT AG
SCHOTT Advanced Optics
Color correction in optical systems
6
Chromatic aberration: color fringes in high resolution lens systems (example tele zoom lens)
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014 ? SCHOTT AG
Chromatic aberration ? show stopper for high resolution optics
SCHOTT Advanced Optics
Color correction in optical systems
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Chromatic aberration of a single lens: ?blue refraction ( B) is stronger than red refraction ( R)“
G
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014 ? SCHOTT AG
R? B
SCHOTT Advanced Optics
Color correction in optical systems
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The size of the chromatic aberration of a single lens is the quotient of the focal length and the Abbe number
?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!
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014 ? SCHOTT AG
SCHOTT Advanced Optics
Color correction in optical systems
9
Correction of chromatic aberration with two lenses:
The achromat
crown glass flint glass
classical: Fraunhofer BK7 and F2
white light
achromat
achromat image
Focal length of two lenses with short distance:
111 +=
Achromatic condition ( R = B): ??= Abbe number
Abbe number is always > 0, 1 or 2 < 0
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014 ? SCHOTT AG
1
1
+
=0
?
?
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Color correction in optical systems
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Achromat: large Abbe number difference between crown and flint glass needed!
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
Focal length, crown, flint
100
50 18.18
33.33
46.15
0 ? 22.22
? 50.00 -50
57.14
66.67
f (crown) f (flint)
-100 -150
? 85.71 ?133.33
-200 10
20
30
40
Abbe # Difference crown-flint
? 200.00 50
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014 ? SCHOTT AG
SCHOTT Advanced Optics
Color correction in optical systems
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The achromat is corrected for two wavelengths: but an error remains, the secondary spectrum!
Color error diagram
Achromat
Example: Achromat with 100 mm focal length (SCHOTT N-BK7?, F2) has an color error of 0.5 mm
e.g. VIS
The single SCHOTT N-BK7? lens has a color error of 15.8 mm
?
?2
Secondary
spectrum
?1
Single lens
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014 ? SCHOTT AG
Pos.
?
SCHOTT Advanced Optics
Color correction in optical systems
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The reason for the secondary spectrum is the different bending of the dispersion curves of ?crown“ and ?flint“ glasses
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
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014 ? SCHOTT AG
Calculated from datasheet Sellmeier coefficients.
SCHOTT Advanced Optics
Color correction in optical systems
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The partial dispersion is a measure for the bending of the dispersion curve
Principle dispersion
nF ? nC
Partial dispersion
ng ? nF
NS-SF6F666 Dispersion Angle of Incidence 65 deg
r-Line C-Line d-Line e-Line F-Line g-Line
Relative Deflection of Rays in 1 m Distance [mm]
Relative partial dispersion
Pg , F
?
ng nF
? nF ? nC
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014 ? SCHOTT AG
SCHOTT Advanced Optics
Color correction in optical systems
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In the diagram relative partial dispersion versus Abbe number, many glasses are located on a line called ?normal line“
The line is given by the glasses K7 and F2 (be careful, other glass vendors have different definitions)
Normal line
?Pg,F ? ng ? nF ? (0,6438? 0,001682??d ) nF ? nC
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014 ? SCHOTT AG
Abbe number ?d
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Color correction in optical systems
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The slope of the normal line is directly proportional to the secondary color error!
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!
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014 ? SCHOTT AG
SCHOTT Advanced Optics
Color correction in optical systems
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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!
PK / FK glasses and short flint glasses (KZFS glasses) have a very pronounced anomalous partial dispersion
Low slopes are possible with this combination
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014 ? SCHOTT AG
SCHOTT Advanced Optics
Color correction in optical systems
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Ideal: position of CaF2, but expensive and sensitive processing. Alternative: Fluoro-phosphate glasses on CaF2 position
CaF2
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014 ? SCHOTT AG
SCHOTT Advanced Optics
Color correction in optical systems
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On the way to CaF2! Extremely low dispersion glasses (XLD) Target: better processability! XLD glass N-FK58 successful production run!
optical position:nd = 1.45600,? d = 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
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014 ? SCHOTT AG
SCHOTT Advanced Optics
Color correction in optical systems
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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
Most anomalous dispersion glasses are available in step 0.5!
Highly accurate and economic metrology is an important prerequisite for the success!
?We are not selling glass, we are selling properties!“
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014 ? SCHOTT AG
SCHOTT Advanced Optics
Color correction in optical systems
20
N-FK58 XLD: A new extremely low dispersion glass with excellent processing properties
? 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.
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014 ? SCHOTT AG