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.
http://www.photocrati.com/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
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
2
Color correction in optical systems
SCHOTT Advanced Optics
3
The refractive index n is a measure for the deflection
of light in transition to a different medium
air n1

The world of
optical glass
glass
n2

n = 1.487
n = 2.02
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
© SCHOTT AG
sin( ) n2

sin(  ) n1
Color correction in optical systems
SCHOTT Advanced Optics
4
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
The higher the Abbe number the lower the dispersion
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
© SCHOTT AG
0.6
0.7
0.8
wavelength in µm
nd 1
d 
nF  nC
Color correction in optical systems
SCHOTT Advanced Optics
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
5
SCHOTT Advanced Optics
Color correction in optical systems
Chromatic aberration: color fringes in high resolution
lens systems (example tele zoom lens)
Chromatic aberration
 show stopper for
high resolution optics
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
© SCHOTT AG
6
SCHOTT Advanced Optics
Color correction in optical systems
7
Chromatic aberration of a single lens:
„blue refraction ( B) is stronger than red refraction (
G
R‒
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
© SCHOTT AG
B
R)“
Color correction in optical systems
SCHOTT Advanced Optics
8
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
Color correction in optical systems
SCHOTT Advanced Optics
9
Correction of chromatic aberration with two lenses:
The achromat
classical: Fraunhofer
BK7 and F2
crown glass
flint glass
white light
achromat
achromat image
Focal length of two lenses with short distance:
Achromatic condition (
= Abbe number
R
=
Abbe number is always > 0,
B):
1
or
2
<0
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
© SCHOTT AG
1
1
∗
+
1
+
=
1
1
∗
=0
SCHOTT Advanced Optics
Color correction in optical systems
10
Achromat: large Abbe number difference between
crown and flint glass needed!
Positive lens: crown glass
Negative lens: flint glass
Large focal length of single lenses
= less lens bending = less
monochromatic image aberrations
46.15
50
57.14
33.33
f (flint)
0
‒ 22.22
‒ 50.00
-50
‒ 85.71
-100
‒133.33
-150
‒ 200.00
-200
10
20
30
40
Abbe # Difference crown-flint
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
© SCHOTT AG
66.67
f (crown)
18.18
Focal length, crown, flint
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.
100
50
Color correction in optical systems
SCHOTT Advanced Optics
11
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
N-BK7®
The single SCHOTT
a color error of 15.8 mm
2
Secondary
spectrum
e.g. VIS
lens has
1

Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
© SCHOTT AG
Single lens
Pos.
SCHOTT Advanced Optics
Color correction in optical systems
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
Calculated from datasheet Sellmeier coefficients.
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
© SCHOTT AG
12
Color correction in optical systems
SCHOTT Advanced Optics
13
The partial dispersion is a measure for the bending
of the dispersion curve
Principle dispersion
nF  nC
SF66
N-SF66
Dispersion
Angle of Incidence 65 deg
r-Line
C-Line
d-Line
e-Line
Partial dispersion
ng  nF
Relative partial dispersion
Pg , F 
ng  nF
nF  nC
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
© SCHOTT AG
F-Line
g-Line
Relative Deflection of Rays in 1 m Distance [mm]
Color correction in optical systems
SCHOTT Advanced Optics
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)
Pg , F 
ng  nF
nF  nC
 (0,6438  0,001682 d )
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
© SCHOTT AG
Normal line
Abbe number d
14
SCHOTT Advanced Optics
Color correction in optical systems
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
15
SCHOTT Advanced Optics
Color correction in optical systems
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
16
SCHOTT Advanced Optics
Color correction in optical systems
17
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
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
18
SCHOTT Advanced Optics
Color correction in optical systems
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
19
SCHOTT Advanced Optics
Color correction in optical systems
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
20
SCHOTT Advanced Optics
Color correction in optical systems
N-FK58 XLD: A new extremely low dispersion (XLD) glass
with excellent processing properties
N-FK58
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
© SCHOTT AG
21
Color correction in optical systems
SCHOTT Advanced Optics
22
N-FK58 XLD: A new extremely low dispersion (XLD) glass
with high internal transmittance!
1,0
internal transmittance
0,9
0,8
N-FK58, 25 mm
competitor 1
competitor 2
competitor 3
competitor 4
0,7
0,6
0,5
0,4
0,3
0,2
200
300
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
© SCHOTT AG
400
500
wavelength [nm]
600
700
Color correction in optical systems
SCHOTT Advanced Optics
23
Supporting glasses: SCHOTT‘s N-KZFS4 shows the largest
deviation from the normal line compared to the competition
0
N-KZFS4
N-KZFS5
N-KZFS8
-0,002
-0,004
PgF
SCHOTT
competitor 1
-0,006
competitor 2
competitor 3
-0,008
-0,01
-0,012
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
© SCHOTT AG
Color correction in optical systems
SCHOTT Advanced Optics
24
SCHOTT’s N-PK52A: High transmission up to 4 µm
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
1.0
0.9
0.8
Spectral transmittance
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
2500
3000
3500
4000
4500
Wavelength (nm)
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
© SCHOTT AG
5000
5500
6000
Color correction in optical systems
SCHOTT Advanced Optics
25
High end applications need glasses with anomalous
partial dispersion
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!
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.
http://www.photocrati.com/nikon-70-200mm-f2-8g-af-s-ed-vr-ii-lens-review-field-test-report/
Color correction in optical systems, Dr. Ralf Jedamzik, May 2014
© SCHOTT AG