Color correction in optical systems or why optical … · Color correction in optical systems or...

© 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



© SCHOTT AG

3


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 AG

The Abbe number is a measure for the change of refractive index with the wavelength (dispersion)

4


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 AG

Refraction of different glasses as seen with a prism

5


Flint glasses:

high refractive index +

high dispersion

Crown glasses:

low refractive index +

low dispersion

N-FK58 XLD


© SCHOTT AG

Chromatic aberration: color fringes in high resolution lens systems (example tele zoom lens)

6


Chromatic aberration show stopper forhigh resolution optics


© SCHOTT AG

Chromatic aberration of a single lens: „blue refraction (�B) is stronger than red refraction (�R)“

7


�G

�R �B‒


© SCHOTT AG

The size of the chromatic aberration of a single lens is the quotient of the focal length and the Abbe number

8


∆� = �� − �� =�

�

� =�� − 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 AG

Correction of chromatic aberration with two lenses:

9


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 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


-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 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


Achromat

Secondary

spectrum

Single lens

Pos.

e.g. VIS

2

1

�


© SCHOTT AG

The reason for the secondary spectrum is the different bending of the dispersion curves of „crown“ and „flint“ glasses

12


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 AG

Fg nn

CF nn

CF

Fg

Fgnn

nnP

,

The partial dispersion is a measure for the bendingof the dispersion curve

13


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 AG

In the diagram relative partial dispersion versus Abbe number, many glasses are located on a line called „normal line“

14


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 AG

The slope of the normal line is directly proportional to the secondary color error!

15


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 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


PK/FK glasses and short flint glasses (KZFS glasses) have a very pronounced anomalous partial dispersion

Low slopes are possible with this combination


© SCHOTT AG

Ideal: position of CaF2, but expensive and sensitive pro-cessing. Alternative: Fluoro-phosphate glasses on CaF2 position

17


CaF2


© SCHOTT AG

On the way to CaF2! Extremely low dispersion glasses (XLD) Target: better processability!XLD glass N-FK58 successful production run!

18


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 AG

19


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 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



© SCHOTT AG

N-FK58 XLD: A new extremely low dispersion (XLD) glass with excellent processing properties

21

N-FK58



© 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



© SCHOTT AG

Supporting glasses: SCHOTT‘s N-KZFS4 shows the largest deviation from the normal line compared to the competition

23


-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 AG

SCHOTT’s N-PK52A: High transmission up to 4 µm

24


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 AG

High end applications need glasses with anomalous partial dispersion

25


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/

Date post:	15-Sep-2018
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Color correction in optical systems or why optical … · Color correction in optical systems or...

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