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 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 

The world of optical glass glass

n2

 n = 1.487 sin() n2 n = 2.02  sin( ) n1

Color correction in optical systems, Dr. Ralf Jedamzik, May 2014 © SCHOTT AG SCHOTT Advanced Optics Color correction in optical systems 4

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

Refractive index

1.53

n n F C nd

1.51

nd 1  d  1.49 nF nC 0.3 0.4 0.5 0.6 0.7 0.8 The higher the Abbe number the lower the dispersion wavelength in µm

Color correction in optical systems, Dr. Ralf Jedamzik, May 2014 © SCHOTT AG SCHOTT Advanced Optics Color correction in optical systems 5

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)

Chromatic aberration  show stopper for high resolution optics

Color correction in optical systems, Dr. Ralf Jedamzik, May 2014 © SCHOTT AG SCHOTT Advanced Optics Color correction in optical systems 7

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

G

R ‒ B

Color correction in optical systems, Dr. Ralf Jedamzik, May 2014 © SCHOTT AG SCHOTT Advanced Optics Color correction in optical systems 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 SCHOTT Advanced Optics Color correction in optical systems 9

Correction of chromatic aberration with two lenses:

The achromat classical: Fraunhofer BK7 and F2 crown glass flint glass

white light

achromat achromat image

1 1 1 Focal length of two lenses with short distance: + =

1 1 Achromatic condition (R = B): + = 0 = Abbe number ∗ ∗

Abbe number is always > 0,1 or 2 < 0

Color correction in optical systems, Dr. Ralf Jedamzik, May 2014 © SCHOTT AG SCHOTT Advanced Optics Color correction in optical systems 10

Achromat: large Abbe number difference between crown and flint glass needed!

Positive lens: crown glass 100

Negative lens: flint glass 66.67 57.14 46.15 50 At fixed focal length of the system 33.33 18.18 f (crown) f (flint) (example 100 mm), the focal length t n i

l 0 f

,

of each single lens is larger if the n ‒ 22.22 w o r c

‒ 50.00 Abbe number difference , -50 h t g n

is large. e l

l ‒ 85.71 a c

o -100 F Large focal length of single lenses ‒133.33 -150 = less lens bending = less ‒ 200.00 monochromatic image aberrations -200 10 20 30 40 50

Abbe # Difference crown-flint

Color correction in optical systems, Dr. Ralf Jedamzik, May 2014 © SCHOTT AG SCHOTT Advanced Optics Color correction in optical systems 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) 2 Secondary has an color error of 0.5 mm spectrum e.g. VIS The single SCHOTT N-BK7® lens has a color error of 15.8 mm Single lens 1

Pos. 

Color correction in optical systems, Dr. Ralf Jedamzik, May 2014 © SCHOTT AG SCHOTT Advanced Optics Color correction in optical systems 12

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 SCHOTT Advanced Optics Color correction in optical systems 13

The partial dispersion is a measure for the bending of the dispersion curve

Principle dispersion n  n NSF66-SF66 Dispersion Angle of Incidence 65 deg F C r-Line C-Line d-Line e-Line Partial dispersion F-Line n  n g-Line g F Relative Deflection of Rays in 1 m Distance [mm]

Relative partial dispersion

ng  nF Pg,F  nF  nC

Color correction in optical systems, Dr. Ralf Jedamzik, May 2014 © SCHOTT AG SCHOTT Advanced Optics Color correction in optical systems 14

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 Normal line F2 (be careful, other glass vendors have different definitions)

ng  nF Pg,F  (0,64380,001682 d ) Abbe number d nF  nC

Color correction in optical systems, Dr. Ralf Jedamzik, May 2014 © SCHOTT AG SCHOTT Advanced Optics Color correction in optical systems 15

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 16

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 17

Ideal: position of CaF2, but expensive and sensitive pro- cessing. 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 18

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 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

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 SCHOTT Advanced Optics Color correction in optical systems 21

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 SCHOTT Advanced Optics Color correction in optical systems 22

N-FK58 XLD: A new extremely low dispersion (XLD) glass with high internal transmittance!

1,0

0,9

e 0,8 c n a t t i 0,7 N-FK58, 25 mm m

s competitor 1 n

a 0,6 competitor 2 r t

l competitor 3 a

n 0,5 r competitor 4 e t n i 0,4

0,3

0,2 200 300 400 500 600 700 wavelength [nm] Color correction in optical systems, Dr. Ralf Jedamzik, May 2014 © SCHOTT AG SCHOTT Advanced Optics Color correction in optical systems 23

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

N-KZFS4 N-KZFS5 N-KZFS8 0

-0,002

-0,004

SCHOTT competitor 1 F g -0,006 P competitor 2  competitor 3

-0,008

-0,01

-0,012

Color correction in optical systems, Dr. Ralf Jedamzik, May 2014 © SCHOTT AG SCHOTT Advanced Optics Color correction in optical systems 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

0.7 e c n

a 0.6 t t i m s n

a 0.5 r t l a r t

c 0.4 e p S

0.3

0.2

0.1

0.0 2500 3000 3500 4000 4500 5000 5500 6000 Wavelength (nm)

Color correction in optical systems, Dr. Ralf Jedamzik, May 2014 © SCHOTT AG SCHOTT Advanced Optics Color correction in optical systems 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