Optics and photonics -- Test method for refractive index of optical glasses

This document specifies the measuring method for the refractive index of optical glasses with the accuracy within 1 × 10−5 used in the spectral range from 365 nm to 2 400 nm. Additional information on how to apply the refractive index in the dispersion and the various dispersion formulae of optical glasses is given in Annex A and Annex B.

Optique et photonique -- Méthode d’essai pour déterminer l’indice de réfraction des verres optiques

General Information

Status
Published
Publication Date
29-Jun-2020
Current Stage
5060 - Close of voting Proof returned by Secretariat
Start Date
30-May-2020
Completion Date
29-May-2020
Ref Project

Buy Standard

Standard
ISO 21395-1:2020 - Optics and photonics -- Test method for refractive index of optical glasses
English language
21 pages
sale 15% off
Preview
sale 15% off
Preview
Draft
ISO/FDIS 21395-1 - Optics and photonics -- Test method for refractive index of optical glasses
English language
21 pages
sale 15% off
Preview
sale 15% off
Preview

Standards Content (sample)

INTERNATIONAL ISO
STANDARD 21395-1
First edition
2020-06
Optics and photonics — Test method
for refractive index of optical
glasses —
Part 1:
Minimum deviation method
Optique et photonique — Méthode d’essai pour déterminer l’indice de
réfraction des verres optiques —
Partie 1: Méthode de la déviation minimale
Reference number
ISO 21395-1:2020(E)
ISO 2020
---------------------- Page: 1 ----------------------
ISO 21395-1:2020(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2020

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2020 – All rights reserved
---------------------- Page: 2 ----------------------
ISO 21395-1:2020(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Principle ........................................................................................................................................................................................................................ 1

5 Measuring apparatus ....................................................................................................................................................................................... 2

5.1 General construction ......................................................................................................................................................................... 2

5.2 Goniometer................................................................................................................................................................................................. 3

5.3 Light source ............................................................................................................................................................................................... 3

5.4 Detector ......................................................................................................................................................................................................... 4

6 Specimen prism ..................................................................................................................................................................................................... 4

6.1 General ........................................................................................................................................................................................................... 4

6.2 Dimensions ................................................................................................................................................................................................. 5

6.3 Apex angle ................................................................................................................................................................................................... 5

6.4 Flatness .......................................................................................................................................................................................................... 5

7 Environmental condition of measurement ............................................................................................................................... 5

7.1 Temperature .............................................................................................................................................................................................. 5

7.2 Humidity ....................................................................................................................................................................................................... 5

7.3 Atmospheric pressure....................................................................................................................................................................... 5

8 Measurement ........................................................................................................................................................................................................... 6

8.1 Adjustment of the measurement specimen prism ................................................................................................... 6

8.2 Measurement of the apex angle, α ........................................................................................................................................ 6

8.3 Measurement of the angle of minimum deviation, δ ..................................................................................... 7

min

9 Indication ..................................................................................................................................................................................................................... 8

10 Test report ................................................................................................................................................................................................................... 8

Annex A (informative) Calculation of principal dispersion, Abbe number, partial dispersion

and relative partial dispersion .............................................................................................................................................................. 9

Annex B (informative) Dispersion formulae for calculation of refractive index at arbitrary

wavelength ...............................................................................................................................................................................................................12

Annex C (informative) Correction of refractive index for temperature, humidity and

atmospheric pressure ..................................................................................................................................................................................14

Annex D (informative) Other measurement methods of the apex angle .....................................................................16

Annex E (informative) Other measurement methods of the angle of minimum deviation ......................18

Bibliography .............................................................................................................................................................................................................................21

© ISO 2020 – All rights reserved iii
---------------------- Page: 3 ----------------------
ISO 21395-1:2020(E)
Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards

bodies (ISO member bodies). The work of preparing International Standards is normally carried out

through ISO technical committees. Each member body interested in a subject for which a technical

committee has been established has the right to be represented on that committee. International

organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.

ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of

electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the

different types of ISO documents should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www .iso .org/ patents).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO's adherence to

the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see

www .iso .org/ iso/ foreword .html.

This document was prepared by Technical Committee ISO/TC 172, Optics and photonics, Subcommittee

SC 3, Optical materials and components.
A list of all parts in the ISO 21395 series can be found on the ISO website.

Any feedback or questions on this document should be directed to the user’s national standards body. A

complete listing of these bodies can be found at www .iso .org/ members .html.
iv © ISO 2020 – All rights reserved
---------------------- Page: 4 ----------------------
ISO 21395-1:2020(E)
Introduction

The refractive index of optical glasses has been measured by various methods, but up to now, an

International Standard for the measurement has not been available. The refractive index of optical

glasses is the most important characteristic for the optical elements to be manufactured from

them. This document defines a suitable method for measuring the refractive index of optical glasses

accurately and also helps to improve communication between raw optical glass suppliers and optical

element manufacturers.
© ISO 2020 – All rights reserved v
---------------------- Page: 5 ----------------------
INTERNATIONAL STANDARD ISO 21395-1:2020(E)
Optics and photonics — Test method for refractive index of
optical glasses —
Part 1:
Minimum deviation method
1 Scope

This document specifies the measuring method for the refractive index of optical glasses with the

accuracy within 1 × 10 used in the spectral range from 365 nm to 2 400 nm.

Additional information on how to apply the refractive index in the dispersion and the various dispersion

formulae of optical glasses is given in Annex A and Annex B.
2 Normative references

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements of this document. For dated references, only the edition cited applies. For

undated references, the latest edition of the referenced document (including any amendments) applies.

There are no normative references in this document.
3 Terms and definitions
No terms and definitions are listed in this document.

ISO and IEC maintain terminological databases for use in standardization at the following addresses.

— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
4 Principle

As shown in Figure 1, when the monochromatic light beam is refracted by the specimen prism at

the angle of minimum deviation, the relative refractive index of the specimen prism to the air at the

wavelength of the monochromatic light beam is described by the following Formula (1):

αδ+
min
sin
n = (1)
rel
sin
© ISO 2020 – All rights reserved 1
---------------------- Page: 6 ----------------------
ISO 21395-1:2020(E)
where
n is the relative refractive index (n = n /n );
rel rel abs air
n is the absolute refractive index of the specimen prism;
abs
n is the refractive index of air;
air
α is the apex angle;
δ is the angle of minimum deviation.
min

As shown in Figure 1, light enters the plane AB of the specimen prism at an angle of incidence (θ) and

exits from the plane AC at an exit angle (θ′). The incident and the exiting light ray form the deviation

angle (δ). With the deviation angle minimized, the incident and the exiting angles are equal. The smallest

angle of deviation is called the angle of minimum deviation. The angle of minimum deviation (δ ) and

min

the apex angle (α) of the specimen prism are measured, and the refractive index is calculated using

those angles. Formulae for the calculation of principal dispersion, Abbe number, partial dispersion and

relative partial dispersion are given in Annex A. The dispersion formulae that calculate the refractive

index at the wavelength different from the measured wavelengths are given in Annex B. The correction

of the refractive index of optical glasses for temperature, humidity and atmospheric pressure of optical

glasses are given in Annex C.

NOTE When measuring the refractive index with this method, it is necessary to consider temperature,

pressure, humidity and measurement errors. Expressions for the relations of these errors are described

in ISO 17328. The dependence of the refractive index of air on temperature and pressure can be found in

ISO 12123:2018, A.3.
Key
θ angle of incidence δ angle of minimum deviation
min
θ′ exit angle 1 incident light
α apex angle 2 transmitted light
A, B, C the vertices of the prism 3 minimum deviation
Figure 1 — Principle of minimum deviation method
5 Measuring apparatus
5.1 General construction
The measuring apparatus is shown in Figure 2.
2 © ISO 2020 – All rights reserved
---------------------- Page: 7 ----------------------
ISO 21395-1:2020(E)
Key
1 light source 4 rotating stage coupled with prism (5)
2 collimator 5 specimen prism
3 goniometer coupled with telescope (6) 6 telescope
and detector (7) 7 detector
Figure 2 — Schematic of minimum deviation method
5.2 Goniometer

The goniometer shall provide the capability of reading the angle within ±1 arc sec.

5.3 Light source

The light source should be a mercury, hydrogen, helium, rubidium, cesium or cadmium lamp, also He-

Ne laser or Nd: YAG laser defined in ISO 7944. The spectral lines and their associated wavelengths are

shown in Table 1.

Light sources and their corresponding wavelengths not defined in Table 1 are also applicable for

measurement, but the spectral bandwidth of the light source and the accuracy/certainty of emission

line (for example D line (589,3 nm)) should be checked before use.
Table 1 — Wavelength and spectral line of light source
Wavelength/nm Spectral line Light source
365,01 i Mercury lamp
404,66 h Mercury lamp
435,83 g Mercury lamp
479,99 F′ Cadmium lamp
486,13 F Hydrogen lamp
543,5 — He-Ne laser
546,07 e Mercury lamp
587,56 d Helium lamp
632,8 — He-Ne laser
643,85 C′ Cadmium lamp
656,27 C Hydrogen lamp
706,52 r Helium lamp
© ISO 2020 – All rights reserved 3
---------------------- Page: 8 ----------------------
ISO 21395-1:2020(E)
Table 1 (continued)
Wavelength/nm Spectral line Light source
780,00 — Rubidium lamp
852,11 s Cesium lamp
1 013,98 t Mercury lamp
1 064,1 — Nd: YAG laser
1 128,7 — Mercury lamp
1 395,1 — Mercury lamp
1 529,6 — Mercury lamp
1 813,1 — Mercury lamp
1 970,1 — Mercury lamp
2 325,4 — Mercury lamp
5.4 Detector

A general-type detector that is capable of detecting each wavelength or easily exchangeable for different

spectral ranges should be used.
6 Specimen prism
6.1 General
An example of the shape of specimen prism is shown in Figure 3.
Key
l length
t thickness
α apex angle
Figure 3 — Shape of the specimen prism
4 © ISO 2020 – All rights reserved
---------------------- Page: 9 ----------------------
ISO 21395-1:2020(E)
6.2 Dimensions

The length of the edges making the apex angle should be between 15 mm and 40 mm and the thickness

should be between 10 mm and 30 mm.
6.3 Apex angle

A reasonable choice of apex angle, α, for a test prism can be calculated from the expected refractive

index of the prism, n , and the angle of incidence, θ.
rel
 
sinθ
α =2arcsin
 
 
rel
α is typically between 35° and 80°.
6.4 Flatness

The plane polished sides should have a peak to valley flatness better than ¼ λ over 80 % of the aperture

at the measurement wavelength of 546 nm or 632,8 nm.
7 Environmental condition of measurement
7.1 Temperature

The temperature shall be between 20 °C and 25 °C. Environmental temperature stability and

uniformity shall be controlled according to the required measurement accuracy. This requirement

shall be calculated so that the temperature variation of the glass contributes no more than 50 % to

the desired total measurement error. Sufficient time should be allowed for the prism to acclimatise to

the test conditions prior to commencing (typically 24 h). Care should be taken in handling the

...

FINAL
INTERNATIONAL ISO/FDIS
DRAFT
STANDARD 21395-1
ISO/TC 172/SC 3
Optics and photonics — Test method
Secretariat: JISC
for refractive index of optical
Voting begins on:
glasses —
2020-04-03
Voting terminates on:
Part 1:
2020-05-29
Minimum deviation method
Optique et photonique — Méthode d’essai pour déterminer l’indice de
réfraction des verres optiques —
Partie 1: Méthode de la déviation minimale
RECIPIENTS OF THIS DRAFT ARE INVITED TO
SUBMIT, WITH THEIR COMMENTS, NOTIFICATION
OF ANY RELEVANT PATENT RIGHTS OF WHICH
THEY ARE AWARE AND TO PROVIDE SUPPOR TING
DOCUMENTATION.
IN ADDITION TO THEIR EVALUATION AS
Reference number
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO-
ISO/FDIS 21395-1:2020(E)
LOGICAL, COMMERCIAL AND USER PURPOSES,
DRAFT INTERNATIONAL STANDARDS MAY ON
OCCASION HAVE TO BE CONSIDERED IN THE
LIGHT OF THEIR POTENTIAL TO BECOME STAN-
DARDS TO WHICH REFERENCE MAY BE MADE IN
NATIONAL REGULATIONS. ISO 2020
---------------------- Page: 1 ----------------------
ISO/FDIS 21395-1:2020(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2020

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2020 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/FDIS 21395-1:2020(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Principle ........................................................................................................................................................................................................................ 1

5 Measuring apparatus ....................................................................................................................................................................................... 2

5.1 General ........................................................................................................................................................................................................... 2

5.2 Goniometer................................................................................................................................................................................................. 3

5.3 Light source ............................................................................................................................................................................................... 3

5.4 Detector ......................................................................................................................................................................................................... 4

6 Specimen prism ..................................................................................................................................................................................................... 4

6.1 General ........................................................................................................................................................................................................... 4

6.2 Dimensions ................................................................................................................................................................................................. 5

6.3 Apex angle ................................................................................................................................................................................................... 5

6.4 Flatness .......................................................................................................................................................................................................... 5

7 Environmental condition of measurement ............................................................................................................................... 5

7.1 Temperature .............................................................................................................................................................................................. 5

7.2 Humidity ....................................................................................................................................................................................................... 5

7.3 Atmospheric pressure....................................................................................................................................................................... 5

8 Measurement ........................................................................................................................................................................................................... 6

8.1 Adjustment of the measurement specimen prism ................................................................................................... 6

8.2 Measurement of the apex angle, α ........................................................................................................................................ 6

8.3 Measurement of the angle of minimum deviation, δ ..................................................................................... 7

min

9 Indication ..................................................................................................................................................................................................................... 8

10 Test report ................................................................................................................................................................................................................... 8

Annex A (informative) Calculation of principal dispersion, Abbe number, partial

dispersion, and relative partial dispersion .............................................................................................................................. 9

Annex B (informative) Dispersion formula for calculation of refractive index at arbitrary

wavelength ...............................................................................................................................................................................................................12

Annex C (informative) Correction of refractive index for temperature, humidity, and

atmospheric pressure ..................................................................................................................................................................................14

Annex D (informative) Another measurement of the apex angle ........................................................................................16

Annex E (informative) Another measurement method of the angle of minimum deviation ..................18

Bibliography .............................................................................................................................................................................................................................21

© ISO 2020 – All rights reserved iii
---------------------- Page: 3 ----------------------
ISO/FDIS 21395-1:2020(E)
Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards

bodies (ISO member bodies). The work of preparing International Standards is normally carried out

through ISO technical committees. Each member body interested in a subject for which a technical

committee has been established has the right to be represented on that committee. International

organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.

ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of

electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the

different types of ISO documents should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www .iso .org/ patents).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO's adherence to

the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see

www .iso .org/ iso/ foreword .html.

This document was prepared by Technical Committee ISO/TC 172, Optics and photonics, Subcommittee

SC 3, Optical materials and components.

Any feedback or questions on this document should be directed to the user’s national standards body. A

complete listing of these bodies can be found at www .iso .org/ members .html.
iv © ISO 2020 – All rights reserved
---------------------- Page: 4 ----------------------
ISO/FDIS 21395-1:2020(E)
Introduction

The refractive index of optical glasses has been measured by various methods, but an International

Standard for the measurement has not existed so far. The refractive index of optical glasses is the

most important characteristic for the optical elements to be manufactured from them. This document

defines a suitable method for measuring the refractive index of optical glasses accurately and also helps

to improve communication between raw optical glass suppliers and optical element manufacturers.

© ISO 2020 – All rights reserved v
---------------------- Page: 5 ----------------------
FINAL DRAFT INTERNATIONAL STANDARD ISO/FDIS 21395-1:2020(E)
Optics and photonics — Test method for refractive index of
optical glasses —
Part 1:
Minimum deviation method
1 Scope

This document specifies the measuring method for the refractive index of optical glasses with the

accuracy within 1 × 10 used in the spectral range from 365 nm to 2 400 nm.

Additional information on how to apply the refractive index in the dispersion and the various dispersion

formulas of optical glasses is given in Annex A and Annex B.
2 Normative references

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements of this document. For dated references, only the edition cited applies. For

undated references, the latest edition of the referenced document (including any amendments) applies.

There are no normative references in this document.
3 Terms and definitions
No terms and definitions are listed in this document.

ISO and IEC maintain terminological databases for use in standardization at the following addresses.

— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
4 Principle

As shown in Figure 1, when the monochromatic light beam is refracted by the specimen prism at

the angle of minimum deviation, the relative refractive index of the specimen prism to the air at the

wavelength of the monochromatic light beam is described by the following Formula (1):

αδ+
min
sin
n = (1)
rel
sin
© ISO 2020 – All rights reserved 1
---------------------- Page: 6 ----------------------
ISO/FDIS 21395-1:2020(E)
where
n is the relative refractive index (n = n /n );
rel rel abs air
n is the absolute refractive index of the specimen prism;
abs
n is the refractive index of air;
air
α is the apex angle;
δ is the angle of minimum deviation.
min

As shown in Figure 1, light enters the plane AB of the specimen prism at an angle of incidence (θ) and

exits from the plane AC at an exit angle (θ′). The incident and the exiting light ray form the deviation

angle (δ). With the deviation angle minimized the incident and the exiting angles are equal. The smallest

angle of deviation is called the angle of minimum deviation. The angle of incidence equals to the angle of

emission when the angle of deviation is minimized. The angle of minimum deviation (δ ) and the apex

min

angle (α) of the specimen prism are measured, and the refractive index is calculated using those angles.

Formulae for the calculation of principal dispersion, Abbe number, partial dispersion, and relative

partial dispersion are given in Annex A. Formulae for calculating calculating the refractive index at

other wavelengths than the measured ones dispersion formulas are given in Annex B. The correction of

the refractive index of optical glasses for temperature, humidity, and atmospheric pressure of optical

glasses are given in Annex C.

NOTE When measuring the refractive index with this method, it is necessary to consider temperature,

pressure, humidity and measurement error. Expressions for the relations of these errors are described

in ISO 17328. The dependence of the refractive index of air on temperature and pressure can be found in

ISO 12123:2018, A.3.
Key
θ angle of incidence δ angle of minimum deviation
min
θ′ exit angle 1 incident light beam
α apex angle 2 transmitted light ray
A,B,C the vertices of the prism 3 minimum deviation
Figure 1 — Principle of minimum deviation method
5 Measuring apparatus
5.1 General
The measuring apparatus is shown in Figure 2.
2 © ISO 2020 – All rights reserved
---------------------- Page: 7 ----------------------
ISO/FDIS 21395-1:2020(E)
Key
1 light source 4 rotating stage coupled with prism (5)
2 collimator 5 specimen prism
3 goniometer coupled with telescope (6) 6 telescope
and detector (7) 7 detector
Figure 2 — Schematic of minimum deviation method
5.2 Goniometer

The goniometer shall provide the capability of the angle reading within ±1 arc sec.

5.3 Light source

The light sources should be used lamps of mercury and hydrogen, helium, rubidium, caesium, cadmium,

also He-Ne laser and Nd: YAG laser defined in ISO 7944. The wavelengths with the spectral lines are

shown in Table 1.

The light sources and the wavelength not defined in Table 1 are also applicable for measurement, but

the spectral bandwidth of light source and the accuracy/certainty of emission line (for example D line

(589,3 nm)) should be checked before use.
Table 1 — Wavelength and spectral line of light source
Wavelength/nm Spectral line Light source
365,01 i Mercury lamp
404,66 h Mercury lamp
435,83 g Mercury lamp
479,99 F′ Cadmium lamp
486,13 F Hydrogen lamp
543,5 — He-Ne laser
546,07 e Mercury lamp
587,56 d Helium lamp
632,8 — He-Ne laser
643,85 C′ Cadmium lamp
656,27 C Hydrogen lamp
706,52 r Helium lamp
© ISO 2020 – All rights reserved 3
---------------------- Page: 8 ----------------------
ISO/FDIS 21395-1:2020(E)
Table 1 (continued)
Wavelength/nm Spectral line Light source
780,00 — Rubidium lamp
852,11 s Cesium lamp
1 013,98 t Mercury lamp
1 064,1 — Nd : YAG l a s er
1 128,7 — Mercury lamp
1 395,1 — Mercury lamp
1 529,6 — Mercury lamp
1 813,1 — Mercury lamp
1 970,1 — Mercury lamp
2 325,4 — Mercury lamp
5.4 Detector

A general-type detector which is capable of detecting each wavelength or easily exchangeable for

different spectral ranges should be used.
6 Specimen prism
6.1 General
An example of the shape of specimen prism is shown in Figure 3.
Key
l length
t thickness
α apex angle
Figure 3 — Shape of the specimen prism
4 © ISO 2020 – All rights reserved
---------------------- Page: 9 ----------------------
ISO/FDIS 21395-1:2020(E)
6.2 Dimensions

The length of edges making the apex angle should be 15 mm to 40 mm and the thickness should be

10 mm to 30 mm.
6.3 Apex angle

A reasonable choice of apex angle, α, for a test prism can be calculated from the expected refractive

index of the prism, n , and the angle of incidence, θ.
rel
sin()θ 
α =2arcsin
 
 
rel
α is typically between 35° to 80°.
6.4 Flatness

The plane polished sides should have a peak to valley flatness better than ¼ λ over 80 % of the aperture

at a measurement wavelength of 546 nm or 632,8 nm.
7 Environmental condition of measurement
7.1 Temperature

The temperature shall be between 20 °C and 25 °C. Environmental temperature stability and uniformity

shall controlled according to th
...

Questions, Comments and Discussion

Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.