ISO 21395-1:2020

Optics and photonics — Test method for refractive index of optical glasses — Part 1: Minimum deviation method

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 — Partie 1: Méthode de la déviation minimale

General Information

• Status: Published
• Publication Date: 29-Jun-2020
• ICS: 37.020 - Optical equipment
• Technical Committee: ISO/TC 172/SC 3 - Optical materials and components
• Drafting Committee: ISO/TC 172/SC 3/WG 1 - Raw optical glass
• Current Stage: 6060 - International Standard published
• Start Date: 30-Jun-2020
• Due Date: 02-May-2020
• Completion Date: 30-Jun-2020

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: [phone]
Email: [e-mail]
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

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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

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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
−5
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
2
n = (1)
rel
α
sin
2
© ISO 2020 – All rights reserved 1

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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

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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

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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

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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
 
n
 
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: [phone]
Fax: [phone]
Email: [e-mail]
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
−5
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
2
n = (1)
rel
α
sin
2
© 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

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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
 
n
 
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
...