ISO 6760-2:2024

Optics and photonics — Test method for temperature coefficient of refractive index of optical glasses — Part 2: Interferometric method

This document specifies a test method for the temperature coefficient of refractive index of optical glass using interferometry. Temperature changes in optical glass lead to changes in the optical path length. The change in optical path length can be measured with an interferometer using the number of cycles of light/dark change of the interference stripe. This document defines a test method to measure the amount of change in the refractive index when the temperature of the specimen is changed continuously. The intended temperature range for the specified measurement method is an arbitrary range. The intended wavelength range for the specified measurement method is 365 nm to 1 014 nm. The intended accuracy for the specified measurement method is within 1 × 10-6 K-1.

Optique et photonique — Méthode d'essai pour déterminer le coefficient de température de l'indice de réfraction des verres optiques — Partie 2: Méthode interférométrique

General Information

• Status: Published
• Publication Date: 05-May-2024
• ICS: 37.020 - Optical equipment
• Technical Committee: ISO/TC 172/SC 3 - Optical materials and components
• Drafting Committee: ISO/TC 172/SC 3 - Optical materials and components
• Current Stage: 6060 - International Standard published
• Start Date: 06-May-2024
• Due Date: 04-Nov-2024
• Completion Date: 06-May-2024

Standards Content (Sample)

International
Standard
ISO 6760-2
First edition
Optics and photonics — Test
2024-05
method for temperature coefficient
of refractive index of optical
glasses —
Part 2:
Interferometric method
Optique et photonique — Méthode d'essai pour déterminer le
coefficient de température de l'indice de réfraction des verres
optiques —
Partie 2: Méthode interférométrique
Reference number
© ISO 2024
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
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 2
5 Measuring apparatus . 2
5.1 General .2
5.2 Light sources .3
5.3 Thermal chamber .4
5.4 Flat plates .4
5.5 Interferometer for optical path length change measurement .5
5.6 Interferometer for expansion measurement .5
5.7 Detectors.5
5.8 Temperature sensor .6
5.9 Barometer .6
6 Measurement specimen . 6
7 Measurement procedure . 6
8 Calculation . 7
8.1 Temperature coefficient of absolute refractive index .7
8.2 Linear expansion coefficient of specimen .8
8.3 Temperature coefficient of relative refractive index .8
9 How to express the temperature coefficient of refractive index . 9
10 Test report . 10
Annex A (informative) Formula for calculating the refractive index of air .11
Annex B (normative) Calculation of absolute refractive index of specimen at a given
temperature using the temperature coefficient of refractive index .13
Annex C (informative) Temperature coefficient of refractive index under continuous
temperature change . 17
Annex D (informative) Sensor specifications . 19
Annex E (informative) How to find the deviation of interference fringes between two
temperatures .21
Annex F (normative) Flat plate requirements .23
Annex G (normative) Heating rate and temperature distribution .25
Bibliography .26

iii
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 document 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).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
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 6760 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
Introduction
Optical glass is widely used in optical devices such as cameras, telescopes, and microscopes, and its refractive
[4]
index is measured by the minimum deviation method (ISO 21395-1 ) and the V-block refractometer method
[5]
(ISO 21395-2 ). Here, when designing an optical apparatus that requires high resolution, it is necessary to
consider the temperature change of the refractive index of the optical glass in the usage environment. This
document proposes a method for measuring the temperature coefficient of refractive index of optical glass
with high accuracy.
v
International Standard ISO 6760-2:2024(en)
Optics and photonics — Test method for temperature
coefficient of refractive index of optical glasses —
Part 2:
Interferometric method
1 Scope
This document specifies a test method for the temperature coefficient of refractive index of optical glass
using interferometry. Temperature changes in optical glass lead to changes in the optical path length. The
change in optical path length can be measured with an interferometer using the number of cycles of light/
dark change of the interference stripe. This document defines a test method to measure the amount of
change in the refractive index when the temperature of the specimen is changed continuously.
The intended temperature range for the specified measurement method is an arbitrary range.
The intended wavelength range for the specified measurement method is 365 nm to 1 014 nm.
-6 -1
The intended accuracy for the specified measurement method is within 1 × 10 K .
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
temperature coefficient of refractive index
ratio of refractive index change to temperature change at a selected wavelength
[3]
[SOURCE: ISO 9802:2022 , 3.4.2.3, 3.4.2.4, modified — term and definition slightly reworded.]
3.2
temperature coefficient of absolute refractive index
Δn /ΔT
abs
ratio of refractive index change in vacuum to temperature change at a selected wavelength
[3]
[SOURCE: ISO 9802:2022 , 3.4.2.3, modified — term reworded.]

3.3
temperature coefficient of relative refractive index
Δn /ΔT
rel
ratio of refractive index change at an air pressure of 1,013 25 × 10 Pa and a relative humidity of 0 % to
temperature change at a selected wavelength
[3] 6
[SOURCE: ISO 9802:2022 , 3.4.2.4, modified — term reworded and "0,101 33 × 10 Pa" and "0 %
humidity" added.]
Note 1 to entry: This definition of Δn /ΔT is for a specific pressure and humidity. Δn /ΔT can be calculated for any
rel rel
other pressure and humidity by understanding the index of air in those conditions.
3.4
thermal chamber
chamber where the temperature of the specimen can be changed and/or maintained to a preset temperature
4 Principle
The temperature coefficient of refractive index is calculated in either Formula (1) or Formula (2) obtained
by Annex C. The derivation of these formulae is described in Annex C. For a calculation method for obtaining
the relative refractive index of glass at an arbitrary temperature and relative humidity, see Annex B.
Δn
1 f ×λ
abs
=× −×α n (1)
labs
ΔΔT 2 LT×
Δn
1 f ×λ
abs
=× −×α n (2)
lrel
ΔΔT 2 LT×
where
Δn
-1
abs
is the temperature coefficient of absolute refractive index of specimen (K );
ΔT
is interferometer scale factor of double-path interferometers;
λ is the wavelength of the refractive index temperature coefficient measurement (m);
L is the measurement specimen length (m);
f is the number of cycles of light/dark change of interference fringes associated with
changes in optical path length of the specimen corresponding to ΔT;
ΔT is the specimen temperature difference (K);
n is the absolute refractive index of the specimen;
abs
n is the relative refractive index of the specimen;
rel
-1
α is the linear expansion coefficient of the specimen (K ).
l
5 Measuring apparatus
5.1 General
The measuring equipment shall be in accordance with the requirements in 5.2 to 5.9.
a) For measuring equipment, use the Fizeau-interference measurement principle in which the measured
sample itself constitutes the interference space.

b) The resolution to read the number of cycles of light and dark changes in the interference space. The
resolution shall be 1/10 cycle or less.
Figure 1 shows an example of a schematic diagram of the measuring equipment.
Key
1 specimen 11 detector for optical path length change of the specimen
measurement
2 thermal chamber shell 12 light source for expansion measurement
3 transmissive reference flat 13 beam splitter for expansion measurement
4 reflective reference flat 14 detector for expansion measurement
5 window 15 vacuum gauge
6 heating/cooling unit 16 barometer
7 temperature sensor 17 cushioning chamber (e.g. flexible plastic bag)
a
8 mechanical length of the specimen Connection to vacuum pump.
b
9 light source for optical path length change of the Dry air inlet.
specimen measurement
10 beam splitter for optical path length change
measurement
Figure 1 — Example of a schematic drawing of a Fizeau-interferometric type of measurement
equipment
5.2 Light sources
For change in optical path length of specimen light source and linear expansion coefficient light source,
use a light source with sufficient intensity, monochromaticity and coherence to obtain interference fringes
with the required precision. The wavelength for the measurement of the change in optical path length of
specimen and linear expansion coefficient do not need to be the same.
NOTE 1 A sufficient light source, such as a laser, has to provide adequate illumination to enable accuracy, precision,
and repeatability for the test.

[2]
NOTE 2 Examples of light sources are listed in ISO 7944:— , Table 1, Table 2 and Table 3.
5.3 Thermal chamber
The thermal chamber has a window for observing changes in the optical path length. Thermal chamber shall
a) have the ability to change the temperature of the specimen between the temperatures to be measured,
b) have the ability to maintain the temperature of the specimen within ±0,5 K,
c) have a thermometer to measure the temperature of the specimen with an accuracy of ±0,2 K or better,
d) have the ability to be filled with dry air at a relative humidity of 0 % or provide a vacuum with a residual
pressure of less than 10 Pa to prevent condensation. When the inside of the thermal chamber is filled
with dry air, the structure shall be such that the air pressure in the thermal chamber is the same as the
atmospheric pressure around the container, and
e) have a window of the thermal chamber, which shall be made of quartz glass with a wedge angle of
approximately 6 arc min (0,1°) on the opposite plane and polished on both sides.
NOTE Quartz glass is used because it has a wide wavelength range with a high transmittance, has a high durability
against temperature changes, and is resistant to breakage.
5.4 Flat plates
Two flat plates, one is a transmission flat plate and the other is a reference flat plate, are used to measure
the linear expansion coefficient of the specimen along with the change of the optical path length by using
interference action. In the thermal chamber, the specimen is sandwiched between two plate's interference
surfaces. Each interference surface shall be parallel with each interference surface of the specimen.
Examples include point contact, line contact, surface contact, etc. An example of point contact is shown in
Figure 2. The contact points of the specimen and the flat plate are on the same plane. The flat plate geometry
is described in Annex F.
The required accuracies of the two flat plates are as follows:
a) The transmission flat plate and reference flat plate shall be made of quartz glass or extremely low-
expansion glass ceramics, where both sides of which have been polished with a wedge angle of
approximately 6 arc min (0,1°).
b) The flatness of the surface used for the interference action shall be λ/2 or less (1/2 of the measurement
wavelength of the linear expansion coefficient).
c) There shall be a hole in the centre of the transmission flat plate to secure the optical path for
measurement of change in the optical path length of the specimen.
d) The back surface not involved in interference of the reference flat plate may be ground glass surface
without polishing. In this case, the wedge angle of the reference flat plate is not required.
e) A suitable method should be used to ensure that reflections from the rear surface of the reference flat
plate do not cause confusion with desired interference pattern. For example, there are sanding and
donut-shaped processing by making a hole in the relevant position.

Key
1 specimen
2 transmissive reference flat plate
3 reflective reference flat plate
4 contact point
Figure 2 — Example of point contact between the specimen and 2 flat plates
5.5 Interferometer for optical path length change measurement
The interferometer used shall be able to measure the optical path length change of the specimen while
changing the temperature of the specimen.
5.6 Interferometer for expansion measurement
The interferometer used shall be able to measure the length change of the specimen while changing the
temperature of the specimen.
5.7 Detectors
The detector used shall be able to detect bright to dark changes due to interference of reflected light from
the specimen, and bright to dark changes due to interference of reflected light from 2 flat plates.

5.8 Temperature sensor
The temperature sensor used to measure the temperature of specimen shall have a measurement accuracy
within 0,2 K or less.
Additionally, when the inside of the thermal chamber is filled with dry air, the temperature in the space
between the two flat plates should be measured by using a second thermometer which also has a measurement
accuracy within 0,2 K or less. However, if there is no difference between the temperature of the space between
the two flat plates and the temperature of the specimen, the temperature of the specimen may be taken as
the temperature between the flat plates. The temperature sensor, which measures the temperature of the
specimen, shall be in contact with the specimen to avoid a break in heat transfer due to vacuum.
5.9 Barometer
If dry air is filled in the thermal chamber, a barometer with a measurement accuracy of 0,05 kPa or better
shall be used. If the structure is such that the air pressure in the thermal chamber is the same as the air
pressure around the container, a barometer independent of the thermal chamber may be used. In a vacuum
the barometer shall be suitable to guarantee that the air pressure is below 10 Pa.
6 Measurement specimen
Measurement specimen shall follow the requirements below:
a) Specimen materials shall be annealed sufficiently to ensure that the phase difference due to birefringence
is not more than 5 nm per cm. Materials containing visible defects, such as striae, bubbles, or inclusions
shall not be used. Materials used should contain striae of grade SW30 or less and bubbles and inclusions
of grade IC10 and IN30 respectively or less.
b) The test specimen shall have two parallel, flat polished surfaces. The dimensions and shape of the parallel
plate should be 20 mm to 25 mm in diameter and approximately 5 mm in sample length, considering
the amount of change in the optical path length to be measured, the temperature distribution in the
measurement sample, and the coherence of the measurement light.
NOTE A longer optical path length is better for measurement, and a shorter sample length is better for
temperature distribution. The sample length that satisfies both the required optical path length and the
temperature distribution is approximately 5 mm.
c) The parallelism between the facing plane shall be 10 arc sec (approx. 0,003°) or less, whereas the
flatness shall be λ/2 (1/2 of the wavelength of light source for optical path length change measurement)
or less.
d) Absolute refractive index n (T ) or relative refractive index n (T ) at reference temperature T
abs 0 rel 0 0
-4
shall be known with an accuracy of 1 × 10 or less. Because the accuracy of temperature coefficient of
refractive index is highly dependent on the accuracy of expansion coefficient, the accuracy of refractive
-4
index of 1 × 10 or less is sufficient. T shall be any temperature within the range of 23 °C ± 2 °C.
NOTE Since it is difficult to measure absolute refractive index n (T ) or relative refractive index n (T ) with
abs 0 rel 0
the same specimen as for temperature coefficient of refractive index, for example, produce a specimen for refractive
index measurement from the glass material part adjacent to the specimen, and the relative refractive index n (T ) at
rel 0
[4] [5]
the reference temperature T can be measured in accordance with ISO 21395-1 or ISO 21395-2 .
7 Measurement procedure
The measurement procedure is as follows:
a) Sandwich the specimen between two flat plates (see Figure 2), place the specimen in the interferometric
type measuring equipment (see Figure 1).
b) Fill the inside of the thermal chamber with dry air at a relative humidity of 0 % or a vacuum of not more
than 10 Pa.
c) Lower the specimen temperature to -50 °C or less and raise the temperature to at least 90 °C or more.
It is also allowed to start the measurement after raising the specimen temperature to 90 °C at least
and lowering the temperature to -50 °C or less. This shall be done at the rate at which the temperature
distribution within the specimen is within 1,0 K. The speed at which the temperature distribution
in the specimen is within 1,0 K depends on the measuring instrument and should be confirmed in a
preliminary test. Typical heating rate and temperature distribution are described in Annex G.
d) During the measurement (in either direction of the temperature change), continuously record the
specimen temperature and the number of cycles, f, that the interference stripe changes from light to
dark. Sensors can be used for recording. See Annex D for sensor specifications. The resolution of the
light/dark change readings for the interference stripes shall be 1/10 cycle or less. At the same time as
c), continuously measure the number of cycles, f , of the light/dark change of the interference stripes of
α
the interference space between the two flat plates. In this case, the reading resolution of the light/dark
change of interference stripes shall be 1/10 cycle or less. See Annex E for an example of the procedure
for determining the amount of number of cycles of light/dark change of interference shift between two
temperatures.
e) If the inside of the thermal chamber is filled with dry air having a relative humidity of 0 %, continuously
record the temperature of the dry air in the space between the flat plates at the same time as c) and d).
f) If the inside of the thermal chamber is filled with dry air having a relative humidity of 0 %, measure and
record the air pressure in the thermal chamber at the same time as c) and d). If the structure is such
that the air pressure in the thermal chamber is the same as the air pressure around the container, the
measurement of the atmospheric pressure in the vicinity of the chamber may be used instead.
8 Calculation
8.1 Temperature coefficient of absolute refractive index
Calculate the temperature coefficient of the absolute refractive index in accordance with Formula (3) using
Formula (C.5). For a formula describing the temperature coefficient of the absolute refractive index as a
[1]
function of temperature and wavelength, see ISO 6760-1 :
Δn
1 λ f
abs
=× ×−α ×nT() (3)
labs 0
ΔΔTL2 T
where
Δn
-1
abs
is the temperature coefficient (K ) of absolute refractive index of the specimen;
ΔT
is interferometer scale factor of double-path interferometers;
λ is the refractive index temperature coefficient measurement wavelength in vacuum (m);
L is the measurement specimen length (m);
f is the number of cycles of light/dark change in interference fringes associated with
changes in optical path length of specimen corresponding to ΔT;
ΔT is the specimen temperature difference (K);
n (T ) is the absolute refractive index of the specimen at ambient temperature T ;
abs 0 0
T is the reference temperature at which the refractive index of the specimen is measured, 23 °C ± 2 °C;
-1
α is the linear expansion coefficient of specimen (K ).
l
NOTE 1 The temperatures for calculation are arbitrary. When the temperature change ΔT of the specimen is 20 K,
calculate the temperature co
...