ISO 21395-2:2022

INTERNATIONAL ISO
STANDARD 21395-2
First edition
2022-02
Optics and photonics — Test method
for refractive index of optical
glasses —
Part 2:
V-block refractometer method
Optique et photonique — Méthode d'essai pour l'indice de réfraction
des verres optiques —
Partie 2: Méthode du réfractomètre à blocs en V
Reference number
ISO 21395-2:2022(E)
© ISO 2022

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ISO 21395-2:2022(E)
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© ISO 2022
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Published in Switzerland
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ISO 21395-2:2022(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principles . 2
5 Measuring equipment . 3
5.1 General . 3
5.2 Light source . 4
5.3 Bandpass filter . 5
5.4 Slit . 5
5.5 Collimator lens . 5
5.6 V-block prism . 5
5.7 Origin-point reference block . 6
5.8 Telescope . 6
5.9 Detector . 7
5.10 Deviation-angle measuring device . 7
5.11 Refractive index matching liquid . 7
6 Shapes of origin-point reference block and specimen . 7
6.1 General . 7
6.2 Apex angle. 7
6.3 Contact face with V-block prism . 7
7 Measurement method . 8
7.1 Measurement environment . 8
7.1.1 Temperature . 8
7.1.2 Atmospheric pressure . 8
7.2 Measurement . 8
7.2.1 General . 8
7.2.2 Preparation for measurement . 8
7.2.3 Setting of deviation-angle origin point (0°) . 8
7.2.4 Adjustment of V-block prism . 9
7.2.5 Measurement of deviation angle . 9
7.2.6 Calculation of refractive index . 10
8 Indication .10
9 Test report .10
Annex A (normative) Refractive index matching liquid .12
Annex B (informative) Telescope with autocollimation function .14
Annex C (normative) Alternative measurement .15
Annex D (normative) Measurement with reference specimen.24
Bibliography .26
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ISO 21395-2:2022(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
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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.
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ISO 21395-2:2022(E)
Introduction
This document specifies the method to determine the refractive index of optical glasses with the
V-block refractometer. Some explanation of the V-block refractometer can be found in Reference [3].
The refractive index of optical glasses is the most important characteristic for the optical elements
manufactured from them.
Regarding the standardization of the method of refractive index measuring method of optical glasses,
the minimum deviation method is defined as ISO 21395-1.
The minimum deviation method is most accurate in refractive index measurement but requires an
advanced technical skill to prepare a specimen with a precise shape and to measure the refractive index.
In contrast the V-block refractometer method is easier and faster when preparing a specimen and
requires less technical skill for measurement. Therefore, this method is commonly used by people
checking the quality of the optical glass products on a daily basis.
This document is intended to aid in measuring the refractive index of optical glasses accurately and
improving the communications between raw optical glass suppliers and optical element manufacturers
as well.
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INTERNATIONAL STANDARD ISO 21395-2:2022(E)
Optics and photonics — Test method for refractive index of
optical glasses —
Part 2:
V-block refractometer method
1 Scope
This document specifies a method to determine the refractive index of optical glass with the accuracy
-5
within 3 × 10 at the wavelength range from 365 nm to 2 400 nm by using the V-block refractometer
method.
While this document can be used for non-glass materials, the user is informed that only optical glass
has been considered in the development of this document, and other materials can have issues, which
have not been taken into consideration.
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.
ISO 280, Essential oils — Determination of refractive index
ISO 9802, Raw optical glass — Vocabulary
ISO 21395-1, Optics and photonics — Test method for refractive index of optical glasses — Part 1: Minimum
deviation method
ISO 80000-1, Quantities and units — Part 1: General
ISO 80000-3, Quantities and units — Part 3: Space and time
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 9802, ISO 80000-1,
ISO 80000-3 and the following apply.
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 https:// www .electropedia .org/
3.1
V-block prism
prism manufactured from optical glass material with a known refractive index
3.2
refractive index matching liquid
transparent liquid having the refractive index close to that of the specimen
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ISO 21395-2:2022(E)
4 Principles
As shown in Figure 1 a), when a beam aligned perpendicular to the entrance face transmits through
the V-block prism, it is refracted at the interface between the V-block prism and the specimen and exits
from the exit face nominally parallel to the entrance face. The relationship among the deviation angle
of the emergent beam to the incident beam, γ, the relative refractive index of the V-block prism to the
measurement atmosphere, N, and the relative refractive index of the specimen to the measurement
atmosphere, n, is expressed by the following Formula (1):
22 22
23×−nN −×Nn−×2
γ =arcsin (1)
2
where
n is the relative refractive index of the specimen to the measurement atmosphere;
N is the relative refractive index of the V-block prism to the measurement atmosphere;
γ is the deviation angle.
NOTE In general, the symbol of the refractive index is the lowercase n. However, this document intentionally
uses the capital letter N for the relative refractive index of the V-block prism to make it more distinct from that of
the specimen.
The sign of the deviation angle of the emergent beam shall be positive (+) for upward deviation (that is,
n > N) and negative (−) for downward deviation (n < N) respectively, relative to the incident beam.
Therefore, by measuring γ, n can be obtained by calculation.
This principle is also applicable when the incident beam angle is controlled so that the emergent beam is
perpendicular to the exit face of the V-block prism. Therefore, as shown in Figure 1b), it is also possible
to calculate the refractive index based on the measurement result of the deviation angle of the incident
beam to the emergent beam.
7.2.6 provides the formula for the calculation of n.
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ISO 21395-2:2022(E)
a)  Deviation angle measurement on exit
b)  Deviation angle measurement on entrance side
side
Key
1 specimen (with relative refractive index, n) 6 emergent beam
2 V-block prism (with relative refractive index, N) 7 refractive index matching liquid
3 exit face γ deviation angle
a
4 entrance face n > N.
b
5 incident beam n < N.
Figure 1 — Principle of V-block method
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In order for Formula (1) to satisfy the required tolerance indicated in Clause 1 (i.e., within 3 × 10 ),
the shape of the V-block prism and the specimen shall be manufactured following to 5.6 and 6.1,
respectively.
5 Measuring equipment
5.1 General
The measuring equipment is composed of the elements and instruments shown in Figure 2.
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ISO 21395-2:2022(E)
Key
1 specimen or origin point reference block 6 collimator lens
2 V-block prism 7 refractive index matching liquid
3 light source 8 deviation-angle measurement device (scale plate)
4 bandpass filter 9 telescope
5 slit 10 detector (or human eye)
Figure 2 — Outline of V-block measurement equipment
Figure 2 illustrates the configuration in which the direction of the incident beam is fixed as shown
in Figure 1 a). When measuring the deviation angle in the case of Figure 1 b), the deviation-angle
measurement device (Key 8) shall be attached not to the telescope but to the light source.
The details of the components are described in 5.2 to 5.10.
5.2 Light source
The sources of the spectral/optical radiation should be discharge lamps of mercury, hydrogen, helium,
[1]
rubidium, caesium or cadmium, or the He-Ne or Nd: YAG laser specified in ISO 7944 . The spectral
lines shown in Table 1 as the measurement wavelength are used with the bandpass filter of respective
wavelength(s). It is necessary to select the light source considering the light transmittance of the
V-block prism and the specimen.
It is permissible to use a light source having a wavelength different from those specified in Table 1,
or a light source combining a continuous spectral light source and a monochromator/bandpass filter.
However, the wavelength deviation due to the finite bandwidth shall be considered when using
alternative light sources.
Table 1 — Wavelength and spectral light source
Wavelength Spectral line Light source
nm
365,01 i Mercury discharge tube
404,66 h Mercury discharge tube
435,83 g Mercury discharge tube
479,99 F' Cadmium discharge tube
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ISO 21395-2:2022(E)
Table 1 (continued)
Wavelength Spectral line Light source
nm
486,13 F Hydrogen discharge tube
543,5 - He-Ne laser
546,07 e Mercury discharge tube
587,56 d Helium discharge tube
632,8 - He-Ne laser
643,85 C' Cadmium discharge tube
656,27 C Hydrogen discharge tube
706,52 r Helium discharge tube
780,00 - Rubidium discharge tube
852,11 s Caesium discharge tube
1 013,98 t Mercury discharge tube
1 064,1 - Nd: YAG laser
1 128,66 - Mercury discharge tube
1 395,1 - Mercury discharge tube
1 529,6 - Mercury discharge tube
1 813,1 - Mercury discharge tube
5.3 Bandpass filter
The bandpass filter transmits only the desired wavelength of light by blocking the light of unnecessary
wavelengths.
5.4 Slit
The slit adjusts the width of the incident beam.
5.5 Collimator lens
The collimator lens changes the divergent beam into a parallel beam.
5.6 V-block prism
The function of the V-block prism on which the specimen is placed is to keep the deviation angle, γ, of
the emergent beam relative to the incident beam within a measurable range. The V-block prism shall be
-5
manufactured from a glass where the refractive index is known with an accuracy of at least 1,0 × 10
according to the measurement method of ISO 21395-1. The refractive index of the V-block prism shall
be selected so that the deviation angle due to the refractive index difference between the V-block prism
and a specimen falls within the range of ±45°. The shape of the V-block prism is shown in Figure 3.
-5
NOTE In order to guarantee a refractive index value with 1,0 × 10 units, the measured refractive index
requires a value of 6 digits or more after the decimal point.
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ISO 21395-2:2022(E)
Key
1 entrance face h height
V
2 exit face W width
V
α angle of V-shaped faces ω roll angle
V1 R
α angle between V-shaped face and entrance or exit face ω tilt angle
V2 T
E lateral edge ω pan angle
L P
d thickness
V
Figure 3 — V-block prism
The required accuracies of the V-block prism are as follows:
a) The angle α of V-shaped faces shall be within 90° ± 5". The angles α between V-shaped face and
V1 V2
entrance or exit face shall be 45° ± 5";
b) The degree of parallelism between the entrance face and the exit face shall be within ±5";
c) The entrance face, exit face and V-shaped surface shall be polished with the flatness approximately
λ/2 at the measurement wavelength λ;
d) The roll angle ω concerning the incident beam optical axis on the V-shaped surface shall be within
R
±20".
If it is difficult to obtain such a fine-shaped V-block prism, a method shown in Annex C can be used, but
in that case the calculation of the refractive index of the specimen is more complicated.
5.7 Origin-point reference block
The origin-point reference block is used for setting the deviation-angle reference point (0°) by placing it
on the V-shaped face of the V-block prism. The origin-point reference block and the V-block prism shall
be manufactured from the same glass lump, and the difference in refractive index between the two
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shall be within 5 × 10 . The shape of the origin point reference block is specified in 6.
5.8 Telescope
The telescope captures the emergent beam from the exit surface of the V-block prism and forms an
image of the light at the measurement wavelength on the detector. It can be rotated around a rotation
axis close to the V-block prism.
It can include an auto-collimation function. Annex B explains the composition of a telescope with an
auto-collimation function.
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ISO 21395-2:2022(E)
5.9 Detector
The detector detects the light of the measurement wavelength as an image or a strength signal. Instead
of using a detector, the light may be detected by looking into the telescope with the human eye.
5.10 Deviation-angle measuring device
The deviation-angle measuring device measures the deviation angle between the incident and the
emergent beam. The resolution of the angle reading on the scale plate shall be smaller than 2”.
5.11 Refractive index matching liquid
The refractive index matching liquid shall be used to fill the air gaps between the specimen (or origin-
point reference block) and the V-block prism. The refractive index value of the matching liquid should
be close to that of the specimen. Annex A provides the information about the selection of appropriate
refractive index matching liquid.
6 Shapes of origin-point reference block and specimen
6.1 General
The shape of the origin-point reference block and the specimen are shown in Figure 4.
Key
1 contact face with V-block prism
α apex angle
s
E lateral edge
L
d thickness
s
Figure 4 — Origin-point reference and specimen
6.2 Apex angle
The apex angle shall be within 90° ± 2'.
6.3 Contact face with V-block prism
The contact face through which the beam is transmitted shall be fine ground and flattened sufficiently.
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ISO 21395-2:2022(E)
7 Measurement method
7.1 Measurement environment
7.1.1 Temperature
The ambient air, the V-block prism, the refractive index matching liquid and the specimen shall
have a temperature between 20 °C and 25 °C with a fluctuation within ±1,0 °C. When the refractive
-5
index temperature coefficient of the specimen is estimated to be more than 1,0 × 10 /K or less than
-5
-1,0 × 10 /K, the temperature fluctuation should be within ±0,5 °C in order to measure with high
accuracy.
7.1.2 Atmospheric pressure
The atmospheric pressure should be maintained between 86 kPa and 106 kPa and the fluctuation range
during the measurement should be within ±1 kPa.
7.2 Measurement
7.2.1 General
In the following subclauses, the measurement procedure is described when the direction of the incident
beam is fixed as shown in Figure 1 a) and Figure 2.
Even if the angle of the incident beam is variable as shown in Figure 1 b), the procedure is almost
the same except that the position of the light source is adjusted instead of the telescope indicated in
Figure 2.
7.2.2 Preparation for measurement
Select a V-block with the specification shown in 5.6 and install it.
In addition, the material of V-block prism should be selected according to the refractive index of the
specimen at the d line (587,56 nm).
7.2.3 Setting of deviation-angle origin point (0°)
Place the origin-point reference block on the V-block prism, filling the air gaps between the specimen
and the V-block prism with the appropriate refractive index matching liquid.
Inject the incident beam into the entrance face of the V-block prism, and detect the emergent beam from
the exit face of the prism by rotating a telescope with a light detector. Set the angular position when
detecting maximum light output as the deviation-angle origin point (0°) for future measurements.
Figure 5 shows an arrangement for setting the deviation-angle origin point.
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ISO 21395-2:2022(E)
Key
1 origin-point reference block 6 collimator lens
2 V-block prism 7 refractive index matching liquid
3 light source 8 deviation-angle measurement device (scale plate)
4 bandpass filter 9 telescope
5 slit 10 detector (or human eye)
Figure 5 — Schematic diagram of setting the deviation-angle origin point
7.2.4 Adjustment of V-block prism
The V-block shall be adjusted by using an autocollimation method so that the emergent beam is aligned
perpendicular to the exit face of the V-block prism. Specifically, the positioning tolerance of pan angle
and tilt angle of V-block prism, which are shown in Figure 3 as ω and ω , shall be within 1’ and 4”,
P T
respectively.
Annex B provides further details of the telescope with an autocollimation function.
7.2.5 Measurement of deviation angle
Figure 6 shows a schematic diagram for the deviation angle measurement.
Place the specimen on the V-block prism, filling the air gaps between the specimen and the V-block
prism with the appropriate refractive index matching liquid.
Inject the incident beam perpendicular to the incident face of the V-block prism and detect the emergent
beam from the exit face of the V-block prism by rotating a telescope with a light detector. The difference
between the angular position of maximum light output and the previously obtained origin point is the
deviation angle for this specimen.
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ISO 21395-2:2022(E)
Key
1 specimen 7 refractive index matching liquid
2 V-block prism 8 deviation-angle measurement device (scale plate)
3 light source 9 telescope
4 bandpass filter 10 detector (or human eye)
5 slit 11 deviation-angle origin point
6 collimator lens γ deviation angle
Figure 6 — Schematic diagram of the deviation angle measurement
7.2.6 Calculation of refractive index
Calculate the refractive index by the following Formula (2):
22 2
nN= +×−sinγγN sin (2)
where
n is the relative refractive index of specimen to the measurement atmosphere;
N is the relative refractive index of V-block prism to the measurement atmosphere;
γ is the deviation angle.
8 Indication
Indicate the calculated refractive index with five digits or more after the decimal point.
9 Test report
The report shall specify the following:
a) a reference to this document, i.e. ISO 21395-2:2022;
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ISO 21395-2:2022(E)
b) the method used (Clause 7 or Annex C or Annex D);
c) details to identify the specimen;
d) temperature and atmospheric pressure for which the refractive index value is valid;
e) measurement wavelength;
f) refractive index value of the specimen;
g) deviations from the procedure;
h) unusual features observed;
i) date of the test.
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ISO 21395-2:2022(E)
Annex A
(normative)

Refractive index matching liquid
A.1 General
Annex A provides the information about the selection of the refractive index matching liquid. A
refractive index matching liquid is a transparent liquid having the same, or sufficiently close, refractive
index as the specimen. The reference refractive index shall be the refractive index at the Fraunhofer d
line. Since the refractive index is measured at multiple wavelengths, it is preferable that the refractive
indices are known at multiple wavelengths.
A.2 Refractive index matching liquid
The refractive index matching liquid used shall be a liquid
...