ISO 23364:2021

Optics and Photonics — Bulk absorption optical filters

This document specifies filter functions of uncoated bulk absorption filters for optical applications excluding ophthalmic optics (spectacles) and gives a standard form for their specification. Additionally, basic definitions and a description of the specification concerning optical bulk absorption filters are given. This document specifies the optical properties of the filters and the test and measurement methods whenever necessary. This document does not specify any material properties (internal quality, homogeneity, etc.) and it does not apply to any production method. This document applies to both the raw material (filter glass, filter plastics, etc.) and the polished component. NOTE 1 Colorimetric parameters for the description of the filter function are specified in e.g. ISO 11664‑1 and ISO 11664‑2. NOTE 2 For filters where the spectral transmission characteristics are achieved by the application of optical coatings, see ISO 9211 series. NOTE 3 In the case of high power applications, further optical effects may occur.

Optique et photonique — Filtres optiques à absorption de masse

General Information

• Status: Published
• Publication Date: 19-Apr-2021
• ICS: 37.020 - Optical equipment
• Technical Committee: ISO/TC 172/SC 3 - Optical materials and components
• Drafting Committee: ISO/TC 172/SC 3/WG 2 - Coatings
• Current Stage: 6060 - International Standard published
• Start Date: 20-Apr-2021
• Due Date: 01-May-2021
• Completion Date: 20-Apr-2021

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 23364
First edition
2021-04
Optics and Photonics — Bulk
absorption optical filters
Optique et photonique — Filtres optiques à absorption de masse
Reference number
ISO 23364:2021(E)
©
ISO 2021

---------------------- Page: 1 ----------------------
ISO 23364:2021(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2021
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 2021 – All rights reserved

---------------------- Page: 2 ----------------------
ISO 23364:2021(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
3.1 Boundary conditions . 1
3.2 Optical properties . 2
3.3 Calculated parameters . 5
3.4 Definition of bulk absorption filters by their function . 6
4 Measurement . 7
4.1 General . 7
4.2 Measurement conditions . . 7
5 Numerical specification and graphical representation of spectral characteristics .7
5.1 General . 7
5.2 Rules for the numerical specification of spectral characteristics . . 8
5.2.1 Rules for the spectral characteristics, τ , τ, τ , a, E, D or θ . 8
i V
5.2.2 Rules for the cut-off wavelength and peak transmittance . 8
5.3 Rules for the graphical representation of spectral characteristics . 9
5.4 Graphical representation of optical functions .10
5.4.1 General.10
5.4.2 Attenuating function (ND) .10
5.4.3 Function bandpass (BP) or bandrejection (BR) .11
5.4.4 Function shortpass (SP) or longpass (LP) .13
Annex A (informative) Graphical representation of transmission using a diabatic scale as
an ordinate .15
Annex B (informative) Recommendation for the thickness of the witness sample .17
Annex C (informative) Spectral weighting function for luminous transmittance .19
Bibliography .20
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ISO 23364:2021(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 on 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 the following
URL: 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 2021 – All rights reserved

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ISO 23364:2021(E)

Introduction
The optical properties of a bulk absorption filter are characterized by spectrophotometric values.
These values relate to the energy transported by electromagnetic waves (radiant or luminous) and they
vary as a function of wavelength. Additional influences can be caused by scattering.
NOTE 1 The functional spectral dependency is generally indicated by including the wavelength, λ, in
parentheses as part of the symbol.
NOTE 2 The wavelength, λ, can be replaced by the wavenumber, σ, or the photon energy, hν, h = Planck
constant; ν = frequency. The units recommended are the nanometre (nm) or the micrometre (µm) for the
−1
wavelength, the reciprocal centimetre (cm ) for the wavenumber and the electron volt (eV) for the photon
energy. Bulk absorption filters are defined according to their function, i.e. according to the nature of the principal
modification of the spectral transmission (see Table 1).
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INTERNATIONAL STANDARD ISO 23364:2021(E)
Optics and Photonics — Bulk absorption optical filters
1 Scope
This document specifies filter functions of uncoated bulk absorption filters for optical applications
excluding ophthalmic optics (spectacles) and gives a standard form for their specification. Additionally,
basic definitions and a description of the specification concerning optical bulk absorption filters are
given.
This document specifies the optical properties of the filters and the test and measurement methods
whenever necessary.
This document does not specify any material properties (internal quality, homogeneity, etc.) and it does
not apply to any production method.
This document applies to both the raw material (filter glass, filter plastics, etc.) and the polished
component.
NOTE 1 Colorimetric parameters for the description of the filter function are specified in e.g. ISO 11664-1 and
ISO 11664-2.
NOTE 2 For filters where the spectral transmission characteristics are achieved by the application of optical
coatings, see ISO 9211 series.
NOTE 3 In the case of high power applications, further optical effects may occur.
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 9211-1, Optics and photonics — Optical coatings — Part 1: Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 9211-1 and the following
terms and definitions 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 http:// www .electropedia .org/ .
3.1 Boundary conditions
3.1.1
optical surface
optically effective surface, into which the radiation enters the bulk absorption filter or from which it
leaves
Note 1 to entry: In general, bulk absorption filters are made as plane parallel plates and have two optical surfaces
which are opposite to each other.
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ISO 23364:2021(E)

3.1.2
angle of incidence
Θ
angle between the normal to the optical surface and the incident ray
Note 1 to entry: Unless otherwise specified, the angle of incidence is equal to 0°; this means the incident rays are
normal to the optical surface.
[SOURCE: ISO 9211-1:2018, 3.1.2.6, modified — Note 1 to entry was added.]
3.1.3
thickness
d
geometrical length that the radiation passes through the bulk absorption filter at normal incidence to
the optical surfaces
Note 1 to entry: At normal incidence onto a plane parallel bulk absorption filter the path length of the radiation
is equal to the thickness, d, of the bulk absorption filter. All parameters and characteristic numbers of this
document are referenced to the case of normal incidence, when the path length and the thickness of the bulk
absorption filter are the same.
Note 2 to entry: For the case of non-normal incidence, the thickness is not equal to the path length of the light.
3.1.4
witness sample
sample, which represents the bulk absorption filter component and which is used for spectral
measurements and environmental testing
Note 1 to entry: The details about the witness sample and the measurement (i.e. material, surface condition,
geometry, number per batch) may be subject to the negotiation between manufacturer and customer.
[SOURCE: ISO 9211-1:2018, 3.1.1.6, modified — In the note “sampling procedures” has been replaced by
“measurement”.]
3.2 Optical properties
3.2.1
spectral transmittance
τ(λ)
ratio of the spectral radiant flux transmitted to that of the incident radiant flux
Note 1 to entry: The spectral transmittance is dependent on the internal absorption properties, especially on the
travel path of the light, as well as on the optical properties of the surface.
Φ
eλ ,2
τλ() = (1)
Φ
eλ ,1
where
Φ is the incident spectral radiant flux;
eλ,1
Φ is the transmitted spectral radiant flux.
eλ,2
Note 2 to entry: See Figure 1.
Note 3 to entry: Wherever the Greek letter τ is mistakable T may be used.
Note 4 to entry: If necessary, the transmittance can be represented as an average over a wavelength range from
λ to λ as follows:
1 2
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ISO 23364:2021(E)

λ
2
m m
τλ()dλ
τλ Δλ τλ
∫ () ()
λ
∑ i ∑ i
1 i=1 i=1
τλ() to λ = ≈ == (2)
ave 12
λλ − λλ− m
21 21
where
Δ=λλ  − mλ /
()
21
Note 5 to entry: The subscript “ave” stands for “average”. As an alternative the subscript “avg” may be used.
NOTE The light falls onto the optical surface at normal incidence, however, it is sketched at an angle in order
to visualize the surface reflection R. As such, path length of the light and geometrical thickness d are the same.
Figure 1 — Sketch for depicting the differences between transmittance and internal
transmittance
3.2.2
spectral internal transmittance
τ (λ)
i
ratio of the spectral radiant flux arriving without reflection at the exit surface of the bulk absorption
filter to that of the entered radiation
Note 1 to entry: The internal transmittance describes the properties inside the material, thus surface effects do
not have an influence.
Note 2 to entry: The subscript “i” stands for “internal”.
Φ
eλ ,4
τλ() = (3)
i
Φ
eλ ,3
where
Φ is the spectral radiant flux, which has entered the volume;
eλ,3
Φ is the spectral radiant flux, which is going to leave the volume.
eλ,4
Note 3 to entry: For bulk absorption filters with a homogeneous distribution of absorption within the material
the following applies:
−α d
l
τ = e (4)
i
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ISO 23364:2021(E)

where
α is the absorption coefficient;
l
d is the thickness of the bulk absorption filter.
From this context, the internal transmittance can be calculated for different thicknesses of the bulk absorption
filter (at normal incidence) by:
d
 
1
 
d
ττ =   (5)
2
()
ii,,dd
12
where
τ is the internal transmittance that corresponds to the thickness d ;
1
i,d
1
τ is the internal transmittance that corresponds to the thickness d .
2
i,d
2
Note 4 to entry: See Figure 1.
3.2.3
cut-off wavelength of the internal transmittance
λ
i0,5
wavelength in the transition between a region of high and a region of low transmittance, where the
transmittance has a value of τ = 0,5
i
Note 1 to entry: The subscript “i” stands for “internal”.
3.2.4
cut-off wavelength of the transmittance
λ
0,5
wavelength in the transition between a region of high and a region of low transmittance, where the
transmittance has a value of τ = 0,5
Note 1 to entry: The Annex A of ISO 9211-2 defines λ' and λ’’ as the wavelength where the transmittance is
0,5 0,5
half of τ or τ , respectively.
A M
3.2.5
spectral absorptance
a(λ)
ratio of the spectral concentration of radiant flux absorbed to that of the incident radiation
3.2.6
refractive index
n(λ)
ratio of the velocity of propagation of electromagnetic radiation in vacuum to the velocity of propagation
in a medium
[SOURCE: ISO 9211-1]
3.2.7
reflection factor
Ρ(λ)
ratio of spectral transmittance (3.2.1) to spectral internal transmittance (3.2.2)
τλ()
P()λ = (6)
τλ
()
i
Note 1 to entry: When neglecting diffuse scattering the following formula applies at normal incidence onto the
bulk absorption filter:
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ISO 23364:2021(E)

2n λ
()
P()λ = (7)
2
n ()λ + 1
Note 2 to entry: Because the spectral dependency of P(λ) is weak, a constant reflection factor is used in most
cases. Unless otherwise specified, a constant reflection factor is used, which is derived at the d-line (587,6 nm).
2n
d
PP = = P(λ = 587,6 nm) = (8)
d
2
n + 1
d
Note 3 to entry: P is the Greek upper case letter Rho.
3.3 Calculated parameters
3.3.1
spectral optical density
D(λ)
logarithm to the base 10 of the reciprocal of the spectral transmittance (3.2.1)
1
D λ = lg (9)
()
τλ()
3.3.2
spectral extinction
E(λ)
logarithm to the base 10 of the reciprocal of the spectral internal transmittance (3.2.2)
1 α d
l
E()λ = lg = (10)
τλ() ln(10)
i
Note 1 to entry: Extinction = Absorbance. Sometimes letter A is used.
3.3.3
spectral diabatic transmittance
θ(λ)
a characterization of the spectral internal transmittance (3.2.2) represented by the following formula:
 
1
θλ() =−1 lg lg (11)
 
τλ
()
 
i
Note 1 to entry: For transformation of spectral internal transmittance and spectral diabatic transmittance see
Annex A.
3.3.4
luminous transmittance
τ
V
ratio of the luminous flux transmitted by an ocular or filter to the incident luminous flux for a specified
illuminant and photopic vision
Note 1 to entry: This is usually expressed as a percentage and is calculated from the following formula:
780 nm
τλ() ⋅⋅SV()λλ () ⋅ dλ
D65
∫
380 nm
τ =×100  (12)
V
780 nm
S ()λ ⋅ VV ()λλ ⋅ d
D65
∫
380 nm
where
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ISO 23364:2021(E)

λ is the wavelength of the light in nanometres;
τ(λ) is the spectral transmittance of the ocular or filter;
V(λ) is the spectral luminous efficiency function for photopic vision;
S (λ) is the spectral distribution of incident radiation of CIE standard illuminant D65 (see ISO 11664-2).
D65
Note 2 to entry: Although τ is defined as above using the spectral distribution of standard illuminant D65, for
V
other purposes, S (λ) can be replaced by the spectral distribution of standard illuminant A or the relevant light
D65
source.
Note 3 to entry: The values of the spectral radiation distribution of CIE standard illuminant D65, S (λ) are
D65
given in ISO/CIE 11664-2.
Note 4 to entry: The values of the spectral luminous efficiency function V(λ) are given in ISO/CIE 11664-1.
Note 5 to entry: The values for the spectral weighting function, SVλλ ⋅ , are given in Annex C.
() ()
D65
[SOURCE: ISO 4007:2012-05]
3.4 Definition of bulk absorption filters by their function
3.4.1 Attenuating function
3.4.1.1
ND
code for a bulk absorption filter which reduces the transmittance independent of wavelength at the
same ratio
3.4.2 Bandpass function
3.4.2.1
BP
code for a bulk absorption filter which has high transmittance in a defined wavelength region and low
transmittance in the neighbouring regions
3.4.3 Bandreje
...

FINAL
INTERNATIONAL ISO/FDIS
DRAFT
STANDARD 23364
ISO/TC 172/SC 3
Optics and Photonics — Bulk
Secretariat: JISC
absorption optical filters
Voting begins on:
2021­01­26
Optique et photonique — Filtres optiques à absorption de masse
Voting terminates on:
2021­03­23
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 23364:2021(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 2021

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ISO/FDIS 23364:2021(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2021
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
CH­1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii © ISO 2021 – All rights reserved

---------------------- Page: 2 ----------------------
ISO/FDIS 23364:2021(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
3.1 Boundary conditions . 1
3.2 Optical properties . 2
3.3 Calculated parameters . 5
3.4 Definition of bulk absorption filters by their function . 6
4 Measurement . 7
4.1 General . 7
4.2 Measurement conditions . . 7
5 Numerical specification and graphical representation of spectral characteristics .7
5.1 General . 7
5.2 Rules for the numerical specification of spectral characteristics . . 8
5.2.1 Rules for the spectral characteristics, τ , τ, τ , a, E, D or θ .8
i V
5.2.2 Rules for the cut-off wavelength and peak transmittance . 8
5.3 Rules for the graphical representation of spectral characteristics . 9
5.4 Graphical representation of optical functions .10
5.4.1 General.10
5.4.2 Attenuating function (ND) .10
5.4.3 Function bandpass (BP) or bandrejection (BR) .11
5.4.4 Function shortpass (SP) or longpass (LP) .13
Annex A (informative) Graphical representation of transmission using a diabatic scale as
an ordinate .15
Annex B (informative) Recommendation for the thickness of the witness sample .17
Annex C (informative) Spectral weighting function for luminous transmittance .19
Bibliography .20
© ISO 2021 – All rights reserved iii

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ISO/FDIS 23364:2021(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 on 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 the following
URL: 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 2021 – All rights reserved

---------------------- Page: 4 ----------------------
ISO/FDIS 23364:2021(E)

Introduction
The optical properties of a bulk absorption filter are characterized by spectrophotometric values.
These values relate to the energy transported by electromagnetic waves (radiant or luminous) and they
vary as a function of the wavelength. Additional influences can be caused by scattering.
NOTE 1 The functional spectral dependency is generally indicated by including the wavelength, λ, in
parentheses as part of the symbol.
NOTE 2 The wavelength, λ, can be replaced by the wavenumber, σ, or the photon energy, hν, h = Planck
constant; ν = frequency. The units recommended are the nanometre (nm) or the micrometre (µm) for the
−1
wavelength, the reciprocal centimetre (cm ) for the wavenumber and the electron volt (eV) for the photon
energy. Bulk absorption filters are defined according to their function, i.e. according to the nature of the principal
modification of the spectral transmission (see Table 1).
© ISO 2021 – All rights reserved v

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FINAL DRAFT INTERNATIONAL STANDARD ISO/FDIS 23364:2021(E)
Optics and Photonics — Bulk absorption optical filters
1 Scope
This document specifies filter functions of uncoated bulk absorption filters for optical applications
excluding ophthalmic optics (spectacles) and gives a standard form for their specification. Additionally,
basic definitions and a description of the specification concerning optical bulk absorption filters are given.
This document specifies the optical properties of the filters and the test and measurement methods
whenever necessary.
This document does not specify any material properties (internal quality, homogeneity, etc.) and it does
not apply to any production method.
This document applies to both the raw material (filter glass, filter plastics, etc.) and the polished
component.
NOTE 1 Colorimetric parameters for the description of the filter function are specified in e.g. ISO 11664-1 and
ISO 11664­2.
NOTE 2 For filters where the spectral transmission characteristics are achieved by the application of optical
coatings see ISO 9211 series.
NOTE 3 In case of high power applications further optical effects may occur.
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 9211­1, Optics and photonics — Optical coatings — Part 1: Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 9211-1 and the following
terms and definitions 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 http:// www .electropedia .org/ .
3.1 Boundary conditions
3.1.1
optical surface
optically effective surface, into which the radiation enters the bulk absorption filter or from which it leaves
Note 1 to entry: In general, bulk absorption filters are made as plane parallel plates and have two optical surfaces
which are opposite to each other.
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ISO/FDIS 23364:2021(E)

3.1.2
angle of incidence
Θ
angle between the normal to the optical surface and the incident ray
Note 1 to entry: Unless otherwise specified, the angle of incidence is equal to 0°; this means the incident rays are
normal to the optical surface.
[SOURCE: ISO 9211-1:2018, 3.1.2.6, modified — Note 1 to entry was added.]
3.1.3
thickness
d
geometrical length that the radiation passes through the bulk absorption filter at normal incidence to
the optical surfaces
Note 1 to entry: At normal incidence onto a plano-parallel bulk absorption filter the path of the radiation is equal
to the thickness d of the bulk absorption filter. All parameters and characteristic numbers of this standard are
referenced to the case of normal incidence of the light, when the path and the thickness of the bulk absorption
filter are the same.
Note 2 to entry: For the case of non-normal incidence of the light, the thickness is not equal to the path of the light.
3.1.4
witness sample
sample, which represents the bulk absorption filter component and which is used for spectral
measurements and environmental testing
Note 1 to entry: The details about the witness sample and the measurement (i.e. material, surface condition,
geometry, number per batch) may be subject of the negotiation between manufacturer and customer.
[SOURCE: ISO 9211-1:2018, 3.1.1.6, modified — In the note “sampling procedures” has been replaced by
“measurement”.]
3.2 Optical properties
3.2.1
spectral transmittance
τ(λ)
ratio of the spectral radiant flux transmitted to that of the incident radiant flux
Note 1 to entry: The spectral transmittance is dependent on the internal absorption properties, especially on the
travel path of the light, as well as on the optical properties of the surface.
Φ
eλ ,2
τλ() = (1)
Φ
eλ ,1
where
Φ is the incident spectral radiant flux;
eλ,1
Φ is the transmitted spectral radiant flux.
eλ,2
Note 2 to entry: See Figure 1.
Note 3 to entry: Wherever the Greek letter τ is mistakable T may be used.
Note 4 to entry: If necessary, the transmittance can be represented as an average over a wavelength range from
λ to λ as follows:
1 2
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ISO/FDIS 23364:2021(E)

λ
2
m m
τλ()dλ
∫ τλ()Δλ τλ()
λ ∑ i ∑ i
1 i=1 i=1
τλ() to λ = ≈ == (2)
ave 12
λλ − λλ− m
21 21
where
Δ=λλ  − mλ /
()
21
Note 5 to entry: The subscript “ave” stands for “average”. As an alternative the subscript “avg” may be used
NOTE The light falls onto the optical surface at normal incidence, however, it is sketched at an angle in order
to visualize the surface reflection R. That means, travel path of the light and geometrical thickness d are the same.
Figure 1 — Sketch for depicting the differences between transmittance and internal
transmittance
3.2.2
spectral internal transmittance
τ (λ)
i
ratio of the spectral radiant flux arriving without reflection at the exit surface of the bulk absorption
filter to that of the entered radiation
Note 1 to entry: The internal transmittance describes the properties inside the material, thus surface effects do
not have an influence.
Note 2 to entry: The subscript “i” stands for “internal”.
Φ
eλ ,4
τλ() = (3)
i
Φ
eλ ,3
where
Φ is the spectral radiant flux, which has entered the volume;
eλ,3
Φ
is the spectral radiant flux, which is going to leave the volume.
eλ,4
Note 3 to entry: For bulk absorption filters with a homogeneous distribution of absorption within the material
the following applies:
−α d
l
τ = e (4)
i
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where
α is the absorption coefficient;
l
d is the thickness of the bulk absorption filter.
From this context, the internal transmittance can be calculated for different thicknesses of the bulk
absorption filter (at normal incidence) by:
 d 
1
 
d
ττ =   (5)
2
()
ii,,dd
12
where
τ is the internal transmittance that corresponds to the thickness d ;
1
i,d
1
τ is the internal transmittance that corresponds to the thickness d .
2
i,d
2
Note 4 to entry: See Figure 1.
3.2.3
cut-off wavelength of the internal transmittance
λ
i0,5
wavelength in the transition between a region of high and a region of low transmittance, where the
transmittance has a value of τ = 0,5
i
Note 1 to entry: The subscript “i” stands for “internal”.
3.2.4
cut-off wavelength of the transmittance
λ
0,5
wavelength in the transition between a region of high and a region of low transmittance, where the
transmittance has a value of τ = 0,5
Note 1 to entry: The Annex A of ISO 9211-2 defines λ' and λ’’ as the wavelength where the transmittance is
0,5 0,5
half of τ or τ .
A M
3.2.5
spectral absorptance
a(λ)
ratio of the spectral concentration of radiant flux absorbed to that of the incident radiation
3.2.6
refractive index
n(λ)
ratio of the velocity of propagation of electromagnetic radiation in vacuum to the velocity of propagation
in a medium
[SOURCE: ISO 9211-1]
3.2.7
reflection factor
Ρ(λ)
ratio of spectral transmittance (3.2.1) to internal transmittance
τλ
()
P()λ = (6)
τλ()
i
Note 1 to entry: When neglecting diffuse scattering the following formula applies at normal incidence onto the
bulk absorption filter:
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ISO/FDIS 23364:2021(E)

2n()λ
P()λ = (7)
2
n λ + 1
()
Note 2 to entry: Because the spectral dependency of P(λ) is weak, a constant reflection factor is used in most
cases. Unless otherwise specified, a constant reflection factor is used, which is derived at the d-line (587,6 nm).
2n
d
PP = = P(λ = 586,7 nm) = (8)
d
2
n + 1
d
Note 3 to entry: P is the greek upper case letter Rho.
3.3 Calculated parameters
3.3.1
spectral optical density
D(λ)
logarithm to the base 10 of the reciprocal of the spectral transmittance (3.2.1)
1
D λ = lg (9)
()
τλ()
3.3.2
spectral extinction
E(λ)
logarithm to the base 10 of the reciprocal of the spectral internal transmittance (3.2.2)
α d
1
l
E()λ = lg = (10)
τλ() ln(10)
i
Note 1 to entry: Extinction = Absorbance. Sometimes letter A is used.
3.3.3
spectral diabatic transmittance
θ(λ)
a characterization of the spectral internal transmittance (3.2.2) represented by the following formula:
 
1
θλ() =−1 lg lg (11)
 
τλ
()
 
i
Note 1 to entry: For transformation of spectral internal transmittance and spectral diabatic transmittance see
Annex A.
3.3.4
luminous transmittance
τ
V
ratio of the luminous flux transmitted by an ocular or filter to the incident luminous flux for a specified
illuminant and photopic vision
Note 1 to entry: This is usually expressed as a percentage and is calculated from the following formula:
780 nm
τλ() ⋅⋅SV()λλ () ⋅ dλ
D65
∫
380 nm
τ =×100  (12)
V
780 nm
S ()λ ⋅ VV ()λλ ⋅ d
D65
∫
380 nm
where
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ISO/FDIS 23364:2021(E)

λ is the wavelength of the light in nanometres;
τ(λ) is the spectral transmittance of the ocular or filter;
V(λ) is the spectral luminous efficiency function for photopic vision;
S (λ) is the spectral distribution of incident radiation of CIE Standard Illuminant D65 (see ISO 11664­2).
D65
Note 2 to entry: Although τ is defined as above using the spectral distribution of Standard Illuminant D65, for other
V
purposes, S (λ) can be replaced by the spectral distribution of Standard Illuminant A or the relevant light source.
D65
Note 3 to entry: The values of the spectral radiation distribution of CIE standard illuminant D65, S (λ) are
D65
given in ISO/CIE 11664­2.
Note 4 to entry: The values of the spectral luminous efficiency function V(λ) are given in ISO/CIE 11664­1.
Note 5 to entry: The values for the spectral weighting function, SVλλ ⋅ , are given in Annex C.
() ()
D65
[SOURCE: ISO 4007:2012-05]
3.4 Definition of bulk absorption filters by their function
3.4.1 Attenuating function
3.4.1.1
ND
code for a bulk absorption filter which reduces the transmittance
...