ISO/FDIS 23364

Optics and Photonics -- Bulk absorption optical filters

Optique et photonique -- Filtres optiques à absorption de masse

Standards Content (sample)

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
---------------------- Page: 1 ----------------------
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
---------------------- Page: 3 ----------------------
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

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
---------------------- Page: 5 ----------------------
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.
© ISO 2021 – All rights reserved 1
---------------------- Page: 6 ----------------------
ISO/FDIS 23364:2021(E)
3.1.2
angle of incidence
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

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.
eλ ,2
τλ() = (1)
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
2 © ISO 2021 – All rights reserved
---------------------- Page: 7 ----------------------
ISO/FDIS 23364:2021(E)
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 /

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)

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”.
eλ ,4
τλ() = (3)
eλ ,3
where
Φ is the spectral radiant flux, which has entered the volume;
eλ,3
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
τ = e (4)
© ISO 2021 – All rights reserved 3
---------------------- Page: 8 ----------------------
ISO/FDIS 23364:2021(E)
where
α is the absorption coefficient;
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 
 
ττ =   (5)
ii,,dd
where
τ is the internal transmittance that corresponds to the thickness d ;
i,d
τ is the internal transmittance that corresponds to the thickness d .
i,d
Note 4 to entry: See Figure 1.
3.2.3
cut-off wavelength of the internal transmittance
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
Note 1 to entry: The subscript “i” stands for “internal”.
3.2.4
cut-off wavelength of the transmittance
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
P()λ = (6)
τλ()

Note 1 to entry: When neglecting diffuse scattering the following formula applies at normal incidence onto the

bulk absorption filter:
4 © ISO 2021 – All rights reserved
---------------------- Page: 9 ----------------------
ISO/FDIS 23364:2021(E)
2n()λ
P()λ = (7)
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).

PP = = P(λ = 586,7 nm) = (8)
n + 1
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)
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
E()λ = lg = (10)
τλ() ln(10)
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:

 
θλ() =−1 lg lg (11)
 
 

Note 1 to entry: For transformation of spectral internal transmittance and spectral diabatic transmittance see

Annex A.
3.3.4
luminous transmittance

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)
780 nm
S ()λ ⋅ VV ()λλ ⋅ d
D65
380 nm
where
© ISO 2021 – All rights reserved 5
---------------------- Page: 10 ----------------------
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

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
code for a bulk absorption filter which reduces the transmittance
...

DRAFT INTERNATIONAL STANDARD
ISO/DIS 23364
ISO/TC 172/SC 3 Secretariat: JISC
Voting begins on: Voting terminates on:
2020-06-10 2020-09-02
Optics and Photonics — Bulk absorption optical filters
Optique et photonique —
ICS: 37.020
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENT AND APPROVAL. IT IS
THEREFORE SUBJECT TO CHANGE AND MAY
NOT BE REFERRED TO AS AN INTERNATIONAL
STANDARD UNTIL PUBLISHED AS SUCH.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL,
This document is circulated as received from the committee secretariat.
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
STANDARDS MAY ON OCCASION HAVE TO
BE CONSIDERED IN THE LIGHT OF THEIR
POTENTIAL TO BECOME STANDARDS TO
WHICH REFERENCE MAY BE MADE IN
Reference number
NATIONAL REGULATIONS.
ISO/DIS 23364:2020(E)
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 SUPPORTING DOCUMENTATION. ISO 2020
---------------------- Page: 1 ----------------------
ISO/DIS 23364:2020(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/DIS 23364:2020(E)
Contents Page

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

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

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

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

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

3.1 General ........................................................................................................................................................................................................... 1

3.2 Boundary conditions ......................................................................................................................................................................... 2

3.3 Optical properties ................................................................................................................................................................................. 2

3.4 Calculated parameters ...................................................................................................................................................................... 5

3.5 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, A, D or θ.........................................................................8

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 ..........................................16

© ISO 2020 – All rights reserved iii
---------------------- Page: 3 ----------------------
ISO/DIS 23364:2020(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 03, Optical materials and components.
iv © ISO 2020 – All rights reserved
---------------------- Page: 4 ----------------------
ISO/DIS 23364:2020(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

wavelength, the reciprocal centimetre (cm−1) 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 2020 – All rights reserved v
---------------------- Page: 5 ----------------------
DRAFT INTERNATIONAL STANDARD ISO/DIS 23364:2020(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. The standard specifies the optical properties of the filters and the test and measurement

methods whenever necessary. This standard does not specify any material properties (inner 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 series.

NOTE 2 For filters where the spectral transmission characteristics are achieved by the application of optical

coatings see ISO 9211 series.
2 Normative references

The following referenced documents are indispensable for the application 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
ISO 9211-2, Optics and photonics — Optical coatings — Part 2: Optical properties
ISO 4007, Personal protective equipment — Eye and face protection — Vocabulary
ISO 11664-1, Colorimetry — Part 1: CIE standard colorimetric observers
ISO 11664-2, Colorimetry — Part 2: CIE standard illuminants
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 General

The terms are subdivided into sublcauses, namely “boundary conditions”, “optical properties”,

“calculated parameters” and the “definition of bulk absorption filters by their function”.

© ISO 2020 – All rights reserved 1
---------------------- Page: 6 ----------------------
ISO/DIS 23364:2020(E)
3.2 Boundary conditions
3.2.1
optical surface

surface area, 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.
3.2.2
angle of incidence
angle between the normal to the 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]
3.2.3
thickness

geometrical length that the radiation passes through the bulk absorption filter at normal incidence

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 does not correspond to the path

of the light.
3.2.4
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, amount per batch) may be subject of the negotiation between manufacturer and customer.

[SOURCE: ISO 9211-1, modified -- In the note “sampling procedures” has been replaced by

“measurement”]
3.3 Optical properties
3.3.1
spectral transmittance

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 inner absorption properties, especially on the

travel path of the light, as well as on the optical properties of the surface.
eλ,2
τλ()= (1)
eλ,1
where
is the incident spectral radiant flux;
eλ,1
is the transmitted spectral radiant flux.
eλ,2
Note 2 to entry: See Figure 1.
2 © ISO 2020 – All rights reserved
---------------------- Page: 7 ----------------------
ISO/DIS 23364:2020(E)
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
m m
τλ()dλ
τλ Δλ τλ
∫ () (()
∑∑i i
1 i==1 i 1
τλ() to λ = ≈ =
ave 12
λλ− λλ− m
21 21
Where
Δ=λλ()−λ /m .
3.3.2
spectral internal transmittance
τ (λ)

ratio of the spectral radiant flux leaving 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”.
eλ,4
τλ()= (2)
eλ,3
where
is the spectral radiant flux, which has entered the volume;
eλ,3
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
τ =e (3)
where
α is the absorption coefficient;
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:
 
 
ττ=   (4)
id,,id
Note 4 to entry: See Figure 1.
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NOTE The light falls onto the 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
3.3.3
cut-off wavelength of the internal transmittance
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
Note 1 to entry: The subscript “i” stands for “internal”.
3.3.4
cut-off wavelength of the transmittance
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: ISO 9211-2 defines λ' and λ’’ as the wavelength where the transmittance is half of τ or τ .

0,5 0,5 A M
3.3.5
spectral absorptance
a(λ)

ratio of the spectral concentration of radiant flux absorbed to that of the incident radiation

3.3.6
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]
3.3.7
reflection factor
Ρ(λ)
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ratio of spectral transmittance to internal transmittance
τλ()
P()λ = (5)

Note 1 to entry: When neglecting diffuse scattering the following formula applies at normal incidence onto the

bulk absorption filter:
2n()λ
P()λ = (6)
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 (589 nm).

PP==P()λ=589nm = (7)
n +1
3.4 Calculated parameters
3.4.1
spectral optical density
D(λ)
logarithm to the base 10 of the reciprocal of the spectral transmittance (3.3.1)
D()λ =lg (8)
3.4.2
spectral extinction
E(λ)

logarithm to the base 10 of the reciprocal of the spectral internal transmittance (3.3.2)

Ed()λ =lg =α (9)
Note 1 to entry: Extinction = Absorbance. Sometimes letter A is used.
3.4.3
spectral diabatic transmittance
θ(λ)

diabatie is a characterization of the spectral transmittance represented by the following equation:

 
θλ()=−1 lg lg (10)
 
τλ()
 i 

Note 1 to entry: For transformation of spectral transmittance and spectral diabatic transmittance see Annex A.

3.4.4
luminous transmittance

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 equation:

780
τλ()⋅SV()λλ⋅ ()⋅dλ
D65
380
τ =×100 (11)
780
SV()λλ⋅ ()⋅dλ
D65
380
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where
λ is the wavelength of the light in nanometres;
τ(λ) is the spectral transmittance of the ocular or filter;
...