ISO/CIE 23539:2023

Photometry — The CIE system of physical photometry

This document specifies the characteristics of the system of physical photometry established by the CIE and accepted as the basis for the measurement of light. It defines the photometric quantities, units and standards that make up the CIE system of physical photometry and that have been officially accepted by the Comité International des Poids et Mesures (CIPM). This comprises: — the definition of photometric quantities, symbols and units; — the definition of CIE spectral luminous efficiency functions for photopic vision, scotopic vision, mesopic vision and 10° photopic vision; — the definition of CIE photometric observers that conforms to these functions; — the definition of maximum luminous efficacy for photopic vision, mesopic vision, scotopic vision and 10° photopic vision.

Photométrie — Le système CIE de photométrie physique

L'ISO 23539:2005 spécifie les caractéristiques du système de photométrie physique établi par la CIE et accepté comme fondement pour la mesure de la lumière. Elle définit les grandeurs photométriques, les unités et les normes qui constituent le système CIE de photométrie physique et qui ont été officiellement acceptées par le Comité international des poids et mesures (CIPM).

General Information

• Status: Published
• Publication Date: 02-Mar-2023
• ICS: 17.180.01 - Optics and optical measurements in general
• Technical Committee: CIE - International Commission on Illumination
• Drafting Committee: CIE - International Commission on Illumination
• Parallel Committee: ISO/TC 274 - Light and lighting
• Current Stage: 6060 - International Standard published
• Start Date: 03-Mar-2023
• Due Date: 19-Dec-2022
• Completion Date: 03-Mar-2023

Relations

• Revises: ISO 23539:2005 - Photometry — The CIE system of physical photometry
• Effective Date: 11-Dec-2021

Standards Content (Sample)

INTERNATIONAL ISO/CIE
STANDARD 23539
First edition
2023-03
Photometry — The CIE system of
physical photometry
Photométrie — Le système CIE de photométrie physique
Reference number
ISO/CIE 23539:2023(E)
© ISO/CIE 2023
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ISO/CIE 23539:2023(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO/CIE 2023

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Published in Switzerland
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ISO/CIE 23539:2023(E)
Contents Page

Foreword ..........................................................................................................................................................................................................................................v

Introduction .............................................................................................................................................................................................................................. vi

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

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

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

4 Photometric quantities and units ...................................................................................................................................................... 2

4.1 Photometric quantities ................................................................................................................................................................... 2

4.2 Photometric units ................................................................................................................................................................................ 3

5 CIE standard spectral luminous efficiency functions .................................................................................................. 3

5.1 General ........................................................................................................................................................................................................... 3

5.2 Photopic vision ....................................................................................................................................................................................... 3

5.3 Scotopic vision ........................................................................................................................................................................................ 4

5.4 Mesopic vision ......................................................................................................................................................................................... 4

5.5 10° photopic vision ............................................................................................................................................................................. 5

6 Names, symbols and units for photometric quantities ............................................................................................... 5

6.1 General ........................................................................................................................................................................................................... 5

6.2 Photopic vision ....................................................................................................................................................................................... 6

6.3 Scotopic vision ........................................................................................................................................................................................ 6

6.4 Mesopic vision ......................................................................................................................................................................................... 6

6.5 10° Photopic vision ............................................................................................................................................................................. 7

6.6 Photometric quantities for other observers ................................................................................................................. 7

7 Basic formulae relating photometric quantities to radiometric quantities ........................................7

7.1 General ........................................................................................................................................................................................................... 7

7.2 General formula ..................................................................................................................................................................................... 7

7.3 General formula for luminous flux........................................................................................................................................ 8

7.4 Maximum luminous efficacy ...................................................................................................................................................... 8

7.4.1 General ........................................................................................................................................................................................ 8

7.4.2 Photopic vision ..................................................................................................................................................................... 9

7.4.3 Scotopic vision ...................................................................................................................................................................... 9

7.4.4 Mesopic vision ...................................................................................................................................................................... 9

7.4.5 10° photopic vision ........................................................................................................................................................... 9

7.4.6 Summary of maximum luminous efficacies ............................................................................................ 10

7.5 (Photopic) luminous flux ............................................................................................................................................................ 10

7.6 Scotopic luminous flux ................................................................................................................................................................. 10

7.7 Mesopic luminous flux .................................................................................................................................................................. 11

7.8 10° photopic luminous flux.......................................................................................................................................................12

8 Physical measurement ...............................................................................................................................................................................12

8.1 General ........................................................................................................................................................................................................12

8.2 Photometers ...........................................................................................................................................................................................13

8.3 Spectroradiometers ........................................................................................................................................................................13

8.3.1 Spectral measurement ...............................................................................................................................................13

8.3.2 Spectral calculations ................................................................................................................................................... 13

9 Tables of values of spectral luminous efficiency functions ................................................................................14

9.1 Photopic vision .................................................................................................................................................................................... 14

9.2 Scotopic vision ..................................................................................................................................................................................... 18

9.3 10° photopic vision .......................................................................................................................................................................... 21

Annex A (informative) Example of a spectral luminous efficiency function for mesopic

vision .............................................................................................................................................................................................................................25

Annex B (informative) Supplementary information on mesopic vision .....................................................................29

iii
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Annex C (informative) Background of the CIE system of physical photometry ..................................................30

Annex D (informative) Guidance on valid description of photometric values .....................................................32

Annex E (informative) Cone-fundamental-based spectral luminous efficiency functions ....................33

Bibliography .............................................................................................................................................................................................................................43

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ISO/CIE 23539:2023(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 the International Commission on Illumination (CIE) in cooperation

with Technical Committee ISO/TC 274, Light and lighting.

This first edition of ISO/CIE 23539 cancels and replaces ISO 23539:2005/CIE S 010:2004, which has

been technically revised.
The main changes are as follows:

— The scope of the document has changed to incorporate the spectral luminous efficiency functions

published by the CIE for a) mesopic vision and b) 10° photopic vision, on the basis of CIE 018:2019.

— The International System of Units (SI) and its reformulation of the definition of the candela –

effective on 20 May 2019 – has been incorporated (Resolution 1, 26th CGPM, 2018).

— A list of normative references has been added.

— Specific requirements have been added regarding the use of units, tabulated values and interpolation

of intermediate values.

— The background of the CIE system of physical photometry, specifically the evolution of the

photometric base unit, has been updated in Annex C.

— The CIE 2015 cone-fundamental-based spectral luminous efficiency functions for a) 2° field size and

b) 10° field size have been added in Annex E based on CIE 170-2:2015.

Any feedback or questions on this document should be directed to the CIE Central Bureau or 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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Introduction

The purpose of photometry is to measure light as perceived by human eyes. The brightness of a

luminous surface depends not only on the amount of radiation it emits, transmits or reflects, but also

on its spectral composition and on the visual response function of the observer viewing it. Because

human visual response varies at different light levels and from person to person, precise photometry

requires the definition of representative standard observers. The CIE system of physical photometry

specifies procedures for the quantitative evaluation of optical radiation in terms of internationally

agreed spectral luminous efficiency functions for human vision. V(λ) represents photopic vision, V'(λ)

represents scotopic vision and V (λ) represents mesopic vision, the latter being intermediate

mes;m

between photopic and scotopic vision. Furthermore, V (λ) represents 10° photopic vision. These

[1] [2]

luminous efficiency functions adopted from CIE 018:2019 and BIPM-2019/05, together with the

SI base unit, the candela, constitute a system that enables the calculation of values of photometric

quantities for optical radiation as well as light-emitting, light-transmitting or light-reflecting surfaces,

to be precisely determined based on the International System of Units (SI), regardless of the spectral

composition of the radiation emitted, transmitted or reflected.

The CIE system of physical photometry has some limitations in respect to the brightness of coloured

surfaces: two light sources of different colour but with the same measured luminance value will

not necessarily be perceived as equally bright. CIE has therefore published a more complex model

[3]

(CIE 200:2011) for specific situations. For eye-mediated non-image-forming effects of light

induced partially or completely by the intrinsically photosensitive retinal ganglion cells (ipRGCs),

[4]
CIE S 026/E:2018 is used.
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INTERNATIONAL STANDARD ISO/CIE 23539:2023(E)
Photometry — The CIE system of physical photometry
1 Scope

This document specifies the characteristics of the system of physical photometry established by the CIE

and accepted as the basis for the measurement of light. It defines the photometric quantities, units and

standards that make up the CIE system of physical photometry and that have been officially accepted

by the Comité International des Poids et Mesures (CIPM). This comprises:
— the definition of photometric quantities, symbols and units;

— the definition of CIE spectral luminous efficiency functions for photopic vision, scotopic vision,

mesopic vision and 10° photopic vision;
— the definition of CIE photometric observers that conforms to these functions;

— the definition of maximum luminous efficacy for photopic vision, mesopic vision, scotopic vision

and 10° photopic vision.
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.

CIE S 017, ILV: International Lighting Vocabulary
3 Terms and definitions

For the purposes of this document, the terms and definitions given in CIE S 017 and the following apply.

ISO and IEC maintain terminology databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/

CIE maintains a terminology database for use in standardization at the following address:

— CIE e-ILV: available at https:// cie .co .at/ e -ilv
3.1
CIE photometric observer
CIE observer

ideal observer having a relative spectral responsivity that conforms to a CIE-defined spectral luminous

efficiency function for human vision and that complies with the summation law implied in the definition

of luminous flux

Note 1 to entry: CIE has defined spectral luminous efficiency functions for photopic vision, V(λ), and scotopic

vision, V'(λ), which are CIE standard photometric observer(s). Furthermore, CIE has defined CIE photometric

observers for mesopic vision, V (λ), and 10° photopic vision, V (λ), as well as published definitions of cone-

mes;m 10
[5]
fundamental-based spectral luminous efficiency functions .

Note 2 to entry: CIE photometric observers are distinct from CIE standard photometric observers, which only

include spectral luminous efficiency functions for photopic vision, V(λ), and scotopic vision, V'(λ).

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Note 3 to entry: Other spectral luminous efficiency functions defined in this document are also intended to define

CIE photometric observers.

Note 4 to entry: Other spectral luminous efficiency functions will possibly be included as CIE photometric

observers. However, only the functions tabled in Clause 8 relate the given spectral radiometric quantity to the

International System of Units (SI).
3.2
10° photopic vision
10° vision
photopic vision based on the CIE 10° photopic photometric observer

Note 1 to entry: 10° photopic vision corresponds to vision by the normal eye in situations where the visual target

has an angular subtense larger than 4° or is seen off-axis.
4 Photometric quantities and units
4.1 Photometric quantities
The most commonly used photometric quantities are:
— luminous flux;
— luminous intensity;
— luminance;
— illuminance.

These quantities are defined in CIE S 017 and their definitions, adopted by the Consultative Committee

for Photometry and Radiometry (CCPR) of the International Committee for Weights and Measures

[6]

(CIPM), are referred to in this clause. After the redefinition of the SI units in 2019, the definition of

the unit of candela is tied to the definitions of the SI units kilogram, second and metre. It is important to

note that the 2019 reformulation of the definition of the candela does not make a numerical difference

in the calculations of the photometric quantities.

To avoid confusion, photometric quantities are distinguished symbolically from their radiometric

equivalents by the subscript “v”, whereas radiometric quantities receive the subscript “e”. The same

subscripts are also applicable to photometric and radiometric quantities other than those listed (e.g.

luminous exposure, radiant exposure). For photometric quantities using spectral luminous efficiency

functions other than that for 2° photopic vision, V(λ), the quantity names and symbols described in

Clause 6 are used.

For many practical purposes the simplest physical quantity used in optical radiometry is the radiant

flux or radiant power, Φ , measured in watts (W), which is emitted by a source of radiation, transmitted

by a medium of propagation or received at a surface. The corresponding photometric quantity is the

luminous flux, Φ , measured in lumen (lm), derived from radiant flux, Φ , by evaluating the radiation

v e

according to its action upon a CIE standard photometric observer or CIE photometric observer. In this

document the relation between radiometric quantities and photometric quantities is shown by example

using the relation between the radiant flux and the (photopic) luminous flux using the CIE standard

photometric observer for photopic vision, as well as other CIE photometric observers. The general

relation between a given luminous flux, Φ , for a specific photometric condition, X, and the spectral

v,X
radiant flux Φ (λ) is given in 7.3, with specific observers shown in 7.5 to 7.8.
e,λ

The most commonly used photometric quantities given in this subclause, as well as others such as

luminous exitance and luminous exposure, are defined in CIE S 017. All photometric quantities can be

formulated in terms of luminous flux and appropriate geometric factors. The defining relationships for

other photopic, scotopic and mesopic photometric quantities are formed from the formulae in 7.5 to

7.8 by replacing the symbols for radiant flux and luminous flux with the appropriate radiometric and

photometric symbols.
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Where a particular spectral luminous efficiency function is not specified, the photopic condition is

implied.
4.2 Photometric units

The General Conference on Weights and Measures (CGPM) has fundamentally revised the SI to be based

[6]

on seven defining constants. In particular, the luminous efficacy of a monochromatic radiation of

12 −1

frequency 540 × 10 Hz, K , is introduced and its value is set to 683 lm·W . This constant relates

the photometric units (lm, cd and lx) directly to the corresponding radiometric units (W, W·sr and

W·m ).

As a consequence, the SI unit of luminous intensity of a source in a given direction, the candela, symbol

cd, is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of

12 −1 −1

frequency 540 × 10 Hz, K , to be 683 when expressed in the unit lm·W , which is equal to cd·sr·W ,

−1 −2 3

or cd·sr·kg ·m ·s , where the kilogram, metre and second are defined in terms of h, c and Δv .

This definition of the candela applies equally to any photometric condition (photopic vision, scotopic

vision, mesopic vision and 10° photopic vision). The evolution of the photometric units is found in

Annex C.
5 CIE standard spectral luminous efficiency functions
5.1 General

Photometric quantities are related to radiometric quantities through internationally agreed spectral

weighting functions defined by the CIE as “spectral luminous efficiency functions”. These spectral

luminous efficiency functions provide representations of the relative spectral sensitivity of the

human visual system under defined conditions and are normalized to unity at the wavelength of peak

sensitivity. The relevant spectral luminous efficiency function is applied as a spectral weighting to the

spectral distribution of the corresponding radiometric quantity in order to calculate the corresponding

photometric quantity (see Clause 7 for further details).
The most common spectral luminous efficiency functions are:
— V(λ): photopic luminous efficiency function;
— V'(λ): scotopic luminous efficiency function;
— V (λ): mesopic luminous efficiency function;
mes;m
— V (λ): 10° photopic efficiency function.
These are described in 5.2 to 5.5 and given as tabled values in Clause 9.

This document defines the spectral luminous efficiency functions for photopic, scotopic, mesopic and

10° photopic photometric conditions. These functions shall be used in the determination of photometric

quantities when the corresponding condition is met.

Outside the specified spectral range, all values of the luminous efficiency functions shall be set to zero.

See also 8.3.2.
5.2 Photopic vision

The spectral luminous efficiency function V(λ) applies to photopic vision and shall be used for

determination of photometric quantities at luminance levels above 5 cd⋅m . It is important to note that

the V(λ) function applies at all luminance levels for foveal vision or for all on-axis visual tasks (objects

seen by the eye are in a narrow field of view in central vision i.e. ≤4 ). It is defined by the numerical

[7]

values given in Table 6, the wavelength being measured in standard air. For numerical computations,

the peak value of the V(λ) function shall be evaluated at 555 nm exactly. For calculation purposes, linear

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interpolation shall be used exclusively to evaluate V(λ) at wavelengths intermediate to those given in

Table 6.
5.3 Scotopic vision

The spectral luminous efficiency function V'(λ) applies to scotopic vision and shall be used for

determination of photometric quantities in situations where the eye is adapted to average luminance

levels less than 0,005 cd⋅m . This function is defined by the numerical values given in Table 7, the

wavelength λ being measured in standard air. For numerical computations, the peak value of the V'(λ)

function shall be evaluated at 507 nm exactly. For calculation purposes, linear interpolation shall be

used exclusively to evaluate V'(λ) at wavelengths intermediate to those given in Table 7.

5.4 Mesopic vision

The spectral luminous efficiency function V (λ) applies to mesopic vision under given adaptation

mes;m

conditions and shall be used for determination of photometric quantities at luminance levels in the

intermediate range between photopic and scotopic vision. The procedure to calculate V (λ) for a

mes;m

given adaptation coefficient, m, is given in this subclause and the calculation of the mesopic luminous

flux is given in 7.7 as an example.

The spectral luminous efficiency function for mesopic vision is denoted by V (λ) and is defined

mes;m
according to Formula (1):
V ()λλ=+mV () ()10−mV ()λ for ≤≤m 1 (1)
mes;m
Mm()
where

m is the adaptation coefficient, the value of which depends on the visual adaptation conditions

(see 7.7);
M(m) is a normalizing function such that V (λ) attains a maximum value of 1.
mes;m

Figure 1 shows the curves of the mesopic spectral luminous efficiency function V (λ) for m = 0,2;

mes;m

0,4; 0,6; 0,8 as examples, plotted with V(λ) and V'(λ). Table A.1 in Annex A shows the values of V (λ)

mes;m

for m = 0,8 as an example, which corresponds to the visual adaptation condition for a typical road

lighting luminance level (≈ 1 cd·m ), considering Formulae (12) and (13).
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Key
λ wavelength in nm
V (λ) spectral luminous efficiency for mesopic vision
mes;m

Figure 1 — The spectral luminous efficiency for mesopic vision, V (λ), at m = 0,2; 0,4; 0,6;

mes;m
0,8 as examples, plotted with V(λ) and V'(λ)
5.5 10° photopic vision

The spectral luminous efficiency function V (λ) applies for 10° photopic vision and shall be used for

measurements in situations where the visual target has an angular subtense larger than 4° or is seen

off-axis. The values of V (λ) are given in Table 8.
6 Names, symbols and units for photometric quantities
6.1 General

This clause describes the relations between the names, symbols and units for photometric quantities

used in the CIE system of physical photometry.

The SI defines the photometric units through the introduction of the constant K = 683 lm·W , i.e. the

luminous efficacy for monochromatic radiation of frequency 540 × 10 Hz. Thus, by construction the

unit of the photometric quantity “luminous flux” is lumen and therefore independent of the photometric

condition (i.e. photopic, scotopic, mesopic, 10° photopic), see also Formula (2). The independence of the

unit from the ob
...