ISO 20589:2025

International
Standard
ISO 20589
First edition
Glass in building — Determination
2025-08
of the emissivity
Reference number
© ISO 2025
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Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols . 2
5 Brief outline of the procedure to determine corrected emissivity . 3
6 Spectral normal reflectance measurements and calculations . 3
6.1 Specimen preparation .3
6.2 Spectral normal reflectance measurements .4
6.2.1 General .4
6.2.2 Test apparatus .4
6.2.3 Measurement .4
6.2.4 Accuracy .5
6.3 Interpolation.5
6.4 Determination of normal reflectance .5
6.4.1 General .5
6.4.2 Calculation method .5
6.4.3 Noise criterion .5
7 Calculation of total normal emissivity and corrected emissivity . 6
7.1 Total normal emissivity . .6
7.2 Corrected emissivity .6
8 Test report . 6
Annex A (normative) Table for determining total normal reflectance . 8
Annex B (informative) Procedures to improve the accuracy of spectral normal reflectance
measurements . 9
Annex C (informative) Transmittance and diffuse reflectance measurements and calculation
of total normal transmittance .11
Annex D (informative) Determination of absolute reflectance by comparing the energy of the
beam reflected from the specimen to that of the incident beam .12
Bibliography .16

iii
Foreword
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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
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This document was prepared by the European Committee for Standardization (CEN) (as EN 12898:2019)
and was adopted, without modification other than those given below by Technical Committee ISO/TC 160,
Glass in building.
— informative parts of the Scope have been moved to the Introduction;
— “this European Standard” was changed to “this document”;
— references to EN standards have been changed to references to the corresponding ISO standards;
— clarifications have been made to Figure D.1, Figure D.2, Figure D.3 and Figure D.4, and their keys;
— the term “sample” has been changed to “specimen”.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
Introduction
This document is based on the use of reflectance measurements using double beam dispersive infrared
spectrometers capable of measuring over almost the entire spectral range of a black body standard reference
[1]
temperature and determining the emissivity by the 30 ordinate method. It also takes account of Fourier
Transform Infrared (FTIR) spectrophotometers where the spectral range is limited. It describes a method
th
whereby spectrophotometers can be used to determine emissivity if they are able to measure up to the 24
ordinate point and if they satisfy a noise criterion for this spectral range. It allows the inclusion of data from
th th
the 25 ordinate point up to the 30 ordinate point.
As FTIR spectrophotometers are single beam instruments as opposed to dispersive spectrophotometers
which are double beam instruments (and thus able to correct for instrument drift), a procedure was
[2]
developed by the European funded project, THERMES, to correct for drift. This procedure is described in
[3] [4]
and . Other categories of ordinate errors using FTIR spectrophotometers are discussed in .

v
International Standard ISO 20589:2025(en)
Glass in building — Determination of the emissivity
1 Scope
This document specifies a procedure for determining the emissivity at room temperature of the surfaces of
glass and coated glass.
The emissivity is necessary for taking into account heat transfer by radiation from surfaces at the standard
temperature of 283 K in the determination of the U value and of the total solar transmittance of glazing
[5] [6] [7] [8]
according to ISO 9050 , ISO 10291 , ISO 10292 , and ISO 10293 .
The procedure, being based on spectrophotometric specular reflectance measurements at near normal
incidence on materials that are non-transparent in the infrared region, is not applicable to glazing
components with at least one of the following characteristics:
a) with rough or structured surfaces where the incident radiation is diffusely reflected;
b) with curved surfaces where the incident radiation is regularly reflected at angles unsuitable to reach
the detector while using specular reflectance accessories;
c) infrared transparent.
However, it can be applied with caution to any glazing component provided its surfaces are flat and non-
diffusing (see non-diffusing glazing component) and it is non-transparent in the infrared region (see glazing
component non-transparent in the infrared region).
Although transmittance measurements are included in this document, they are only necessary to check if the
specimen is non-transparent in the infrared region in the context of this document (see glazing component
non-transparent in the infrared region). If the specimen is transparent in the infrared region, this document
is not applicable.
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 10292, Glass in building — Calculation of steady-state U values (thermal transmittance) of multiple glazing
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at http:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
infrared
5 µm to 50 µm spectral range
3.2
emissivity
ratio of the energy emitted by a given surface at a given temperature to that of a perfect emitter (black body
with normal and corrected emissivity = 1,0) at the same temperature
Note 1 to entry: Two different definitions of emissivity should theoretically be used to describe radiation exchange
between:
a) glass surfaces facing each other in multiple glazing (effective emissivity);
b) a glass surface facing a room (hemispherical emissivity).
[9]
However, in practice numerical differences were found to be negligibly small. Thus, corrected emissivity is used to
describe both types of heat exchange with a close approximation.
3.3
specular reflectance
regular reflectance
reflectance according to the laws of geometrical optics, without the diffuse component
Note 1 to entry: The measurement arrangement should be such that the instrument beam reaches the detector after
being specularly reflected on the surface of the specimen (reference mirror) at an angle of incidence ≤ 10°.
3.4
diffuse reflectance
reflectance not containing any regular component, due to rough surfaces and/or transparent materials
containing inhomogeneous particles
3.5
total reflectance
sum of specular reflectance (3.3) and diffuse reflectance (3.4)
3.6
non-diffusing glazing component
glazing component with a diffuse reflectance (3.4) ≤ 0,05, measured at the near infrared (3.1) wavelength of 2 µm
Note 1 to entry: The purpose of this measurement is to ensure that the specimen is non-diffusing in the measurement
range. Most integrating spheres sold with visible/near infrared spectrophotometers have a port designed to measure
diffuse reflectance. Diffuse reflectance measurements in the infrared range are difficult to perform.
Note 2 to entry: See Annex C.
3.7
glazing component non-transparent in the infrared region
glazing component with a total normal transmittance ≤ 0,05 at 283 K, measured spectrophotometrically
4 Symbols
For the purposes of this document, the following symbols apply.
ε total corrected emissivity at 283 K
ε total normal emissivity at 283 K
n
E reading of the spectrophotometer with the specimen placed on the specimen support of the re-
flectance accessory
E instrument reading without placing anything on the specimen support
E instrument reading with the reference mirror replacing the specimen
st
R total normal reflectance at 283 K
n
R (λ ) spectral normal reflectance at wavelength λ
n i i
R (λ ) spectral normal reflectance of the reference mirror at wavelength λ
n,st i i
T (λ ) spectral normal transmittance at wavelength λ
n i i
T total normal transmittance at 283 K
n
N number of measurement points to determine total normal reflectance
5 Brief outline of the procedure to determine corrected emissivity
The procedure for determining the corrected emissivity of coated glass surfaces includes the following steps:
a) the spectral specular reflectance of a glazing component non-transparent in the infrared region at
near normal incidence, R (λ ), shall be determined with an infrared spectrophotometer in the range
n i
(5 to 50) µm (see Clause 6);
b) total normal reflectance at 283 K, R , shall be calculated using the integration procedure specified in 6.4
n
from the corresponding spectral reflectance values measured according to step a);
c) total normal emissivity, ε , shall be calculated from the total normal reflectance as specified in Clause 7;
n
d) the corresponding corrected emissivity, ε, shall be determined from the normal emissivity in accordance
with Formula (6).
NOTE 1 The corrected emissivity, calculated from the normal emissivity with the help of a multiplicative correction
factor, takes into account the effect of the angular distribution of emissivity in the heat transfer calculations of glazing
[5] [7] [6] [8]
according to ISO 9050 , ISO 10292 , ISO 10291 and ISO 10293 .
NOTE 2 Both the normal and the corrected emissivity are total emissivities at 283 K, i.e. they are integrated over
[1]
the relevant spectral range using as a weighting function Planck's radiation function for a black body at 283 K.
For uncoated soda lime silicate glass surfaces or for soda lime silicate glass surfaces with coatings which
[5]
have no effects on the emissivity, the normal emissivity to be used in the calculations specified in ISO 9050
[8] [10]
to ISO 10293 shall be 0,89 . For all other glazing materials or components it shall be measured.
NOTE 3 With reasonable confidence ε = 0,89 can be used for uncoated borosilicate glass, glass ceramics, alkaline
n
[10]
earth silicate glass and alumino silicate glass.
NOTE 4 For temperatures included in the range 253 K to 313 K emissivity is not strongly dependent on the
[10][11]
temperature.
6 Spectral normal reflectance measurements and calculations
6.1 Specimen preparation
Specimens shall be of a size suitable for being inserted into the specimen compartment or placed on the
reflectance accessory. In doing so, care shall be taken to ensure that the portion of the coated surface probed
by the instrument beam is free of damage or any surface contamination. The procedures recommended by
the producer for storing the specimens and cleaning their surfaces shall be followed.
The specimen shall be supported in a suitable way to ensure that the measuring spot during transmittance
and reflectance measurements falls on a flat part.

6.2 Spectral normal reflectance measurements
6.2.1 General
The spectral specular reflectance curve of the specimen at near normal incidence between 5 µm and 50 µm
shall be determined with an infrared spectrophotometer equipped with a specular reflectance accessory at
near normal incidence.
6.2.2 Test apparatus
The following equipment shall be used for the measurements:
— A spectrophotometer covering the spectral range 5 µm to 50 µm. Alternatively, a spectrophotometer
that covers the spectral range from 5 µm to a wavelength less than 50 µm can be used as long as it is
th
capable of measuring up to the 24 point (23,3 µm) and satisfying the noise criterion (see 6.4.3) for all
th
points up to the 24 point inclusive.
[1][11][12]
— A reference mirror (free of surface scratches and contamination ) whose spectral specular
reflectance at near normal incidence R (λ) shall be traceable to a standard material from a metrological
n,st
[13]
laboratory.
— A specular reflectance accessory consisting of a suitable array of mirrors and a specimen support. When
the accessory is placed in the specimen compartment of the spectrophotometer and the specimen (or
reference mirror) placed on the specimen support, the instrument beam reaches the detector after being
specularly reflected on the surface of the specimen (reference mirror) at an angle of incidence ≤10°.
As an alternative, Annex D provides a method for determining absolute reflectance by comparing the energy
of the beam reflected from the specimen to that of the incident beam. However, these accessories can be
difficult to align and should be used with caution.
6.2.3 Measurement
The spectral specular reflectance curve of the specimen at near normal incidence shall be determined with
the relative method. The following measurements are required to determine the spectral normal reflectance
of the specimen R (λ ) at each wavelength λ reported in Table A.1 of Annex A:
n i i
— E (the instrument reading with the specimen placed on the specimen support of the reflectance
accessory);
— E (the instrument reading with the standard reference mirror replacing the specimen);
st
— E (the instrument reading without placing anything on the specimen support).
These wavelengths are selected at the centre of equal energy wavelength intervals of Planck’s radiation
[1]
function at 283 K .
Measurements shall be taken at each wavelength λ reported in Table A.1 over the wavelength range for
i
which the spectrophotometer is capable.
At each wavelength λ the specimen normal
...