IEC 62862-3-5:2026

IEC 62862-3-5 ®
Edition 1.0 2026-09
INTERNATIONAL
STANDARD
Solar thermal electric plants -
Part 3-5: Laboratory reflectance measurement of solar reflectors
ICS 27.160  ISBN 978-2-8327-1472-0

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CONTENTS
FOREWORD . 2
1 Scope . 4
2 Normative references . 4
3 Terms and definitions . 4
3.1 Solar definitions . 4
3.2 Collector components . 4
3.3 Angle definitions . 5
3.4 Optical properties . 5
3.5 Figures and tables . 9
4 Preparation of measurement . 10
4.1 Sampling . 10
4.2 Sample preparation . 11
4.3 Reference coupons . 11
4.4 Instrumentation . 11
5 Measurement method . 12
5.1 General . 12
5.2 Spectral near-normal sun-conic near-specular reflectance . 12
5.3 Spectral near-normal hemispherical reflectance . 13
6 Weighting calculation. 13
6.1 General . 13
6.2 Solar-weighted reflectance . 13
6.3 UV-weighted reflectance . 13
6.4 Calculation equation . 14
7 Uncertainty of the measurement . 14
8 Reporting . 15
8.1 General . 15
8.2 General information . 15
8.3 Methodology . 15
8.4 Results . 16
8.4.1 Uncertainty . 16
8.4.2 Standard method . 16
8.4.3 Simplified method . 16
8.4.4 Indirect method . 17
8.5 Conclusions . 17
Annex A (informative) Simplified method for reflectors with high specularity . 18
Annex B (informative) Indirect method . 19
Bibliography . 20

Figure 1 – Angles of incidence, divergence, reflection and acceptance . 9

Table 1 – Different cases related to reflectance nomenclature. . 9
Table 2 – Range and weight of the different UV classes, calculated with the full data
set from IEC 60904-3 . 14
Table 3 – Wavelength range, depending on the type of the desired average . 14

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
Solar thermal electric plants -
Part 3-5: Laboratory reflectance measurement of solar reflectors

FOREWORD
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IEC 62862-3-5 has been prepared by IEC technical committee TC 117: Solar thermal electric
plants. It is an International Standard.
The text of this International Standard is based on the following documents:
Draft Report on voting
117/253/FDIS 117/255/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.
A list of all parts in the IEC 62862 series, published under the general title Solar thermal electric
plants, can be found on the IEC website.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
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stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
– reconfirmed,
– withdrawn, or
– revised.
1 Scope
This document specifies laboratory methods to measure reflectance of all types of reflectors for
use in concentrating solar thermal (CST) plants.
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.
IEC 60904-3, Photovoltaic devices - Part 3: Measurement principles for terrestrial photovoltaic
(PV) solar devices with reference spectral irradiance data
IEC TS 62862-1-1, Solar thermal electric plants - Part 1-1: Terminology
ISO/IEC Guide 98-3:2008, Uncertainty of measurement — Part 3: Guide to the expression of
uncertainty in measurement (GUM:1995)
ISO 4628-2:2016, Paints and varnishes — Evaluation of degradation of coatings — Designation
of quantity and size of defects, and of intensity of uniform changes in appearance — Part 2:
Assessment of degree of blistering
ISO 9488:2022, Solar energy — Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 9488:2022, IEC TS
62862-1-1 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
– IEC Electropedia: available at https://www.electropedia.org/
– ISO Online browsing platform: available at https://www.iso.org/obp
3.1 Solar definitions
3.1.1
direct solar radiation
G
b
solar radiation incident on a given plane, and originating from a small solid angle centred on
the sun's disk
[SOURCE: ISO 9488:2022 [1], 3.2.17.]
3.2 Collector components
3.2.1
reflector
optical device made of several materials, suitably chosen and arranged to make a surface with
proper reflectance (3.4.1) features
Note 1 to entry: The term "mirror" is typically used as a synonym of reflector.
3.3 Angle definitions
3.3.1
angle of incidence
incidence angle
incident angle
θ
i
angle between the line connecting the center of the light source (solar disc or the light source
of the measurement equipment) to a point on an irradiated surface, and the normal to that
surface at the same point
Note 1 to entry: Unit: the non-SI unit is degrees.
Note 2 to entry: SEE: Figure 1.
[SOURCE: IEC TS 62862-1-1:2018 [2], 3.2.3, modified - The subscript "i" is added to the symbol
and the type of light source is broadened.]
3.3.2
divergent angle
angle of divergence
φ
i
half-apex angle of the cone that encloses the bundle of light rays illuminating a specific point
on the reflector (3.2.1)
Note 1 to entry: Unit: the non-SI unit is mrad.
Note 2 to entry: SEE: Figure 1.
3.3.3
angle of reflection
reflection angle
θ
r
angle between the normal to the irradiated surface at a given point and the reflected ray
Note 1 to entry: SEE: Figure 1.
Note 2 to entry: For specular reflection, the incidence ray, normal and reflected ray lie in the same plane, and the
( )
angle of reflection equals the angle of incidence (3.3.1) 𝜃𝜃 =𝜃𝜃 .
𝑟𝑟 𝑖𝑖
Note 3 to entry: Owing to the divergence of solar radiation and surface irregularities, the reflected radiation can be
distributed within a cone centered around the specular reflection direction.
3.3.4
angle of acceptance
acceptance angle
φ
half-apex angle of the cone containing the rays reflected at a given point of the reflector (3.2.1)
and intercepted by the receiving surface (detector of measurement device or receiver of CST
system)
Note 1 to entry: Unit: the non-SI unit is mrad.
Note 2 to entry: SEE: Figure 1.
Note 3 to entry: This angle depends on the geometry (size, distance, etc.) of the reflector (3.2.1) and the receiving
surface.
3.4 Optical properties
3.4.1
reflectance
ρ(P , P , P , P , P )
λ θi φi φ Ts
ratio of the radiant flux reflected from a surface to that of the incident radiation
Note 1 to entry: The reflectance is a dimensionless variable ranging from 0 to 1.
Note 2 to entry: The reflectance depends on five parameters, named as P and a subscript which refers to the
specific parameter influencing the reflectance. These parameters are P , P , P , P and P , and shall be indicated
λ θi φi φ Ts
in parentheses after ρ. P refers to the influence of the wavelength, λ. Four types of reflectance are defined in relation
λ
to P (see spectral reflectance (3.4.2) to UV reflectance (3.4.5)). P refers to the angle of the incident light source,
λ θi
θ. One special case is defined in relation to P (see near-normal reflectance (3.4.6)); in other cases, the specific
i θi
value of θ shall be indicated. P refers to the divergent angle (3.3.2) of the incident radiation, φ . One special case
i φi i
is defined in relation to P (see sun-conic reflectance (3.4.8)); in other cases, the specific value of φ shall be indicated
φi i
(see conic reflectance (3.4.7)). P refers to the dependence of the reflectance on angle of acceptance (3.3.4)
φ
considered, φ. One special case (see specular reflectance (3.4.9)) and three types of reflectance (see near-specular
reflectance (3.4.10) to diffuse reflectance (3.4.12)) are defined in relation to P . P refers to the influence of the
φ Ts
temperature of the surface in which the radiation is falling on, T . Table 1 summarizes the different cases.
s
SEE: Table 1.
Note 3 to entry: Unless otherwise specified, the temperature T taken into consideration is the ambient temperature.
s
In this case, P can be omitted and only the first 4 parameters are indicated in parentheses.
Ts
Note 4 to entry: Talking about optical properties of materials used in solar applications (reflectance in this particular
case) only the ending "-ance" is permitted, because it means the amount of such a feature; that amount can be
degraded by the thermal process, soiling, mechanical damage, etc.
Note 5 to entry: To properly refer to the reflectance, the different options for the four/five parameters in parentheses
(see spectral reflectance (3.4.2) to diffuse reflectance (3.4.12)) shall be mentioned sequentially. Example: "SW"
means solar-weighted, "n" means near-normal, "sc" means sun-conic and "φ" means near-specular; therefore,
ρ(SW,n,sc,φ) is the solar-weighted near-normal sun-conic near-specular reflectance (3.4.10), measured at ambient
temperature.
Note 6 to entry: The first time that the reflectance is mentioned in a document, the value(s) of the four/five
parameters in parentheses (or the corresponding symbol(s) or letter(s) for the special cases) as well as the full name
shall be specified. For the rest of the document, the parameters in parentheses may be substituted by the
corresponding subscripts, provided that it is clearly stated in the text. For example, ρ(SW,n,sc,φ) may be named in
ρ . Also, using equivalent short names with the most representative parameters is
the rest of the document as
s,φ
allowed, provided that it is clearly stated in the text. For example, the "solar-weighted near-normal sun-conic near-
specular reflectance (3.4.10)" may be named as the "solar specular reflectance".
3.4.2
spectral reflectance
ρ({λ ,…, λ },P ,P ,P ,P )
1 i θi φi φ Ts
ρ([λ λ ],P ,P ,P ,P )
a, b θi φi φ Ts
reflectance (3.4.1) values measured across a specified wavelength range
Note 1 to entry: The wavelength range can be discrete, represented as a set of wavelengths, {λ ,…,λ}; or
1 i
continuous, represented by an interval [λ , λ ], with measurements taken at step of a few nanometres or less.
a b
Note 2 to entry: The values of the wavelengths used for the measurement shall be given the first time that it is
mentioned in a document. For the rest of the document, the symbols {λ ,…,λ } or [λ , λ ] can be used.
1 i a b
λ , λ ], the wavelength step within such
Note 3 to entry: When the wavelengths interval is given in a range, that is [
a b
range shall be indicated in the text.
3.4.3
monochromatic reflectance
single wavelength reflectance
ρ(λ,P ,P ,P ,P )
θi φi φ Ts
reflectance (3.4.1) measured at a single wavelength, λ, within a defined wavelength range, ∆λ,
centred at λ
Note 1 to entry: The value of the wavelength used for the measurement shall be given the first time that it is
mentioned in a document. For the rest of the document, the symbol λ can be used.
Note 2 to entry: The ∆λ depends on the optics of the measurement equipment and shall be specifically indicated in
the text.
3.4.4
solar reflectance
solar-weighted reflectance
ρ(SW,P ,P ,P ,P )
θi φi φ Ts
spectral reflectance (3.4.2) spectrum weighted over the solar spectrum
Note 1 to entry: See 6.2 for the method to calculate the solar reflectance, with the spectrum from IEC 60904-3 [3].
Note 2 to entry: If several solar spectra are used to calculate the solar reflectance, it shall be indicated with a
number in subscript (SW , SW , …) and the meaning of the different options shall be clearly indicated in the text (or
1 2
in the table foot).
3.4.5
UV reflectance
UV-weighted reflectance
ρ(UVW,P ,P ,P ,P )
θi φi φ Ts
spectral reflectance (3.4.2) spectrum weighted over the ultraviolet (UV) range of the solar
spectrum
Note 1 to entry: See 6.3 for the method to calculate the UV reflectance.
Note 2 to entry: If several UV parts are used to calculate the UV reflectance, it shall be indicated (UVAW, UVBW
…).
3.4.6
near-normal reflectance
ρ(P ,n,P ,P ,P )
λ φi φ Ts
reflectance (3.4.1) measured when the incidence angle (3.3.1) is sufficiently small so that the
reflectance (3.4.1) differs from the value measured at normal incidence by less than the
measurement error
Note 1 to entry: Although the letter "n" can be used for the parameter P to simplify the nomenclature when this
θi
condition for near-normal is accomplished, the specific value of the θ shall be indicated in the text.
i
Note 2 to entry: Typically, the incidence angle (3.3.1) is considered "near-normal" if it is less than or equal to 20°.
3.4.7
conic reflectance
ρ(P ,P ,φ ,P ,P )
λ θi i φ Ts
reflectance (3.4.1) when the divergent angle (3.3.2) of the incident radiation is greater than zero
3.4.8
sun-conic reflectance
ρ(P ,P ,sc,P ,P )
λ θi φ Ts
conic reflectance (3.4.7) when the divergent angle (3.3.2) of the incidence radiation is equal to
the mean value of the divergent angle (3.3.2) of the solar radiation on Earth under clear sky
conditions, typically 4,7 mrad
3.4.9
specular reflectance
ρ(P ,P ,P ,0,P )
λ θi φi Ts
ratio of the radiant flux incident on a surface as parallel beams (i.e. a plane wave), to the radiant
flux reflected by the surface in the specular direction, also as a plane wave (φ = 0)
Note 1 to entry: Specular reflectance is a theoretical parameter that can only be obtained through modeling, since
real light source emits radiation with divergence angles, and the detectors of measurement devices have finite
acceptance angles.
Note 2 to entry: The specular direction is on the same plane as the incident radiation and the normal to the surface,
making an angle equal to the incidence one, in the opposite direction.
Note 3 to entry: The divergent angle (3.3.2) does not apply when the specular reflectance is referred. Therefore, in
this case "0" shall be indicated in P . That is, specular reflectance shall be named as ρ(P ,P ,0,0,P ).
φi λ θi Ts
3.4.10
near-specular reflectance
ρ(P ,P ,P ,φ,P )
λ θi φi Ts
specular reflectance (3.4.9) when the acceptance angle (3.3.4) is not null (φ > 0)
Note 1 to entry: Near-specular reflectance can be named as specular reflectance (3.4.9) for simplification, once it
is clearly indicated in the text, to avoid confusion with the case of φ = 0.
3.4.11
hemispherical reflectance
ρ(P ,P ,P ,h,P )
λ θi φi Ts
ratio of the total radiant flux reflected and diffused by a surface across the entire hemisphere
above it, to the total radiant flux incident on the surface
Note 1 to entry: The divergent angle (3.3.2) does not apply when the hemispherical reflectance is measured.
Therefore, in this case "-" shall be indicated in P . That is, the hemispherical reflectance shall be named as ρ(P ,P ,-
φi λ θi
,h,P ).
Ts
3.4.12
diffuse reflectance
ρ(P ,P ,P ,d,P )
λ θi φi Ts
ratio of the radiant flux diffusely reflected by a surface across the entire hemisphere above it,
excluding the flux reflected within the solid angle 2πφ around the specular direction, to the total
radiant flux incident on the surface
Note 1 to entry: The diffuse reflectance is equal to the hemispherical reflectance (3.4.11) minus the specular
reflectance (3.4.9).
3.4.13
specularity
χ
ratio between near-specular reflectance (3.4.10) and hemispherical reflectance (3.4.11)
3.5 Figures and tables
Key
incidence angle
θ
i
specular reflectance angle
θ
r
angle of divergence of the incidence radiation
φ
i
angle of acceptance of specular reflectance
φ
1 reflecting surface
2 light source
Figure 1 – Angles of incidence, divergence, reflection and acceptance
Table 1 – Different cases related to reflectance nomenclature.
Parameter affecting a Section Nomenclature
Symbol
reflectance
spectral Spectral reflectance
ρ({λ ,.,λ }, P , P ,P ,P );
1 i θi φi φ Ts
reflectance
ρ([λ ,λ ], P , P , P ,P )
a b θi φi φ Ts
monochrom Monochromatic reflectance
ρ(λ, P , P , P , P )
θi φi φ Ts
atic
P
λ
reflectance
solar Solar reflectance, solar-weighted
ρ(SW, P , P , P , P )
θi φi φ Ts
reflectance reflectance
UV UV reflectance, UV-weighted
ρ(UVW, P , P , P , P )
θi φi φ Ts
reflectance reflectance
near-normal Near-normal reflectance
ρ(P , n, P , P ,P )
λ φi φ Ts
reflectance
P
θi
---
ρ(P , θ , P , P ,P ) Reflectance at a certain θ , different
λ i φi φ Ts i
from near-normal
conic Conic reflectance
ρ(P , P ,φ ,P ,P )
λ θi i φ Ts
reflectance
P
φi
sun-conic Sun-conic reflectance
ρ(P , P , sc, P ,P )
λ θi φ Ts
reflectance
specular Specular reflectance
ρ(P , P , P , 0, P )
P λ θi φi Ts
φ
reflectance
Parameter affecting a Section Nomenclature
Symbol
reflectance
near- Near-specular reflectance
ρ(P , P , P , φ, P )
λ θi φi Ts
specular
reflectance
hemispheric
ρ(P , P , P , h, P ) Hemispherical reflectance
λ θi φi Ts
al
reflectance
ρ(P , P , P , d , P ) diffuse Diffuse reflectance
λ θi φi Ts
reflectance
a
The specific parameter affecting the reflectance that is addressed in each case is marked in bold type.
4 Preparation of measurement
4.1 Sampling
The following rules shall be followed to prepare the reflector samples before they are measured.
– Unless the instrument is able to measure curved samples or the curvature of the sample is
so small that the measurement result is not affected by it, the samples shall be cut to a size
that allows measurement on different points on the surface but also fits into the instru
...