ISO/PRF 7668

Anodizing of aluminium and its alloys -- Measurement of specular reflectance and specular gloss of anodic oxidation coatings at angles of 20°, 45°, 60° or 85°

Anodisation de l'aluminium et de ses alliages -- Mesurage des caractéristiques de réflectivité et de brillant spéculaires des couches anodiques à angle fixe de 20°, 45°, 60° ou 85°

General Information

• Status: Published
• ICS: 25.220.20 - Surface treatment
• Technical Committee: ISO/TC 79/SC 2 - Organic and anodic oxidation coatings on aluminium
• Drafting Committee: ISO/TC 79/SC 2/WG 15 - Anodic oxidation coatings on aluminium

RELATIONS

• Revised: ISO 7668:2018 - Anodizing of aluminium and its alloys -- Measurement of specular reflectance and specular gloss of anodic oxidation coatings at angles of 20 degrees, 45 degrees, 60 degrees or 85 degrees
• Effective Date: 10-Jul-2021

Standards Content (sample)

INTERNATIONAL ISO
STANDARD 7668
Fourth edition
Anodizing of aluminium and its
alloys — Measurement of specular
reflectance and specular gloss of
anodic oxidation coatings at angles of
20°, 45°, 60° or 85°
Anodisation de l'aluminium et de ses alliages — Mesurage des
caractéristiques de réflectivité et de brillant spéculaires des couches
anodiques à angle fixe de 20°, 45°, 60° ou 85°
PROOF/ÉPREUVE
Reference number
ISO 7668:2021(E)
ISO 2021
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ISO 7668:2021(E)
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© ISO 2021

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ISO 7668:2021(E)
Contents Page

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

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

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

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

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

4 Principle ........................................................................................................................................................................................................................ 2

5 Apparatus ..................................................................................................................................................................................................................... 2

6 Geometric conditions ...................................................................................................................................................................................... 2

7 Optical standards ................................................................................................................................................................................................. 6

7.1 Reference standards ........................................................................................................................................................................... 6

7.1.1 Black glass ............................................................................................................................................................................. 6

7.1.2 Glass prism (for Method E only)......................................................................................................................... 6

7.2 Working standards .............................................................................................................................................................................. 7

7.2.1 Description .................. .................................................... ...................................................................................................... 7

7.2.2 Zero point check ............................................................................................................................................................... 7

8 Preparation and calibration of apparatus ................................................................................................................................. 7

9 Measurement of specular reflectance and specular gloss ........................................................................................ 8

9.1 General ........................................................................................................................................................................................................... 8

9.2 Measurement of specular reflectance ................................................................................................................................. 8

9.3 Measurement of specular gloss ................................................................................................................................................ 8

10 Expression of results .....................................................................................................................................................................................11

10.1 General ........................................................................................................................................................................................................11

10.2 Specular reflectance ........................................................................................................................................................................11

10.3 Specular gloss .......................................................................................................................................................................................11

11 Test report ................................................................................................................................................................................................................11

Annex A (normative) Specular reflectance and specular gloss of black glass ........................................................12

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ISO 7668:2021(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 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 79, Light metals and their alloys,

Subcommittee SC 2, Organic and anodic oxidation coatings on aluminium, in collaboration with the

European Committee for Standardization (CEN) Technical Committee CEN/TC 132, Aluminium and

aluminium alloys, in accordance with the Agreement on technical cooperation between ISO and CEN

(Vienna Agreement).

This fourth edition cancels and replaces the third edition (ISO 7668:2018), which has been technically

revised. The main changes compared with the previous edition are as follows:
— Tables 4 and 5 have been revised.

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.
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ISO 7668:2021(E)
Introduction

Specular reflectance and specular gloss are not unique physical properties of a surface. They vary with

the angle of measurement, and with the aperture dimensions that define the incident and the reflected

beams, such that measurements of these properties are not independent of the apparatus being used.

The specular reflectance of most surfaces increases with the angle of measurement and accounts for the

use of reflectometers with various angles as, for example, for painted surfaces. The specular reflectance

characteristics of anodized aluminium, however, do not always behave in the normal manner and,

because of its property of double reflection, reflected light comes partly from the film surface and

partly from the underlying metal. It is advisable to measure the specular reflectance characteristics at

20°, 45°, 60° and 85° to obtain a complete understanding of the specular reflectance properties of the

anodized surface, and careful thought should be given to which method or methods are most relevant

in any particular situation. The specular reflectance of bright-anodized aluminium with a mirror finish

is best measured using 45° or 20° geometry.
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INTERNATIONAL STANDARD ISO 7668:2021(E)
Anodizing of aluminium and its alloys — Measurement of
specular reflectance and specular gloss of anodic oxidation
coatings at angles of 20°, 45°, 60° or 85°
1 Scope

This document specifies methods for the measurement of specular reflectance and specular gloss of

flat samples of anodized aluminium using geometries of 20° (Method A), 45° (Method B), 60° (Method

C) and 85° (Method D) and of specular reflectance by an additional 45° method (Method E) employing a

narrow acceptance angle.

The methods described are intended mainly for use with clear anodized surfaces. They can be used

with colour-anodized aluminium, but only with similar colours.
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 7583, Anodizing of aluminium and its alloys — Terms and definitions
ISO/CIE 11664-1, Colorimetry — Part 1: CIE standard colorimetric observers
ISO 11664-2, Colorimetry — Part 2: CIE standard illuminants
CIE Publication No. 15, Colorimetry
3 Terms and definitions

For the purposes of this document, the terms and definitions given in ISO 7583 and the following 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 https:// www .electropedia .org/
3.1
specular reflectance

ratio of the luminous flux, reflected in the specular direction for a specified source and receptor angle,

to the luminous flux of the incident light
Note 1 to entry: Usually expressed as a percentage.
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ISO 7668:2021(E)
3.2
specular gloss

ratio of the luminous flux, reflected from an object in the specular direction for a specified source

and receptor angle, to the luminous flux reflected from glass with a refractive index of 1,567 at a

wavelength of 546,1 nm (which is the central wavelength of the spectral luminous efficiency function)

in the specular direction

Note 1 to entry: To set the specular gloss scale, polished black glass with a refractive index of 1,567 is assigned

the value of 100 for geometries of 20°, 45°, 60° and 85° (see Table 5). The phenomenon of light reflectance by

anodized aluminium is very different to that of black glass and the choice of a black-glass standard is arbitrary

and made to allow comparison of different qualities of anodized aluminium.
4 Principle

The specular reflectance and specular gloss of anodized aluminium surfaces are measured under

defined conditions using, as required, geometries of 20°, 45°, 60° and 85°.
5 Apparatus
The usual laboratory apparatus and, in particular, the following shall be used.

Approximate comparisons between surfaces of the same colour can be made, but an accurate

measurement requires the combination of light source, photoelectric cell and associated colour filters

to give a spectral sensitivity, approximating to the CIE spectral luminous efficiency, V(λ), weighted

for CIE standard illuminants C or D65. V(λ) shall conform to the requirements of ISO/CIE 11664-1. CIE

standard illuminants C and D65 shall conform to the requirements of CIE 15 and ISO 11664-2.

NOTE Since specular reflection is in general spectrally non-selective, the spectral characteristics of the light

source (5.1) and the receptor (5.3) do not affect the measurement of normal uncoloured anodized surfaces.

5.1 Polychromatic light source and housing, with a lens that directs a parallel, or very slightly

converging, beam of light onto the surface under test.

5.2 Means for locating the specimen surface, in the correct position for measurement.

5.3 Receptor housing containing a lens, a receptor aperture and a photoelectric cell, to receive

the cone of reflected light.

5.4 Sensitivity control, for setting the photocell current to any desired value on the instrument scale

or digital indicator.

5.5 Receptor meter, capable of giving an indication proportional to the light flux passing the receptor

aperture within 1 % of the full-scale reading. Spectral corrections are not usually required (see NOTE

above).
6 Geometric conditions

The incident angle, ε , which is the angle between the axis of the incident beam and the perpendicular

to the surface under test, shall have the following values and tolerances:
— for Method A: 20° ± 0,1°;
— for Method B: 45° ± 0,1°;
— for Method C: 60° ± 0,1°;
— for Method D: 85° ± 0,1°;
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ISO 7668:2021(E)
— for Method E: 45° ± 0,1°.
There shall be no vignetting of rays that lie within the angles specified above.

The axis of the receptor shall, as far as possible, coincide with the mirror image of the axis of the incident

beam; the receptor angle, ε , which is the angle between the axis of the receptor and the perpendicular

to the surface under test, shall be for all methods such that
|ε − ε | ≤ 0,1°
1 2

With a flat piece of polished glass or other front-surface mirror in the test specimen position, an image

of the source shall be formed at the centre of the receptor aperture. The width of the illustrated area of

the test specimen shall be not less than 10 mm.

The angular dimensions of the receptor apertures shall be measured from the receptor lenses. The

dimensions and tolerances of the sources and receptors shall be as indicated in Tables 1 and 2. Figures 1,

2 and 3 give generalized illustrations of these dimensions. Table 1 gives both angles and corresponding

dimensions calculated for lenses of a focal length of 50 mm for Methods A, B, C and D. Table 2 gives

the angles and aperture dimensions for Method E. The angles are mandatory and the aperture sizes

have been calculated from the corresponding angle, δ, as 2f (tan δ/2), where f is the focal

...