ISO/TR 28642:2011

TECHNICAL ISO/TR
REPORT 28642
First edition
2011-07-15

Dentistry — Guidance on colour
measurement
Médecine bucco-dentaire — Directives relatives au mesurage de la
couleur




Reference number
ISO/TR 28642:2011(E)
©
ISO 2011

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ISO/TR 28642:2011(E)
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ISO/TR 28642:2011(E)
Contents Page
Foreword .iv
Introduction.v
1 Scope.1
2 Normative references.1
3 Terms and definitions .1
4 Visual and instrumental colour assessment .3
4.1 Devices .3
4.2 Setting.3
5 Observers.5
5.1 Evaluation of colour competency of candidate for observer in studies on acceptability or
perceptibility in dentistry.5
5.2 Guidelines for observer selected for acceptability or perceptibility evaluation in dentistry .5
6 Testing of acceptability and perceptibility thresholds.6
6.1 Materials to be tested for perceptibility and acceptability thresholds .6
6.2 Test methods .6
7 Application and interpretation .6
7.1 Colour compatibility.6
7.2 Colour stability .7
7.3 Colour interactions.7
8 Reporting of colour and colour difference assessment.7
8.1 Illuminant.8
8.2 Object.8
8.3 Observer/Instrument .8
Bibliography.10

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ISO/TR 28642:2011(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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
In exceptional circumstances, when a technical committee has collected data of a different kind from that
which is normally published as an International Standard (“state of the art”, for example), it may decide by a
simple majority vote of its participating members to publish a Technical Report. A Technical Report is entirely
informative in nature and does not have to be reviewed until the data it provides are considered to be no
longer valid or useful.
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.
ISO/TR 28642 was prepared by Technical Committee ISO/TC 106, Dentistry, Subcommittee SC 2,
Prosthodontic materials.
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ISO/TR 28642:2011(E)
Introduction
The colour appearance of teeth and other dentistry-related tissues needs to be matched and reproduced in
order to achieve acceptable aesthetics in an efficient manner. Three major groups of issues, related to colour
compatibility, colour stability and colour interactions, are identified and considered in this Technical Report.
Interpretation of colour differences associated with these three groups through 50:50 % perceptibility and
acceptability visual thresholds is suggested. Colour is a psychophysical phenomenon that is assessed by both
visual and instrumental methods. Other elements of appearance, including gloss and translucency, affect
aesthetics and may influence the characterization of colour appearance.
The International Commission on Illumination (CIE) colour difference formulae and resources, in particular CIE
Pub No 15.3, were used in this Technical Report.
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TECHNICAL REPORT ISO/TR 28642:2011(E)

Dentistry — Guidance on colour measurement
1 Scope
This Technical Report identifies three types of topics related to shade conformity and interconvertibility of
monochromatic and polychromatic tissues and materials related to the discipline of dentistry; it describes
visual and instrumental methods for assessment of these topics.
This Technical Report suggests interpretation of the findings through colour difference thresholds, and
provides guidelines for future standardization related to dental shade conformity and interconvertibility. It also
includes guidelines related to colour vision of persons undertaking visual colour assessments and instructions
for reporting of colour and colour difference assessments.
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 1942, Dentistry —Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 1942 and the following apply.
3.1
colour difference
single number or metric expressing the distance from complete match in colour or shade
NOTE 1 A colour distance metric defined by the International Commission on Illumination (CIE) is called delta E (∆E).
NOTE 2 Two formulae for calculating ∆E are recommended in this Technical Report: CIE76 (denoted ∆E* ) and
ab
CIEDE2000 (denoted ∆E ).
00
3.2
50:50 % perceptibility threshold of colour difference
PT
difference in colour that can be detected by 50 % of observers, with the other 50 % of observers noticing no
difference in colour between the compared objects
NOTE A nearly perfect colour match in dentistry is a colour difference at or below the 50:50 % perceptibility threshold.
3.3
50:50 % acceptability threshold of colour difference
AT
difference in colour that is considered acceptable by 50 % of observers, with the other 50 % of observers
replacing or correcting the restoration
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ISO/TR 28642:2011(E)
NOTE An acceptable colour match in dentistry is a colour difference at or below the 50:50 % acceptability threshold.
3.4
colour standards in dentistry
sets of polychromatic or monochromatic samples (tabs, chips, or patches), most frequently made of dental
ceramic or resin (tooth shade guides), or silicone elastomer (skin shade guides in maxillofacial prosthodontics)
NOTE The polychromatic and monochromatic samples are fabricated for the purposes of colour matching hard and
soft oral tissues, and human skin (see Reference [2]).
3.5
coverage error
index that describes the minimal colour differences (∆E* ) between the specimens of one set (e.g. a shade
min
guide) to each specimen of another set (e.g. natural teeth)
NOTE The coverage error is calculated as the mean ∆E* for all best matches as follows:
min
Coverage error = Σ∆E* /n
min
Where n is the number of best matches.
The smaller the coverage error, the higher the chances of successful shade matching.
3.6
tooth whitening
change in tooth colour caused by bleaching or stain-removal agents and manifested by the increase in the
value of lightness and the decrease of chroma
3.7
colour shifting
change in perceived colour that is a sum of a blending effect and an effect of physical translucency
3.7.1
colour shifting due to blending
change in perceived colour of a material due to a change in surroundings
NOTE Colour shifting due to blending is a psycho-physical phenomenon, and is not modelled by the current CIELAB
mathematical model; it is an optical illusion — visually perceptible, but not currently quantifiable or measurable by any
instrument.
3.7.2
colour shifting due to physical translucency
change in colour of translucent dental restorations caused by surroundings and background (underlying layers
of hard dental tissues or other restorative materials)
3.8
opacity
measure of the ability of a material to block the passage of light
NOTE A material with high opacity is one with low translucency/transparency.
3.9
transparency
physical property of allowing the transmission of light through a material
NOTE 1 A material with high transparency is one with low opacity.
NOTE 2 Transparency is the extreme value of high translucency.
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ISO/TR 28642:2011(E)
NOTE 3 A transparent material allows light to pass through undiminished, while a negligible portion of the transmitted
light is scattered.
3.10
translucency
ability of a material to allow light to pass through it
NOTE 1 A material with high translucency is one with low opacity.
NOTE 2 Translucent materials allow light to pass through only diffusely (they cannot be seen through clearly).
3.11
gloss
capacity of a surface to reflect more light in some directions than in others
3.12
specular gloss
ratio of flux reflected in the specular direction to incident flux (i.e. the angle of the reflected light is equal and
opposite to the angle of the incident beam) for a specified angle of incidence, source, and receptor angular
aperture

NOTE These reflections normally have the highest reflectances (see References [2] and [3]).
4 Visual and instrumental colour assessment
4.1 Devices
Frequently used devices for visual and instrumental colour assessment in dentistry are spectrophotometers,
spectroradiometers, colourimeters, imaging systems for traditional digital imaging and spectral imaging,
viewing booths, and different types of hand-held shade matching lights.
Spectral measurement is performed using a spectrophotometer, a device designed to measure spectral
transmittance and spectral reflectance factor of objects. Compared to colorimetric measurements, spectral
measurements enable more flexible measurements to calculate color differences under arbitrary illuminants
and observers.
Spectrometer parameters relevant for dentistry include wavelength range, wavelength resolution, integration
time and spectral sensitivity. In addition to these parameters, method of measurement, technology and
geometry have to be taken in account when selecting the equipment or designing a system to measure
spectra.
Spectral measurement accuracy can be measured in terms of root mean square error (rms) or degree of
correlation between spectral data, and other weighted spectral measurements that emphasize the wavelength
that are more relevant for human vision. Having spectral measurements of both reference and test samples it
is possible to calculate how robust is the pair of samples to changes in illumination and observers by means of
index of metamerism.
4.2 Setting
Colour assessment requires careful control of factors that affect colour perception and/or measurement. The
following aspects of the test conditions should be specified in reporting visual assessments of colour
appearance.
4.2.1 llluminant
The characteristics of the light source or sources that illuminate the object of a colour measurement as
specified preferably by the spectral power distribution (SPD) and correlated colour temperature (CCT)
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ISO/TR 28642:2011(E)
specified according to ISO 11644-2:2007. CIE standard illuminant D65 (representing noon daylight, CCT of
6500 K) with a Colour Renderin
...