Perceptibility and acceptability thresholds for colour differences in dentistry

Abstract

Introduction

Data on acceptability (AT) and perceptibility thresholds (PT) for colour differences vary in dental literature. There is consensus that the determination of ΔE* is appropriate to define AT and PT, however there is no consensus regarding the values that should be used. The aim of this clinical review was to provide a systematic approach to the topic of colour science of high clinical relevance to dental research.

Materials and methods

MEDLINE/PubMed, WoS and EBSCO databases were searched up to January 7, 2013; the outcome was restricted to English, and to clinical studies were spectrophotometers were used for measurement.

Results

Forty-eight studies were eligible and met the inclusion criteria. Of the 48 studies there appeared to be a trend in their source references: 44% referred to the same study for the PT (ΔE* = 1); and 35% referred to the same article for the AT (ΔE* = 3.7).

Conclusions

More than half the studies defined PT as ΔE* = 1, and one third of the studies referred to ΔE* = 3.7 as the threshold at which 50% of observers accepted the colour difference. Most clinical studies refer to the same few in vitro literature that have attempted to determine PT and AT from decades ago.

Introduction

Colour science is an area within dental research that has been extensively studied over the last three decades. An initial search for the words ‘colour’ or ‘colour’ within dental journals included in PubMed yields over 5400 articles. Colour has become an increasingly important topic partially because many different research areas such as prosthodontics, aesthetics and dental materials science, use colour quantification to gain scientific data. The clinical relevance of such studies depends on how much colour change is considered perceptible and/or acceptable. Considering the fact that colour perception and/or acceptation, being subjective, can vary significantly among people, it is important to agree upon perceptibility and acceptability threshold values to be used within dental colour research.

Different colour difference formulas exist which are designed to provide a quantitative representation of the perceived colour difference between two objects within dental research. The most extensively used colour difference formula within dental research is derived from the CIE-L*a*b* system¹ which approximates uniformed distances between colour coordinates while entirely covering the visual colour space:

$$\Delta E^{*}=\sqrt{({(\Delta L^{*})}^2+{(\Delta a^{*})}^2+{(\Delta b^{*})}^2)}$$

ΔL*, Δa*, and Δb* are the differences in lightness–darkness, green–red coordinate and blue–yellow coordinate, respectively. ΔE* is the colour difference between two objects, where the higher the value the bigger the difference in colour and hence the more perceptible the difference is to the human eye. The mere determination of a colour difference between two objects is of little clinical value² without an understanding of the magnitude of colour difference that is visually detectable (perceptibility threshold) and the magnitude that constitutes an unacceptable alteration to dental aesthetics (acceptability threshold). The ISO Standardization Technical Report³ also suggests such interpretation through colour difference thresholds, and provides guidelines for future standardization related to dental shade conformity and interconvertibility.

All dental research that evaluates colour, involves the author selecting a perceptibility and/or acceptability threshold with which to compare the results. The quantitative assessment of a colour difference is worthless unless the following two thresholds are identified; the difference that is perceivable to an observer and the colour difference that is clinically acceptable.² Ample literature exists on perceptibility and acceptability thresholds in dentistry;4, 5, 6 however there has never been a review on this subject, and therefore this will be a key element of evidence-based dentistry. As more and more research is performed on colour science in dentistry there appears to be no consensus on the thresholds of perceptibility and acceptability. This review of the colour literature is intended to define current practices and trends in the use of ΔE* and to provide a basis for setting an acceptable and consistent standard for determining colour in a practical setting. It will also help researchers improve the accuracy of clinical interpretations of colour differences and aid research targeted at refining the colour reproduction process in dentistry.⁵

This review aims to establish, through the available literature, what is currently the colour difference value used in dental research as the perceptibility threshold (PT) and the acceptability threshold (AT). The general objective is to suggest the need for a uniformed PT and AT to be used in clinical research. The specific review questions to be addressed are:

• Which PT and AT values are currently used in dental research.

• What are the original source references for these ΔE* values.