Visual and instrumental shade matching using CIELAB and CIEDE2000 color difference formulas

Abstract

Objectives

To compare visual and instrumental shade matching performances using two shade guides and three color difference formulas.

Methods

One hundred dental students (DS) volunteers (35 males and 65 females) with normal color vision participated in the study. The spectral reflectance of 4 extracted human upper central incisors (UCI) and shade tabs from Vita Classical (VC) and Vita Toothguide 3D-Master (3D) shade guides were measured using a spectroradiometer (SP) under D65 illuminant (diffuse/0° geometry) inside a viewing booth with a gray background. Color coordinates (CIE L *, a *, b *, C * and h °) were calculated according to CIE D65 illuminant and CIE 2° Standard Observer. Color coordinates of UCI were also evaluated using a dental spectrophotometer (EA – Easyshade Advance). DS used VC and 3D to visually select the best shade match for each UCI, under same experimental conditions used for the SP evaluation. Three color difference metrics (CIELAB, CIEDE2000(1:1:1) and CIEDE2000(2:1:1)) were used to calculate the best instrumental shade matching based on minimum color difference.

Results

The agreement between visual and instrumental shade matching was greater using SP (25–75%) than EA (0–25%). The percentage of best match for the visual assessment was more consistent using VC (23–55%) than 3D (19–34%). Considering the best performance (using SP and VC), the CIEDE2000(2:1:1) color difference formula showed the best estimate to the visual perception from DS.

Significance

Within the limitations of this study, combining the use of SP, CIEDE2000(2:1:1) and Vita Classical shade guide most closely represented the visual perception of DS. Instrumental shade determination should be accompanied by experienced human visual assessment.

Introduction

The ability to reproduce the exact shade of natural teeth using restorative dental materials is one of the most challenging goals in clinical dentistry . Visual shade matching using commercially available shade guides is the most common method to select the shade in dentistry . The most frequently used shade guides are Vita Classical (VC) and Vita 3D Master (3D) . Ordinary visual determination of teeth shade is subjective and inconsistent because many observer-related variables (e.g. gender, color deficiency, experience and eye fatigue) can influence on the final result . Color perception does not only vary between persons, it also fluctuates for the same individual over time .

Recent developments on technology and materials offer the potential to improve shade-matching skills in dentistry . The main advantage of dental shade-matching instruments is their ability to reduce the imperfections and inconsistencies of visual shade matching , which have been discussed in several studies .

CIE (Commission Internationale de l’Eclairage) has been traditionally involved in colorimetry for dental materials. It has been responsible for introducing the main color systems, illumination patterns and color difference (Δ E ) concepts used in color science . When considering CIELAB color space ( L *: value axis; a *: red-green axis; b *: yellow-blue axis), the <SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='ΔEab*’>ΔEabΔEab*
Δ E a b *
has been classically the standard parameter for total color difference between two objects. In the CIELAB metric, the greater the value, the larger the color difference and, consequently, the more perceptible the difference to the human eye . Aiming to improve the correction between computed and perceived color differences, the CIE recommended the use of CIEDE2000 color-difference formula (Δ E 00 ) , which incorporates specific corrections for non-uniformity of CIELAB color space (the so called weighting functions: S L , S C , S H ), a rotation term ( R T ) that accounts for the interaction between chroma and hue differences in the blue region and a modification of the a * coordinate of CIELAB that mainly affects colors with low chroma (neutral colors) and parameters accounting for the influence of illuminating and vision conditions in color difference evaluation (the so-called parametric factors: K L , K C , K H ) . The parametric factor ratio was proposed as a way to control changes in the magnitude of tolerance judgments and as a way to adjust for scaling of acceptability rather than perceptibility . Studies on visual judgments performed in textile color acceptability and dental ceramics acceptability showed that using K L = 2 resulted in color differences better correlated to observations from a subset of average observers.

The purpose of the present study was to compare visual shade matching performed by dental students (DS) to instrumental shade matching based on measurements from a spectroradiometer and a dental spectrophotometer, using two dental shade guides (VC and 3D) and three different total color difference metrics (CIELAB, CIEDE2000(1:1:1) and CIEDE2000(2:1:1)). Therefore, this study tested the following hypotheses: (1) the performance of a spectroradiometer is similar to a dental spectrophotometer on estimate the best visual shade matching performed by DS, (2) the color difference metrics (CIELAB, CIEDE2000(1:1:1) and CIEDE2000(2:1:1)) have no influence on the instrumental shade matching; and (3) shade matching is not influenced by the type of dental shade guide.

Material and methods

The study was approved by the local Ethics in Research Committee and is registered in the national platform for ethically approved studies involving human beings (# 767.915).

Instrumental shade matching assessment setup

The spectral reflectance of 4 extracted human upper central incisors (UCI), 16 shade tabs from Vita Classical shade guide (VC) and 26 shade tabs from Vita Toothguide 3D-Master shade guide (3D) (VITA Zahnfabrik, Bad Säckingen, Germany) were measured using a spectroradiometer (SP – SpectraScan ® PR-704, Color Research Inc., Chatsworth, CA, USA), under CIE D65 illuminant (diffuse/0° illuminating/measuring geometry) inside a viewing booth (CAC 60, VeriVide Limited, Leicester, United Kingdom) with a gray background (Flexipalette Color Match). Color coordinates (CIE L *, a *, b *, C * and h °) were calculated according to the CIE D65 Standard illuminant and the CIE 2° Standard Observer .

Short-term repeated (3×) measurements without replacement were performed in each specimen. Color measurements were performed in the middle third of all UCIs and shade tabs. As for other studies , a stand was built to support the specimens and avoid specular reflection from the glossy surface. Since the spectroradiometer field of measurement is 1°, it was placed 35 cm from the specimen, allowing for the measurement of the whole specimen.

A dental spectrophotometer (EA – Easyshade Advance, VITA Zahnfabrick, Bad Säckingen, Germany) was also used to evaluate the CIE L *, a *, b *, C * and h ° coordinates from the same UCIs and shade tabs measured by spectroradiometer. EA measurements were performed by a single trained operator in complete darkness. The spectrophotometer was calibrated before each measurement (within instrument calibration tip) to minimize measurement uncertainty. VC and 3D nominal shades were also recorded by the EA from each UCI.

Color differences

For both SP and EA measurements, color differences (Δ E ) between the UCI and the shade tabs from VC and 3D shade guides were evaluated using two different total color difference metrics: CIELAB ( <SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='ΔEab*’>ΔEabΔEab*
Δ E a b *
) and CIEDE2000 (Δ E 00 ).

Computations for the CIELAB ( <SPAN role=presentation tabIndex=0 id=MathJax-Element-3-Frame class=MathJax style="POSITION: relative" data-mathml='ΔEab*’>ΔEabΔEab*
Δ E a b *
) color difference formula were made according to the following equation :

<SPAN role=presentation tabIndex=0 id=MathJax-Element-4-Frame class=MathJax style="POSITION: relative" data-mathml='ΔEab*=(ΔL*)2+(Δa*)2+(Δb*)2′>ΔEab=(ΔL)2+(Δa)2+(Δb)2ΔEab*=(ΔL*)2+(Δa*)2+(Δb*)2
Δ E a b * = ( Δ L * ) 2 + ( Δ a * ) 2 + ( Δ b * ) 2

where ΔL *, Δa * and Δb * are the differences in the respective coordinates for a pair of samples.

Computations for the CIEDE2000 (Δ E 00 ) color difference formula were made according to the following equation :

<SPAN role=presentation tabIndex=0 id=MathJax-Element-5-Frame class=MathJax style="POSITION: relative" data-mathml='ΔE00=ΔL′KLSL2+ΔC′KCSC2+ΔH′KHSH2+RTΔC′KCSCΔH′KHSH1/2′>ΔE00=((ΔLKLSL)2+(ΔCKCSC)2+(ΔHKHSH)2+RT(ΔCKCSC)(ΔHKHSH))1/2ΔE00=ΔL′KLSL2+ΔC′KCSC2+ΔH′KHSH2+RTΔC′KCSCΔH′KHSH1/2
Δ E 00 = Δ L ′ K L S L 2 + Δ C ′ K C S C 2 + Δ H ′ K H S H 2 + R T Δ C ′ K C S C Δ H ′ K H S H 1 / 2
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Nov 23, 2017 | Posted by in Dental Materials | Comments Off on Visual and instrumental shade matching using CIELAB and CIEDE2000 color difference formulas
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