Dental ceramics: A CIEDE2000 acceptability thresholds forlightness, chroma and hue differences

Marıa del Mar Perez a,*, Razvan Ghinea a, Luis Javier Herrera b, Ana Maria Ionescu a,Hector Pomares b, Rosa Pulgar c, Rade D. Paravina d

aDepartment of Optics, Faculty of Science, University of Granada, Campus Fuentenueva s/n 18071, Granada, SpainbDepartment of Computer Architecture and Computer Technology, E.T.S.I.I.T. University of Granada, s/n 18071 Granada, SpaincDepartment of Stomatology, Faculty of Odontology, University of Granada, Campus de Cartuja s/n 18071, Granada, SpaindHouston Center for Biomaterials and Biomimetics & Department of Restorative Dentistry and Biomaterials, The University of Texas Health

Science Center at Houston School of Dentistry, 6516 M. D. Anderson Boulevard, Houston, TX 77030, USA

j o u r n a l o f d e n t i s t r y 3 9 s ( 2 0 1 1 ) e 3 7 – e 4 4

a r t i c l e i n f o

Article history:

Received 22 June 2011

Received in revised form

12 September 2011

Accepted 18 September 2011

Keywords:

CIEDE2000

Dental ceramics

Lightness

Chroma

Hue

Acceptability thresholds

Parametric factors

a b s t r a c t

Objectives: To determine the visual 50:50% acceptability thresholds for lightness, chroma

and hue for dental ceramics using CIEDE2000(KL:KC:KH) formula, and to evaluate the formula

performance using different parametric factors.

Methods: A 30-observer panel evaluated three subsets of ceramic samples: lightness subset

(jDL0/DE00j � 0.9), chroma subset (jDC0/DE00j � 0.9) and hue subset (jDH0/DE00j � 0.9). A Takagi–

Sugeno–Kang Fuzzy Approximation was used as fitting procedure, and the 50:50% accept-

ability thresholds were calculated. A t-test was used in statistical analysis of the thresholds

values. The performance of the CIEDE2000(1:1:1) and CIEDE2000(2:1:1) color difference

formulas against visual results was tested using PF/3 performance factor.

Results: The 50:50% CIEDE2000 acceptability thresholds were DL0 = 2.92 (95% CI 1.22–4.96;

r2 = 0.76), DC0 = 2.52 (95% CI 1.31–4.19; r2 = 0.71) and DH0 = 1.90 (95% CI 1.63–2.15; r2 = 0.88). The

50:50% acceptability threshold for color difference (DE0) for CIEDE2000(1:1:1) was 1.87, whilst

corresponding value for CIEDE2000(2:1:1) was 1.78. The PF/3 values were 139.86 for

CIEDE2000(1:1:1), and 132.31 for CIEDE2000(2:1:1).

Conclusions: There was a statistically significant difference amongst CIEDE2000 50:50%

acceptability thresholds for lightness, chroma and hue differences for dental ceramics.

The CIEDE2000(2:1:1) formula performed better than CIEDE2000(1:1:1).

# 2011 Elsevier Ltd. All rights reserved.

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journal homepage: www.intl.elsevierhealth.com/journals/jden

1. Introduction

Instruments for color measurement in dentistry, such as

spectrophotometers, colorimeters and spectroradiometers,

can help overcoming some shortcomings of visual method

by bringing accuracy and reducing the chair side time for color

matching, communication and reproduction. In addition,

color measuring instruments can be a valuable tool in shade

* Corresponding author at: Office 137, Department of Optics, Faculty oGranada, Spain. Tel.: +34 958246164; fax: +34 958248533.

E-mail address: mmperez@ugr.es (M.d.M. Perez).

0300-5712/$ – see front matter # 2011 Elsevier Ltd. All rights reserveddoi:10.1016/j.jdent.2011.09.007

verification (quality control). Colorimeters shown good mea-

surement repeatability but they are subject to systematic

errors due to edge-loss effect related with sample surface

whilst spectrophotometers precisely measures color from

reflectance or transmittance data but they are hard to use for

in vivo tooth color measurements.1 In this sense, the recent

incorporation of spectroradiometers for color measurements

in dental research provided accurate and highly repeatable

non-contact measurements.2 The development of the CIELAB

f Science, University of Granada, Campus Fuentenueva s/n 18071.

.

j o u r n a l o f d e n t i s t r y 3 9 s ( 2 0 1 1 ) e 3 7 – e 4 4e38

color space and the associated DE*ab, have done much to aid in

this process and is extensively accepted in dentistry. Recent

studies on perceptibility and acceptability color difference

thresholds using computer-simulated teeth and dental ce-

ramics suggested that there is a variance in sensitivity with

respect to lightness (L*), green–red coordinate (a*) and blue–

yellow coordinate (b*) of CIELAB color space.3,4

The dependence of direction of CIELAB color differences on

color sensitivity is not influenced by the experimental

conditions of measurement. It occurs due to the fact that

the formulas which convert CIE1931 colorimetric values into

CIELAB L*a*b* coordinates do not adequately capture the

perceived color differences.5 Advanced CIELAB-based color

difference formulas were introduced to improve the correla-

tion with visual color differences through the implementation

of various corrections of the original CIELAB color difference

formula.6

Various practical applications of CIELAB, which assumed

that CIELAB is a uniform color space, have shown the need of

using weighting factors to predict color differences.7 CMC(l:c),

CIE94(KL:KC:KH) and CIEDE2000(KL:KC:KH) each employ such

weighting factors to adjust the inaccuracies.

CIEDE2000 color-difference formula incorporates specific

corrections for non uniformity of CIELAB color space (the so-

called weighting functions: SL, SC, SH), a rotation term (RT) that

accounts for the interaction between chroma and hue

differences in the blue region and a modification of the a*

coordinate of CIELAB, which 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: KL,

KC, KH).8 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.9 Several authors assumed that texture only

affects lightness tolerances but not chroma or hue toler-

ances,10,11 and therefore the value KL = 2 was proposed.12 It

was found that the rotation term (RT), introduced in CIEDE2000

to weight the interaction between chroma and hue differ-

ences, is close to zero for the dental color space.13

Recent dental investigations found that CIEDE2000 color

difference formula provided better fit than CIELAB formula in

the evaluation of color difference, therefore providing

better indicators of human perceptibility an acceptability of

color differences between tooth colors.2,14 Nonetheless, it

seems appropriate to continue studying the CIEDE2000

weighting functions (SL, SC, SH) and parametric factors (KL,

KC, KH), which may result in an even better fit with the visual

judgments. In addition, no data is available on CIEDE2000

acceptability thresholds for lightness, chroma and hue

differences (DL0, DC0 and DH0, respectively). These thresholds

can lead to a valid and applicable formula to improve the

modelling of tooth colored aesthetic materials and, ultimately,

patient satisfaction.

The specific aims of this study were to determine the visual

acceptability thresholds for lightness, chroma and hue for

dental ceramics using CIEDE2000(KL:KC:KH) formula and to

evaluate the performance of this color difference formula

using different parametric factors. The following hypotheses

were tested: (i) there were no difference amongst CEIDE2000

50:50% acceptability thresholds for lightness, chroma and hue;

(ii) there was no difference in performance of CIEDE2000(2:1:1)

and CIEDE2000(1:1:1) in evaluation of color differences of

dental ceramics.

2. Materials and methods

2.1. Sample preparation

A total of 58 ceramic discs, 14-mm in diameter and 3-mm

thick, were fabricated using mixtures of Vita Omega 900,

Vitapan 3D-Master opaque powders, and pink, white, and

mauve color opaque powders (VITA Zahnfabrik, Bad Sackin-

gen, Germany).14 The surface to be observed for each disc was

polished using silica paper (Struers A/S Ballerup, Denmark)

sequentially up to 800-grit paper. The range of the color

coordinates of the ceramic discs were L0 = 56.09–75.30,

C0 = 5.60–28.89 and h0 = 62.16–85.00. All discs were within the

color range of central and lateral incisor and canine teeth as it

is reflected on a published study.15 The 58 ceramic disks were

combined to create a total of 1653 disc pairs, with CIEDE2000

color differences ranging from 0.10 to 10.02 units. Three

subsets were defined for visual judgments as follows:

lightness subset – pairs of samples that met jDL0/DE00j � 0.9

– where the total color difference is mainly due to the changes

in luminance (40 pairs with DL0 ranging from 0.32 to 8.03);

chroma subset – pairs of samples that met jDC0/DE00j � 0.9 –

where the total color difference is mainly due to changes in

chroma (40 pairs with DC0 ranging from 0.99 to 7.89); hue

subset – pairs of samples that met jDH0/DE00j � 0.9 – where the

total color difference is mainly due to the differences in hue (31

pairs with DH0 ranging from 0.17 to 3.36). As expected, the

range of hue variation is significantly smaller since the

samples are intended to mimic color of human natural

teeth.15 The total number of selected samples was 111.

The values of DL0, DC0 and DH0 for each pair of samples are

shown in Fig. 1.

2.2. Color measurement

A non-contact SpectraScan PR-704 spectroradiometer (Photo

Research, Chatsworth, USA) was used to measure the spectral

reflectance of the ceramic disks. This device measure color in a

way that matches the geometry of the visual assessments, and

has been previously used in dental research.2,16,17 The ceramic

discs were placed in the centre of a viewing cabinet (VeriVide

CAC60, VeriVide Limited, Leicester, United Kingdom) on a 458

tilted base and a light source simulating the spectral relative

irradiance of CIE D65 standard illuminant were employed to

provide consistent illuminating/viewing conditions. The discs

were positioned 40 cm away from the spectrorradiometer and

measured at 08 (corresponding to diffuse/08 illuminating/

measuring geometry). The CIE 1931 28 Standard Colorimetric

Observer was used to calculate color coordinate values.

Since teeth are translucent and the oral cavity is dark, a

Munsell black background (L* = 2.8, a* = 0.7, b* = 1.9) was used

for measurements in this study. Similar to previous studies,2,17

a triangular stand was built to hold samples to avoid the

specular reflection from the glossy surface.

Fig. 1 – DL0, DC0 and DH0 values of each sample pair of

ceramic disks.

j o u r n a l o f d e n t i s t r y 3 9 s ( 2 0 1 1 ) e 3 7 – e 4 4 e39

The CIEDE2000 color difference (DE00) was calculated as

follows:6,8

DE00 ¼DL0

KLSL

� �2

þ DC0

KCSC

� �2

þ DH0

KHSH

� �2

þ RTDC0

KCSC

� �DH0

KHSH

� �" #1=2

In addition, the CIEDE2000 lightness (DEL), chroma (DEC)

and hue (DEH) color differences were defined as follows:18

DEL ¼DL0

ðKL � SLÞ

DEC ¼DC0

ðKC � SCÞ

DEH ¼DH0

ðKH � SHÞ

where DL0, DC0, DH0 are metric differences between the corre-

sponding values of the samples, computed on the basis of

uniform color space used in CIEDE2000, and KL�SL, KC�SC and

KH�SH are empirical terms used for correcting (weighting) the

metric differences to the CIEDE2000 differences for each coor-

dinate. Parametric factors were set to 1 for CIEDE2000(1:1:1)

and KL = 2, KH = 1 and KC = 1 for CIEDE2000(2:1:1). When calcu-

lating the CIEDE2000 color-difference formula, the disconti-

nuities due to mean hue computation and hue-difference

computation as pointed out and characterised by Sharma

et al.,19 were taken into account.

2.3. Psychophysical experiment

The three subsets (lightness, chroma and hue) of sample pairs

were judged by a panel of 30 non-dental professional

observers (12 females and 18 males, aged between 19 and

55). All observers were screened for normal color vision using

the Ishihara charts (Ishihara Color Vision Test. Kamehara

Trading Inc., Tokio, Japan 2004) and all had previous

experience in color discrimination experiments. The psycho-

physical experiment was approved by the Institutional Review

Board. During the visual judgments, the observers were

positioned approximately 40 cm away from the ceramic disc

pairs, which was the same distance used for instrumental

color measurements. Each observer was instructed to focus

their attention on the centre of the ceramic disks and answer

the following question: ‘‘Would you rate the color difference

between the two discs as acceptable’’. The responses for each

pair of ceramic disks and each observer (DV – visual color

difference) were processed.2

2.4. Fitting procedure

A Takagi–Sugeno–Kang (TSK) Fuzzy Approximation model20,21

with Gaussian membership functions and constant conse-

quents was used as fitting method (Matlab 7.1 Fuzzy Logic

Toolbox, MathWork Inc., Natick, MA). In the approximations

performed, the TSK models took the rule centres equally

distributed along the input space, and the rule consequents

were optimally obtained using their derivatives with respect to

the model output in the minimisation of the value of r (Least

Squares LSE approach).22 The number of rules in each case was

selected using a 10-fold cross-validation procedure; the

number of rules for which the model provided a lowest

cross-validation error was chosen to perform the approxima-

tion using all data. The 95% confidence intervals (CI, 95%

Lower Confidence Limit – LCL and the 95% Upper Confidence

Limit – UCL) were estimated and the 50:50% (50% of positive

answers and 50% negative answers) thresholds were calculat-

ed. The 50:50% point was defined as the difference at which an

j o u r n a l o f d e n t i s t r y 3 9 s ( 2 0 1 1 ) e 3 7 – e 4 4e40

observers would have a 50% probability of making dichoto-

mous judgement, and represents the level of acceptability for

these types of judgments.

Acceptability thresholds were calculated for the following

set of samples: (a) lightness subset (DL0), chroma subset (DC0)

and hue subset (DH0); (b) CIEDE2000 lightness subset (DEL),

CIEDE2000 chroma subset (DEC) and CIEDE2000 hue subset

(DEH); (c) CIEDE2000(1:1:1) and CIEDE2000(2:1:1) for the whole

set of 111 disc pairs.

2.5. Statistical analysis

A t-test was used to evaluate the differences amongst the

lightness, chroma and hue thresholds, both for the metric

differences (DL0, DC0 and DH0) and CIEDE2000 lightness (DEL),

chroma (DEC) and hue (DEH) color differences, assuming the

50:50% values as normal distributions with variance estimated

according to the confidence intervals of the respective fitting

curves (SPSS 15.0.1, SPSS, Chicago, USA).

To test the performance of CIEDE2000(1:1:1) and

CIEDE2000(2:1:1) color difference formulas against visual

results, the performance factor PF/3 was calculated.23 This

parameter allows statistical comparison of two data sets by

means of combining three measures of fit: gamma factor g, CV

and VAB.24

The computation of PF/3 is given as:PF3¼ 100 � ½ðg � 1Þ þ VAB þ ðCV=100Þ�

3

Fig. 3 – TSK Fuzzy Approximation fitted curve for visual accepta

(b) DEC of pairs of ceramic discs.

Fig. 2 – TSK Fuzzy Approximation fitted curve for visual accepta

(b) DEL of pairs of ceramic discs.

A PF/3 value of zero indicates perfect agreement between

computed and perceived color differences; higher values

correspond to worse agreement. From the mathematical

point of view, there is no maximum limit for PF/3 values

(can be greater than 100%).

3. Results

The TSK Fuzzy Approximation fitted curves of the percentage

of DV answers (% acceptable) against the instrumentally

measured metric differences (DL0, DC0, and DH0 respectively)

and against the CIEDE2000 lightness, chroma and hue color

differences (DEL, DEC, DEH) with their corresponding 95%

confidence curves are presented in Figs. 2–4. For DL0, the

determined 50:50% acceptability threshold was DL0 = 2.92 with

95% CI 1.22–4.96 and r2 = 0.76 (Fig. 2a). From the fitted curve of

acceptable percentages versus DEL (Fig. 2b), the threshold

value found for lightness was 2.86 (95% CI 1.20–4.84; r2 = 0.75).

In the case of the DC0, the 50:50% acceptability threshold was

DC0 = 2.52 units, with 1.31–4.19 95% CI and r2 = 0.71 (Fig. 3a). For

chroma, from the fitted curve of acceptance percentages

against DEC, we calculated a threshold value of 1.34 (95%CI

0.22–2.96, r2 = 0.56) (Fig. 3b). The DH0 value corresponding to

50:50% acceptability threshold was 1.90, with 95% CI 1.63–2.15;

r2 = 0.88 (Fig. 4a), whilst for DEH (Fig. 4b), the corresponding

threshold was 1.65 (95% CI 1.10–1.87 and r2 = 0.82).

bility in percentages versus (a) DC0 of pairs of ceramic discs;

bility in percentages versus (a) DL0 of pairs of ceramic discs;

Fig. 4 – TSK Fuzzy Approximation fitted curve for visual acceptability in percentages versus (a) DH0 of pairs of ceramic discs;

(b) DEH of pairs of ceramic discs.

j o u r n a l o f d e n t i s t r y 3 9 s ( 2 0 1 1 ) e 3 7 – e 4 4 e41

The t-test confirmed that there were statistically significant

differences between threshold values when calculated with

the metric differences in lightness (DL0), chroma (DC0) and hue

(DH0) ( p < 0.001). For the CIEDE2000 lightness (DEL), chroma

(DEC) and hue (DEH), the statistical analysis showed significant

differences between DEL and DEC threshold values and

between DEL and DEH threshold values, with a negligible p-

value ( p < 0.001). However, when comparing DEC and DEH

thresholds, the p-value was higher ( p = 0.1), showing that it is

not possible to confirm the difference amongst these two

thresholds values.

When considering KL = 2, KC = 1 and KH = 1, the values of the

DEL, DEC, and DEH thresholds were found to be 1.43, 1.34 and

1.65 respectively, and the statistical analysis showed no

significant differences between threshold values of lightness

and chroma or between lightness and hue ( p > 0.1 for both).

The dependence of the threshold value on the color

coordinates is presented in Fig. 5. The percentages of visual

acceptability against DL0 and the average value of lightness of

each judged pair of samples is presented in Fig. 5a. The visual

acceptability percentages of chroma against DC0 and average

CIEDE2000 chroma is shown in Fig. 5b, whilst the visual

acceptability percentages of hue against DH0 and average

CIEDE2000 hue angle for each judged pair of samples is

illustrated in Fig. 5c.

TSK Fuzzy Approximation fitted curves of the percentage of

DV answers (% acceptability) against the instrumentally

measured differences CIEDE2000(1:1:1) and CIEDE2000(2:1:1)

respectively, with their corresponding 95% confidence curves,

are plotted for the three data subsets jointly (Fig. 6). For the

CIEDE2000(1:1:1) the determined 50:50% acceptability thresh-

old was 1.87 (r2 = 0.62; Fig. 6a) and optimal number of rules for

the TSK Fuzzy Approximation equal to 3. In the case of the

Table 1 – Fit parameters and performance factors for the CIEDagainst visual judgments for the whole set of samples.

Color-difference formula Factor g

CIEDE2000(1:1:1) 2.98

CIEDE2000(2:1:1) 2.87

CIEDE2000(2:1:1), the 50:50% acceptability threshold was 1.78,

with r2 = 0.68 and optimal number of rules for the TSK Fuzzy

Approximation equal to 4 (Fig. 6b).

Table 1 shows the values of g, CV, VAB and PF/3 for the two

color-difference formulas.

Lower values of the gamma factor, CV and VAB are obtained

in the case of the CIEDE2000(2:1:1) color-diffrence formula,

indicating a better adjustment of this formulas to the visual

data.

4. Discussion

The threshold values should be used in interpretation of

clinical and dental laboratory results in terms of acceptabil-

ity of color differences between natural teeth and dental

restoration. Many industries have established industry color

tolerances (textile, car industry, money printing) and there

is a rising interest to establishing these tolerances for

dentistry, especially if using the most advanced methods

and means. Practical application of technology that quan-

tifies color and color differences in dentistry requires that

color difference formulas provide a quantitative represen-

tation of the visual color difference. Recent works2,14,25

reported that it is expected that the CIEDE2000 color

difference formula should be used in dental research and

other dental applications.

It has been reported that the DEab and DE00 might be used

interchangeably for the evaluation of color differences in

dentistry.26 It should be noted that this interchangeability may

be valid for specific region of color space that corresponds to

human teeth, but it is not necessarily generally valid. Another

study13 reported a significant correlation between DEab and

E2000(1:1:1) and CIEDE (2:1:1) color-difference formulas

CV VAB PF/3

88.98 1.33 139.86

83.03 1.27 132,31

Fig. 5 – Acceptance percentages against: (a) lightness

differences and average lightness of pairs of ceramic discs;

(b) chroma differences and average chroma of pairs of

ceramic discs; (c) hue differences and average hue angle of

pairs of ceramic discs.

j o u r n a l o f d e n t i s t r y 3 9 s ( 2 0 1 1 ) e 3 7 – e 4 4e42

DE00, even if a significant involvement of weighting functions

for lightness, chroma and hue components was observed, thus

recommending the use of DE00.

A set of color samples of dental ceramic was created for this

study. The 50:50% lightness, chroma and hue acceptability

thresholds and the CIEDE2000(1:1:1) and CIEDE2000(2:1:1)

acceptability thresholds were calculated using TSK Fuzzy

Approximation. Data fuzzy modelling represents a flexible

and effective method to model an unknown function from a

set of observed data relative to that function or phenome-

non.20,21 It was previously reported that it enabled soft and

accurate approximations, without limiting the expected shape

of the objective function, thus allowing the adaptation to

unknown shapes on the available data, and being a reliable

alternative for the color threshold calculation procedure.2

The first null hypothesis was rejected. Our results have

shown differences in sensitivity for changes in lightness (DL0),

chroma (DC0) and hue (DH0) in dental color space and the t-test

confirmed that these differences were statistically significant.

This corresponds to the findings obtained using the CIELAB

formula.3,4

Recent experimental results showed that the L* scale gives

too large DL* values for lightness differences both for dark and

for light samples.27 Therefore, in CIEDE2000, lightness is

corrected with a specific weighting function (SL). However, our

results (Fig. 5a) suggest no dependence between DL0 and L0 and

therefore, the lightness correction of CIEDE2000 does not

seem to be significant within the studied area of color space

(the dental color space). This result is in agreement with the

study that showed that the lightness correction is not

statistically significant for color pairs having mainly lightness

difference.28

It is well documented in the literature29 that in the

Euclidean metric of CIELAB, there is an increase of the

tolerance when color differences originated essentially from

differences in croma or hue, or when the difference in chroma

was very large. Our results suggest no dependence between

DH0 and H0 (Fig. 5c), but there was a slight dependence between

DC0 and C0 (Fig. 5b). Further studies on this topic, with new

experimental data, would be beneficial. CIEDE2000 tolerances

for chroma and hue differences are corrected with the specific

weighting functions: SC and SH, respectively.

The results obtained using TSK Fuzzy Approximation

suggested that there were differences in sensitivities between

CIEDE2000 lightness, chroma and hue difference: 2.86 in

lightness, 1.34 in chroma and 1.65 in hue and the statistical

analysis confirmed the differences between lightness and

chroma and between lightness and hue. However, there were

no statistically significant differences between chroma and

hue. This difference between DEL and DEC or DEH could be due

to the fact that the parametric factor of lightness (KL) should

have a different value than KC or KH. The CIEDE2000 DEL, DEC

and DEH were calculated to establish the influence of the

weighting functions, thus providing a basis to justify consid-

ering the use of KL = 2 instead of KL = 1.

The factor PF/3 has been widely used as an indicator for the

performance of color-difference formulas in comparison with

visual results. The results showed that the factor PF/3 for

CIEDE2000(2:1:1) color difference formula had lower values

than for the CIEDE2000(1:1:1) color difference formula.

Moreover, the 50:50% CIEDE2000(1:1:1) and CIEDE2000(2:1:1)

acceptability thresholds calculated using TSK Fuzzy Approxi-

mation were similar (Fig. 6), but CIEDE2000(2:1:1) acceptability

threshold exhibited slightly better r2-value. These results

showed that CIEDE2000(2:1:1) color difference formula per-

formed better than CIEDE2000(1:1:1) in evaluation of accept-

ability thresholds of dental ceramics. Therefore, the second

null hypothesis was rejected.

Fig. 6 – TSK Fuzzy Approximation fitted curve for visual acceptability in percentages versus DE00(1:1:1) and DE00(2:1:1) of

pairs of ceramic discs.

j o u r n a l o f d e n t i s t r y 3 9 s ( 2 0 1 1 ) e 3 7 – e 4 4 e43

The three parametric factors introduced in CIEDE2000 are

set to 1 for so-called reference conditions6 which include

homogeneous samples. The appearance of a surface (texture)

structure may influence color-difference sensation. Some

publications have clearly shown the effect of texture on

perceived color differences.30,31 In general, it has been

assumed that texture affects only lightness tolerance but

does not affect chroma or hue tolerance.11,12,32 Our experi-

mental results showed higher values for CIEDE2000 accept-

ability thresholds for lightness compared to chroma or hue

thresholds, which, as mentioned above, might be due to

surface texture of the sample.

The results of this study suggested considering the use of

KL = 2 instead of KL = 1 when evaluating CIEDE2000 lightness

difference in dentistry due to the high value of the lightness

tolerance compared with chroma and hue tolerances. Consid-

ering KL = 2, the threshold value for CIEDE2000 lightness was

1.43 and the statistical test confirmed that there were no

significant differences between threshold values of lightness

and chroma or hue ( p > 0.1 for both).

Nevertheless, further research is needed to test the

performance of this color difference formula and its adequacy

to accurately predict the perceived color difference in

dentistry. In particular, would be of great interest to expand

the number of colors and types of materials used (dental resin

composites, in vivo teeth, etc.) and comparatively analyse

data, thereby evaluating the degree of disagreement between

observers and calculated color differences.

The parametric factors KL, KC and KH proposed in this study

correspond exclusively to color differences in lightness,

chroma, or hue, which means that the parametric factors

have been computed independently. In the general case, a

color difference includes simultaneously lightness, chroma

and hue differences; thus the three factors must be combined,

and possible interactions should be investigated.

5. Conclusions

Within the limitations of this study, it was found that:

� Differences in sensitivities between CIEDE2000 50:50%

acceptability thresholds for lightness, chroma and hue

differences for dental ceramics (2.92, 2.52 and 1.90,

respectively), were statistically significant.

� The CIEDE2000 formula performed better using KL, KC and KH

parametric factors set to 2:1:1 than 1:1:1, which recom-

mends its usage in dental research and in vivo instrumental

color analysis.

Conflict of interest statement

The author declares there are no conflict of interest.

Acknowledgments

Special thanks to the observers who took part in this

experiment. The authors acknowledge funding support from

the research projects MAT2009-09795 and SAF2010-20558 of

Spanish Ministry of Science and Innovation.

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