The optical parameters—translucency (TP), contrast ratio (CR), and opalescence (OP)—were recorded under D65 standard illumination on polished and glazed surfaces before and after aging using a spectrophotometer (Easyshade IV; Vita Zahnfabrik, Bad Säckingen, Germany); it is a clinical device that only works in “tooth mode” [37 (link)]. The instrument was calibrated before each measurement, and the probe tip was held at 90° to the surface of the sample. The recording was accepted when two identical consecutive readings were obtained on each area (cervical, incisal, medial) in three randomly selected zones of the polished or glazed surfaces. The measurements were performed by a single operator.
To determine the CIE L*a*b* coordinates—the black (b) and white (w) background of the grey card WhiBal G7 (White Balance Pocket Card) was used.
L* is the lightness coordinate (L* = 0 perfect black, L* = 100 perfect white); a* = is the chromatic coordinate in the red (positive value)/green (negative value) axis, and b* = is the chromatic coordinate in the yellow (positive value)/blue (negative value) axis [38 ,39 (link),40 ].
TP values result by calculating the color difference on black and white backgrounds according to the formula:
TP values may range from 0 (totally opaque) to 100 (totally transparent).
CR defines the opacity and is calculated over a black and white background according to the formula:
CR values may range from 0 (totally transparent) to 1 (totally opaque).
OP values estimate the difference in red–green and yellow–blue color coordinates between transmitted and reflected light; the values are obtained using the formula:
ΔE*—The total color change value, signifying the color difference between two stages, was achieved using the formula:
The recordings were performed for each group in three areas (cervical, medium, incisal).
To report the color change to a clinical standard, ΔE* was converted to NBS units (NBS—The National Bureau of Standards) according to the formula: NBS = ΔE* × 0.92 [34 ,35 ,36 (link),37 (link)]. In conformity with NBS, the levels of color changes (expressed in NBS units) are: extremely slight change (0.0–0.5), slight change (0.5–1.5), perceivable (1.5–3.0), marked change (3.0–6.0), extremely marked change (6.0–12.0), and change to another color (12.0–more).
To determine the CIE L*a*b* coordinates—the black (b) and white (w) background of the grey card WhiBal G7 (White Balance Pocket Card) was used.
L* is the lightness coordinate (L* = 0 perfect black, L* = 100 perfect white); a* = is the chromatic coordinate in the red (positive value)/green (negative value) axis, and b* = is the chromatic coordinate in the yellow (positive value)/blue (negative value) axis [38 ,39 (link),40 ].
TP values result by calculating the color difference on black and white backgrounds according to the formula:
TP values may range from 0 (totally opaque) to 100 (totally transparent).
CR defines the opacity and is calculated over a black and white background according to the formula:
CR values may range from 0 (totally transparent) to 1 (totally opaque).
OP values estimate the difference in red–green and yellow–blue color coordinates between transmitted and reflected light; the values are obtained using the formula:
ΔE*—The total color change value, signifying the color difference between two stages, was achieved using the formula:
The recordings were performed for each group in three areas (cervical, medium, incisal).
To report the color change to a clinical standard, ΔE* was converted to NBS units (NBS—The National Bureau of Standards) according to the formula: NBS = ΔE* × 0.92 [34 ,35 ,36 (link),37 (link)]. In conformity with NBS, the levels of color changes (expressed in NBS units) are: extremely slight change (0.0–0.5), slight change (0.5–1.5), perceivable (1.5–3.0), marked change (3.0–6.0), extremely marked change (6.0–12.0), and change to another color (12.0–more).
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