Color stability testing

Theisc samples prepared for color change studies were 20 mm in diameter and 2 mm in thickness.¹⁴ Digital images were acquired from control and experimental specimens to distinguish the contrast from the transparent specimens of the silicone samples. A digital imaging approach with a single-lens reflex camera (Canon, Japan) and a 105 mm camera macro lens (Sigma, Japan) was used.¹⁴

The digital camera was mounted perpendicularly on a stand clamp holder and set to manual mode to control the shutter speed, ISO, and f-stop settings of 1/60 and 5.6. During photography, these measurements remained constant (Figure 4 A, B).

A) Digital camera setting. B) The camera was fixed perpendicularly at a 10 cm distance from the specimen. C) Reading in the color picker palette tab for (L, a & b) parameters.

The digital photographs were downloaded to a laptop and saved as TIFF files. Adobe Photoshop CS6, Ver. 13.0.1, was used to evaluate the images (Adobe Systems, USA). Red, green, and blue values were extracted and converted into (L-a-b) or (v-h-c) values through mathematical modeling.¹⁴

The RGB Lab system was used in the color study, and the Commission Internationale de l'Eclairage (CIE) was used to assess color change (ΔE). Lightness value (L), hue (a), and chroma (b) were measured to assess the color stability of each sample after the addition of CNF to VST-50 silicone.¹⁵ The silicone sample was attached and removed from the surveyor's table, which had been placed in a stable location; the sample was positioned parallel to the camera lens between time points to ensure optimal and repeatable positioning for each measurement.¹⁶

A measurement template with a 60-pixel square area was made in the center of the sample for standardizing calculations. The color information was collected directly from the L, a, and b parameters’ color picker palette tab.¹⁴ (Figure 4, C).

Color coordinates (L, a, and b) of each sample were measured in three groups as follows: