Interference pattern deviations analysis

According to Eqs. (¹) and (²), we hypothesized that interference pattern could be used to estimate the aragonite tablet thickness of nacreous pearl. Conchiolin layer is extremely thin so its thickness and index (d₂, n₂) can hardly be accurately measured. We generated the simulations with the range of these two parameters showing the possible interference pattern deviations in terms of hue (Fig. 6a–d), with the approximation of n₁ being constant 1.685.

Interference pattern simulations with (a)–(d) showing the possible hue patterns; (c) the high saturation associated with thicker conchiolin layer; (e) the low saturation associated with lower conchiolin index; and (f) similar patterns with D65 as illuminant.

With an additional insight into the interaction between light waves and the repetitive double layers (^{Supporting Information} Section ²), we concluded that slightly thicker conchiolin layer results higher chroma in color, and higher conchiolin index results lower saturation. So Fig. 6c shows the highest chroma and Fig. 6e illustrates a possibly lowest saturation with an imaging processing of whiteness enhancement as the ‘dilution’ from the wavelength-independent reflection.

As was hypothesized and theoretically confirmed that the interference pattern on round nacre surface is pretty sustainable to slight nanostructure variations. We further showed its sustainability under different illumination condition by simulating the patterns with illuminant D65 in Fig. 6f, which is the only series with illuminant D65 in this article.