The grazing angle broadened surface features for a more detailed analysis. HEMA (hydroxyethylmethacrylate) was identified as the base material of the lens and the near surface had two pigment components, Fe oxide and Ti oxide, distributed as particulate (Fig. 2).
The BSE (backscattered electron) image (Fig. 3) of the normal angle cross-section helped to visualize the spatial distribution of pigments, present as particles of different sizes and distributed at various depths in different locations. The range of depth distribution of pigments obtained from the BSE image was approximately from 0.4µm to 7.6µm.
The XPS analysis was started on the outer surface of the darkest circle near the circumference of the lens. Only carbon, nitrogen, oxygen, and silicon were detected in the surface survey spectrum, while no trace of pigment components were observed (Fig. 4). The XPS depth profile was carried out using a C60 ion gun to minimize damage to organic materials during sputtering. The profile showed that the Fe oxide pigment was present at much higher concentration than the Ti oxide one (Fig. 5a). Additionally, the Ti oxide signal was not observed until much deeper than the Fe oxide (Fig.5b). Both were found underneath the organic layer without any surface exposure.
In conclusion, a combination of analytical methods showed that the examined pigmented lens contained majority Fe oxide and minority Ti oxide pigments present in the form of particles distributed over a wide depth and lateral range, however, the pigment particulates were not exposed at the surface of the lens; therefore, they were not likely to cause abrasive damage to the eye or surrounding tissue.