Evaluation of pigment location in tinted soft contact lenses

The purpose of this study was to investigate efficient methods to evaluate the pigment layer location of tinted soft contact lenses and to assess various lens products on the market using those methods.

Two types of tinted soft contact lenses with known pigment location embedded or back surface were manufactured and examined. Light microscopy (LM), focused ion beam milling and scanning electron microscopy (FIB-SEM), and Fourier-domain optical coherence tomography (FD-OCT) were used to examine the pigment layer. Lens surface roughness was also measured using atomic force microscopy. In the second part of the experiment, pigment location and surface roughness of a clear lens (Lens A) and eight commercially-available tinted soft contact lenses (Lens B-I) were evaluated using FIB-SEM and FD-OCT.

Pigment location could be reliably determined with FIB-SEM and FD-OCT. With LM, 40% of the lens sections were broken or deformed during slide preparation. The pigment particles in Lens B were buried below the front surface and there were no significant differences of roughness between the front and back surfaces. However, all tinted lenses with surface pigment had significant difference of roughness between front and back surfaces at the pigmented area.

The FIB-SEM and FD-OCT could reliably locate the pigment layer of tinted soft contact lenses. In addition, lens surface roughness was influenced by pigment layer location.