| Abstract |
The daily use of Contact Lenses (CL) is widely spread, and leads to the production of a huge number of CLs every year. Therefore, the need for the development of techniques being able to characterize CLs' parameters both qualitatively and quantitatively is huge. In order to have an accurate and comprehensive evaluation of CLs at industrial level, several parameters have to be taken into account such as lens fitting, material hydration, oxygen permeability, edge fitting, etc. Besides precision, accuracy and repeatability, this new technique should be easy to use, relatively fast and with low cost.
In the context of this master thesis, a novel characterization methodology of CLs geometrical and optical properties is presented, based on Optical Resolution Photoacoustic Microscopy (OR-PAM). The OR-PAM imaging is achieved through the detection of the acoustic waves generated in a medium, following the absorption of irradiation emitted by a pulsed or intensity-modulated laser source. Despite the fact that OR-PAM has been employed so far exclusively on biological specimens, we demonstrate that it has the potential to reconstruct both the anterior and posterior surface of a CL, after their respective coating with thin pigmented layers, characterized by different optical absorption properties. More specifically, for the coating of the anterior surface, we used black ink with high absorption at both the near infrared and the visible wavelengths. On the contrary, the posterior surface was covered with red ink of negligible absorption in the near infrared; however of high absorption, mainly in the green spectral region. With the use of two optical wavelengths (1064 and 532 nm), the sequential imaging of the CL's surfaces was achieved, whereas from the product of the temporal difference for the acoustic signals generated at the CL center with the speed of sound, we estimated the respective central thickness. By fitting a second order polynomial to the calculated surfaces, the maps of local radius of curvature and dioptric power were extracted, allowing for the determination of the CL's total power.
For the demonstration of the proposed methodology as a powerful metrology, as well as, a qualitative inspection tool, a large variety of CLs was used such as cosmetic lenses, industrially colored lenses for aniridia cases, soft CLs and semi-rigid CLs. For the latter case, the obtained results through OR-PAM were compared to the values provided by the manufacturer, to show an excellent agreement for the geometrical and optical parameters measured, proving thus the reliability of our approach.
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In conclusion, the implementation simplicity of such an imaging system, in combination to the reliability and the low cost of the followed methodology, render it suitable for broad industrial application concerning the quality inspection and optimization of CLs parameters.
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