# Developments in Contact Lens Imaging: New Applications of Optical Coherence Tomography
## 3 CL Metrology  
The authors of a few published papers have reported new applicability of OCT as a tool for the production of CLs, quality control during the CL production process, and the evaluation of new lens designs. Potentially, an SSOCT-based instrument could measure the base curve of the lenses in the production line, as well as verify the shape and surface quality [57; 58; 59; 28]. In 2010, [PERSON] et al. introduced a novel suitable research tool for imaging an entire soft CL in vitro (immersed in CL solution) and on the eye. They detected the surfaces of the lens and generated thickness maps of the soft CL (Figure 5). It was the first report using SOCT with an extended scan depth and width for directly imaging the entire CL in three dimensions (3D) [28]. [PERSON] and [PERSON] also used OCT to measure the mechanical prism of CLs 0.3 mm from the edge. The prism was defined as the maximum difference in thickness at opposing points on the lens edge and considered to be a significant metric for the quality control of the CL manufacturing process. Figure 5: The whole area of CL in vitro and on the ocular surface imaged by SOCT. Cross-sectional OCT image of a toric soft CL immersed in CL solution (**A**) and in vivo (**D**) and their segmented surfaces: anterior with red color and posterior with green color can be seen in (**B**) and (**E**), respectively. Thickness map of the toric soft CL is the same in vivo (**F**) and in vitro (**C**). The resolution is 6.0 \(\upmu\)m axially and 15.0 \(\upmu\)m transversally. Adapted from [PERSON] et al. [28]. The thickness at this fixed distance from the edge was measured for over 360\({}^{\circ}\). Their automated algorithm and analysis established the capability of SOCT as a tool for nondestructive, fast, and reliable CL metrology [57]. The first method to allow whole-lens, 3D metrology of CL based on SSOCT was reported by [PERSON] et al. in 2014. This quantitative inspection of the lens was performed by the measurement of the samples in a dedicated wet cell with enhanced contrast by increased liquid reflectivity. An auto-positioning and auto-levelling method was developed on the basis of lateral capillary interactions between the wet cell wall and the lens floating on the liquid surface (Figure 6) [58]. The method allows for the visualization of the whole lens with its edges and the extraction of the information about the diameter, thickness, radius of curvature, and optical power in one simple measurement. The quantitative topographic results show good repeatability and match the manufacturers' specifications (Figure 7) [58]. The technique can be used to assess the quality of soft, rigid, and toric CLs and is free from the disadvantages of the standard methods used in lens metrology [57; 58; 59]. Figure 6: Experimental confirmation of the capillary forces between wet cell walls filled with a solution of increased reflectivity and a CL. Concave (**A**) and convex meniscus (**D**) formed at a partially-filled and overfilled cuvette as cross-sectional images. Cross-sections (**B**,**E**) and en face projections (**C**,**F**) showing attraction and repulsion effects (W, cuvette wall; L, contact lens). Adapted from [PERSON] et al. [58]. Figure 7: SSOCT of a CL’s measurable estimation. Averaged thickness map (**A**), standard deviation map of the thickness from 10 tests of the spherical soft CL (**B**), and thickness of the toric soft CL shown as maps (**C**); there is a brightly visible bottom weighting ballast with the axis rotated by \(\alpha=8.9\) deg. Astigmatism of the toric soft CL (**D**). Adapted from [PERSON] et al. [58]. The capabilities of OCT (a custom-built OCT system) have been extended to provide precise thickness evaluation and high-definition 3D imaging of the edge of the soft CL immersed in different environments [60]. [PERSON] et al.