<prompt> In specular microscopy, endothelial density is measured byOptical doublingFixed frame analysisOptical focusingNone  <response>: In specular microscopy, endothelial density is measured by Fixed frame analysis. Image : Fixed-frame counting principle andillustration of the three types of endothelial mosaics analyzed with the fixed-frame method. aOnly grey cells included within theframe and cut exclusively bytwo adjacent borders werecounted. bComputer-generated regular hexagonal mosaics, ccomputer-generated irregular mosaic,in order to mimicpolymorphic and pleomorphic mosaics, drealhuman mosaic after Alizarinred staining of cornealendothelium with illustration of the eleven grids . Reference : AK kh 