Practical Ophthalmic Lenses |
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180 meridian angle of incidence aperture apical angle astigmatic lens axis meridian back vertex power base curve base setting bifocal lenses calculated centimetres concave convex lens crossline chart cylindrical surface d₁ datum centre DC X 90 decentration determine deviation diameter distance optical centre equation experiment F₁ F₂ filters focal length focal power focimeter front surface power fused bifocal glass block Hence horizontal meridian horizontal prismatic effect incident ray lens measure lens surface lenses light main lens master curve metres millimetres negative lens neutralizing set object obtain ophthalmic optical axis optical bench parallel plane mirror positive lens power meridian powers F₁ prescription principal meridians prism dioptres Prism due Procedure protractor radius of curvature ray box Record your results reflected refractive index segment top shown in Figure spectacle sph.-cyl spherometer straight edge tangent scale thickness thin lens toroidal toroidal surface transverse test unknown lens visual point