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ciba uses molds made of transparent quartz glass, which can be reused, unlike the plastic molds used for hydrogel lenses. a drop of nelfilcon a is placed onto one half of the mold, and contact the complementary half is suspended a few micrometers away. a circular mask is placed around the edge of the lens, and ultraviolet light is shined through the molds to cure the polymer. the mask blocks the uv light and creates a precise, smooth rim. this allows for a consistent shape with an edge half the thickness of lenses other lenses. a thinner edge reduces what''s known as edge sensation, discomfort caused by the eyelid rubbing against the lens rim. a lens is about 14 millimeters in diameter, but only the center portion, 8 mm across, is shaped to correct vision.

ciba uses molds made of transparent quartz glass, which can be reused, unlike the plastic molds used for hydrogel lenses. a drop of nelfilcon a is placed onto one half of the mold, and contact the complementary half is suspended a few micrometers away. a circular mask is placed around the edge of the lens, and ultraviolet light is shined through the molds to cure the polymer. the mask blocks the uv light and creates a precise, smooth rim. this allows for a consistent shape with an edge half the thickness of lenses other lenses. a thinner edge reduces what''s known as edge sensation, discomfort caused by the eyelid rubbing against the lens rim. a lens is about 14 millimeters in diameter, but only the center portion, 8 mm across, is shaped to correct vision.

another phenomenon associated primarily with hard lenses is "spectacle blur." when a hard lens is placed on the cornea, the surface topography of the eye is disposable contact lenses altered, so the patient may have visual disturbances with glasses immediately after removing the lens. spectacle blur, in general, is cleared in about 30 minutes, but use of more oxygen-permeable lenses minimizes the problem. advantages of hard lenses include favorable specific gravity (1.18-1.20) and refractive index (1.49-1.50), excellent visual acuity, durability, ability to withstand weak alkalis or weak acids, and no sensitivity reactions when placed on the cornea. hard lenses are also relatively resistant to the absorption of disinfectants, surfactant cleaners and environment pollutants; they are easy to handle during insertion and removal. disadvantages of hard lenses include a longer adaptation time, poor oxygen permeation contact and ocular discomfort.

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