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These days, typical values of oxygen permeability for hydrogel contact lenses range from 25 to 50. For example, Nelfilcon A has a Dk value of 26, and the Omafilcon A has a Dk of 25. While those numbers are typical of hydrogel contact lenses, many contact lenses are made of silicone hydrogel , which has a much higher oxygen permeability.
Ultra-high temperature processing (UHT), ultra-heat treatment, or ultra-pasteurization [1] is a food processing technology that sterilizes liquid food by heating it above 140 °C (284 °F) – the temperature required to kill bacterial endospores – for two to five seconds. [2]
The examiner adds convex lenses in +0.25 increments until the patient first reports that they become blurry. [2] The total value of the lenses added to reach this point is the NRA value. High NRA values (above +2.50) might be evidence to over minus, uncorrected hyperopia or latent hyperopia.
The aim of an accurate intraocular lens power calculation is to provide an intraocular lens (IOL) that fits the specific needs and desires of the individual patient. The development of better instrumentation for measuring the eye's axial length (AL) and the use of more precise mathematical formulas to perform the appropriate calculations have significantly improved the accuracy with which the ...
The industry standard is to report Dk/t values measuring lens thickness at the center of a -3.00 D lens. [12] [13] The higher a contact lens wearer's prescription is, the thicker their lens is overall, which decreases Dk/t. Minus lenses are thicker away from the center, while plus lenses are thicker at the center, which decreases Dk/t in those ...
BVD Back vertex distance is the distance between the back of the spectacle lens and the front of the cornea (the front surface of the eye). This is significant in higher prescriptions (usually beyond ±4.00D) as slight changes in the vertex distance for in this range can cause a power to be delivered to the eye other than what was prescribed.
Values of range from below 25 for very dense flint glasses, around 34 for polycarbonate plastics, up to 65 for common crown glasses, and 75 to 85 for some fluorite and phosphate crown glasses. Most of the human eye's wavelength sensitivity curve, shown here, is bracketed by the Abbe number reference wavelengths of 486.1 nm (blue) and 656.3 nm (red)
Opticians may recommend "anti-reflection lenses" because the decreased reflection enhances the cosmetic appearance of the lenses. Such lenses are often said to reduce glare, but the reduction is very slight. [3] Eliminating reflections allows slightly more light to pass through, producing a slight increase in contrast and visual acuity.