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A prescription of −1.00 +0.25 × 180 describes a lens that has a horizontal power of −1.00 D and a vertical power of −0.75 D. Only ophthalmologists write prescriptions in + cylinder. An optometrist would write a prescription in - (minus) cylinder. All spectacle and contact lenses would be made in minus cylinder.
According to the company, 50 percent of its revenue came from repeat customers in 2013. [citation needed] Customers must have a valid prescription prior to purchasing prescription eyeglasses from Eyebuydirect. [9] The company sells multiple types of lenses, including bifocal, progressive, and Transitions lenses. [5]
Ballistic sunglasses or prescription eyeglasses must meet the same requirements. In brief, the U.S. military standard requires that ballistic eyewear must be able to withstand up to a 3.8 mm (.15 caliber) projectile at 195 m/s (640 ft/s)) for spectacles and 5.6 mm (.22 caliber) projectile at 168–171 m/s (550–560 ft/s) for goggles.
The U.S. military standard (MIL-PRF-31013), requires (at a minimum) that ballistic eyewear can always withstand a 0.15 caliber, 5.8 grain, T37 shaped projectile at a velocity of 640 to 660 feet per second (approximately 3.8 mm 0.376 g at a velocity of 195 – 201 m/s). Goggles are required to stop a 17-grain fragment simulating projectile ...
Higher-quality optical-grade glass materials exist (e.g. Borosilicate crown glasses such as BK7 ( n d = 1.51680 , V d = 64.17 , D = 2.51 g/cm³ ), which is commonly used in telescopes and binoculars, and fluorite crown glasses such as the best optical quality low dispersion glass currently in production, N-FK58 made by the German company Schott ...
Comparison of browline (left) and wayfarer horn-rimmed glasses, with prescription lenses for myopia. A variant of horn-rimmed glasses, browline glasses, became one of the defining eyeglass styles of the 1940s and 1950s. Invented in 1947, the style combined the aesthetics of horn-rimmed glasses with the stability of metal frames by fitting ...
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