A-A-53088
Refractive power tolerance for the finished lens shall be as follows:
Sphere 0.00 thru 6.50 diopter is ± 0.3D
Greater than +6.50 diopter or less than -6.50 diopter is ±2%
Cylinder -0.25 thru -2.00 diopter is ± 0.13D
-2.12 thru -4.50 diopter is ±0.15D
Less than -4.50 diopter is ± 4%
The base curve on minus lens shall have certain dioptric power commensurate with the curve established by the major lens manufacturers. Flat base curve is not acceptable. Shall be to specified axial back vertex power within a tolerance of plus or minus 0.06 diopter. Center thickness of minus lens shall not be less than 2.2mm. The corrective curve shall be generated on the inside of the lens.
The semifinished lens shall have the base curve located on convex side only.
The white glass lens shall be made of high grade white ophthalmic quality crown glass with an index of refraction of 1.5230 ± 0.0015. The lens shall be free of bubbles, inclusions, striae and cloudiness. When exposed to sunlight, it shall remain clear and free of color or shade.
The neutral absorption lens shall be neutral gray density commonly referred as gray No. 3, daylight transmission with dominant wavelength between 570 to 600 millimicrons range shall be 20 ± 1 percent, near infra-red transmission shall be 20 percent maximum and erythermal ultra-violet transmission shall be 0.2 percent maximum, each to be determined on a glass thickness of 2.2 ± 0.2 mm.
The 25 mm straight top bifocal lens blank shall have the major blank made of white ophthalmic crown glass with an index of refraction of 1.523 ± .0015. the segment shall also be white, made of barium glass. The dispersion of the segment glass, Nu value (V) shall be equal to, or greater than the value defined by the formula:
Where n is the index of refraction of the segment glass for the sodium D line.
Segment size shall be 25 mm (+0.75/-0.50) wide by 17-1/2 mm (±0.5) high and decentered 3.00 mm (+ 1.5/1.0) to the right and left of the vertical midline. The distance from the geometric center of the major blank to the top of the segment shall be 3 to 6 mm. The optical center of the segment shall be 5 mm ± 0.5 below the flat top and midway between the lateral edges of the segment. Finished distance power shall be within ± 0.06 diopter of specified power. Segment addition shall be within + 0.06D/-0.09D of specified power.
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