ANSI Standards for Cylinder, Lens Clock, and Assembled Lenses · 5 min read
ANSI Standards for Cylinder, Lens Clock, and Assembled Lenses
Introduction to ANSI Z80.1
ANSI Z80.1 is the primary standard governing prescription ophthalmic lenses in the United States. This standard establishes tolerances for lens power, cylinder axis, prism, and other optical parameters. Understanding these tolerances is essential for verifying that finished eyewear meets quality standards.
The standard applies to all prescription ophthalmic lenses intended for use in dress eyewear. It covers single vision lenses, multifocal lenses, and progressive addition lenses. The tolerances specified represent the maximum acceptable deviation from the prescribed values.
Spherical Power Tolerances
Spherical power tolerances depend on the magnitude of the prescribed power. The tolerances become tighter as the power increases because small errors in high-power lenses have a greater effect on vision.
For meridian powers from 0.00 to ±6.50 diopters, the tolerance is ±0.13 diopters. For meridian powers greater than ±6.50 diopters, the tolerance is ±2% of the meridian power. These tolerances apply to each principal meridian of the lens.
Cylinder Power Tolerances
Cylinder power tolerances also depend on the magnitude of the cylinder. The cylinder power is the difference between the two principal meridians of a lens with astigmatic correction.
For cylinder powers from 0.00 to 2.00 diopters, the tolerance is ±0.13 diopters. For cylinder powers from 2.12 to 4.50 diopters, the tolerance is ±0.15 diopters. For cylinder powers greater than 4.50 diopters, the tolerance is ±4% of the cylinder power.
Cylinder Axis Tolerances
Cylinder axis accuracy is critical because axis errors cause visual distortion and discomfort. The tolerance for axis accuracy depends on the cylinder power because the effect of axis error is proportional to cylinder magnitude.
For cylinder powers from 0.00 to 0.37 diopters, the axis tolerance is ±7 degrees. For cylinder powers from 0.50 to 0.75 diopters, the axis tolerance is ±5 degrees. For cylinder powers from 0.87 to 1.50 diopters, the axis tolerance is ±3 degrees. For cylinder powers greater than 1.50 diopters, the axis tolerance is ±2 degrees.
Understanding the Lens Clock
A lens clock, also known as a lens measure or Geneva lens gauge, is a mechanical device used to measure the surface curvature of a lens. It consists of three pins arranged in a straight line. The two outer pins are fixed, and the middle pin moves to conform to the lens surface. The movement of the middle pin is translated into a diopter reading on the dial.
Principles of Operation
The lens clock measures the sagittal depth of a surface over a fixed chord length. The chord length is the distance between the two outer pins. The sagittal depth is the vertical distance from the chord to the lens surface at the midpoint.
The relationship between surface curvature and sagittal depth is expressed mathematically, and the lens clock is calibrated to display the result in diopters. However, this calibration is based on a specific refractive index, typically 1.53, which represents crown glass.
Calibration Considerations
Because the lens clock is calibrated for a refractive index of 1.53, readings must be converted when measuring lenses made from materials with different refractive indices. The formula for conversion is:
True Surface Power = (Lens Clock Reading × (n - 1)) / 0.53
Where n is the actual refractive index of the lens material.
For example, when measuring a polycarbonate lens with a refractive index of 1.586, a lens clock reading of +6.00 would correspond to a true surface power of approximately +6.63 diopters.
Using the Lens Clock
To use a lens clock, place the instrument perpendicular to the lens surface with all three pins making contact. Read the diopter value from the dial. Positive values indicate convex surfaces, and negative values indicate concave surfaces.
When measuring the front and back surfaces of a lens, the approximate lens power can be estimated by adding the front surface power and back surface power. This method provides only an approximation because it does not account for lens thickness, which affects power in higher-power lenses.
Assembled Lens Tolerances
Assembled lenses, meaning lenses mounted in frames, must meet additional tolerances beyond those for lenses alone. These tolerances address the relationship between the two lenses and their positioning within the frame.
Horizontal Prism Imbalance
When measuring horizontal prism at the distance reference point, the allowed imbalance between the two lenses depends on the combined power in the horizontal meridian. For vertical prism, the tolerance is generally 0.33 prism diopters for prescriptions without prescribed vertical prism.
Pupillary Distance Tolerances
The distance between the optical centers of the two lenses must correspond to the patient's pupillary distance within specified tolerances. The binocular PD tolerance is ±2.5 mm for distance PD measurements. Monocular PD tolerance is ±1.0 mm for each eye measured individually.
Segment Height Tolerances
For multifocal lenses, the segment height must be within ±1.0 mm of the specified measurement. The difference in segment height between the two lenses should not exceed 1.0 mm unless intentionally prescribed.
Optical Center Vertical Position
The optical centers of both lenses should be at the same vertical height within 1.0 mm unless vertical prism is prescribed. Vertical imbalance can cause discomfort and diplopia.
Verification Procedures
Verification of finished eyewear requires systematic measurement of all specified parameters. The basic verification procedure includes measuring lens power in both meridians, calculating sphere and cylinder power, measuring cylinder axis, measuring optical center position, measuring pupillary distance, measuring segment height for multifocals, measuring prism if prescribed, and inspecting lens quality for defects.
Lensometer Use
The lensometer, also called a focimeter or vertometer, is the primary instrument for measuring lens power. It provides readings for sphere power, cylinder power, cylinder axis, and prism.
To use a manual lensometer, place the lens in the instrument with the back surface against the lens stop. Focus the target by adjusting the power wheel. Read the sphere power when one set of lines is in focus and the cylinder power and axis when both sets of lines are focused. The difference between the two readings is the cylinder power.
Automated lensometers perform these measurements electronically and display the results digitally. They are faster but should be verified periodically against calibration standards.
Quality Inspection Standards
Beyond optical measurements, lenses must meet quality standards for physical defects. ANSI standards specify acceptable levels of scratches, pits, bubbles, inclusions, and other defects.
The central 30mm diameter zone of the lens, which corresponds to the primary viewing area, has stricter requirements than the peripheral zone. Defects in the central zone are more likely to interfere with vision and should be rejected.
Surface defects are evaluated under controlled lighting conditions. A defect that is visible under normal wearing conditions is generally unacceptable, regardless of its size.
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