Bench Checks of Contact Lens Parameters: Radius and Power

What is a bench check of a contact lens?

A bench check is the off-eye measurement of a finished lens against the order that produced it.

It answers one question: did the laboratory deliver what was specified? Four parameters carry most of that answer, and each has its own measurement.

• Back optic zone radius — the back surface curvature, read on a radiuscope
• Back vertex power — the dioptric power, read on the bench power instrument
• Total diameter — the overall edge-to-edge size, read on a gauge or a comparator
• Center thickness — the axial thickness, read on a dial gauge

None of those four is a fit assessment. Fit is what the eye does with the lens, and it is observed at the slit lamp on Contact Lens Fit Assessment. The bench answers the narrower and earlier question, and answering it first is what stops a manufacturing deviation being investigated as a fitting problem for two visits.

The instrument names below are the classical ones from the standard and the textbooks. This page names an instrument in order to name the parameter it reads. It does not review, price, rank or recommend one.

How is base curve radius read on the bench?

Back optic zone radius is read on a radiuscope — or radiusgauge — from the separation between the real and the aerial image of a spoke target.

According to Ento Key, Verification of Gas-Permeable Lenses (Vinita Allee Henry, Clinical Manual of Contact Lenses), the base curve radius is one of the most important parameters to check because it sets the lens-to-cornea fitting relationship, and the radiuscope or radiusgauge is the most commonly used method of determining it. The lens sits clean and dry, concave side upward, on a small amount of water in the depression of the lens mount; submerging the lens or letting fluid onto the concave surface gives an inaccurate reading. Viewed through the oculars, the real image appears as a spoke pattern near zero on the scale and the aerial image roughly 6 to 9 mm along it. The instrument is zeroed on the focused real image, and the radius is read at the focused aerial image.

That reading is taken on a millimeter scale, to the nearest hundredth. A typical result is recorded as 8.20 mm. The same chapter notes that several automated keratometers have been found to measure gas permeable base curve radius within tolerance.

Two timing rules travel with the measurement:

• Read the radius before dispensing, and again after 12 to 24 hours of soaking in an approved disinfecting solution, because gas permeable lenses may flatten on hydration
• Read the front (convex) radius as well when the design is a front toric or bitoric; a different lens mount is used and the image order reverses

The radiuscope also separates three things that look alike from the chair. If the spokes do not all focus together, the lens carries toricity or warpage: one set of spokes focuses at the steeper curve, the set 90 degrees away at the flatter. Flexure imitates both — a flexing lens can look warped immediately on removal and return to its spherical state soon after — and any of the three can produce a spherocylindrical over-refraction. Read the radius before treating blur as a power problem.

Peripheral curve radius is measurable by the same method only when the curve is at least 1 mm wide, which is why it is typically not a parameter the practitioner checks.

The full gas permeable procedure, in sequence and with the ordered-parameter context around it, is on RGP Contact Lens Parameters and Starting Power. What the radius means as a fitting parameter, rather than as a measurement, is on Base Curve of Contact Lenses.

How is back vertex power checked on the bench?

Back vertex power is read with the lens clean, dry and concave against the lens stop of the bench power instrument, using a contact lens support.

Back vertex power is the value normally checked. Ento Key notes that back and front vertex power differ very little in low-power lenses but can differ by as much as 1 D in high prescriptions, so where there is doubt about which surface a laboratory specified, the laboratory is the place to settle it.

The handling matters more than the arithmetic:

• Tilt the instrument upright so the lens rests on the stop under its own weight, or hold it gently against the stop with thumb and forefinger
• Take care not to flex the lens; a flexed lens reads as a power error
• Most bench instruments accept a contact lens accessory device on the lens stop, which improves accuracy. ANSI Z80.20 illustrates two of these supports — an annular cylinder stop and a paddle stop — in its own figures

Toric lenses are recorded differently depending on how they were made. A front toric with prism is placed with the prism base down and the result is written in spherocylindrical notation; Ento Key’s worked example records one prism diopter base down with drum powers of −1.00 D and −3.00 D at 90 as −1.00 −2.00 × 90. A bitoric or back toric carries no prism to orient it, so the power is not recorded as a spherocylinder at all — it is recorded as the value found in each meridian, alongside the two radii.

Prism itself is read with the lens centred, concave side against the stop, against the concentric reticule rings that mark 1 to 5 prism diopters. Prism is used mainly in front toric designs to resist rotation; what that rotation looks like once the lens is on an eye is on Assessing Toric Contact Lens Rotation at the Slit Lamp.

What changes when the lens is soft?

A soft lens dehydrates in air, and every parameter moves while it does.

According to Ento Key, Soft Lens Measurement (Contact Lens Practice, chapter 7), back vertex power is measured over a 4.5 mm central aperture in air, and the preparation is the measurement: clean the lens, dab both surfaces dry, centre it on the support, and read it quickly. Exposing the lens for longer than around 10 seconds dehydrates it enough to change the optical power.

Repeatability is correspondingly modest. That chapter cites a ring test across five laboratories in which standard deviations of 0.10 to 0.30 D were achieved for sphere and cylinder over 10 repeated measurements of low-powered toric lenses on a manual bench instrument. A single reading that disagrees with the order by a quarter of a diopter is not, by itself, evidence of a manufacturing deviation.

Laboratory-grade measurement avoids the problem by conditioning the lens and keeping it wet:

• Soak in standard phosphate-buffered saline at pH 7.4 ± 0.1 and 310 ± 5 mOsm/kg for at least 30 minutes, and measure at 20 °C ± 0.5 °C. Sterile 0.9% saline is the practice substitute
• Measure in a saline-filled cuvette rather than in air, then convert the in-saline power back to in-air power

That conversion carries a warning worth knowing even if you never perform it. Because the refractive index of saline (1.3345) sits close to that of lens materials (about 1.39 to 1.42), an in-saline measurement has to be roughly five times more accurate than the equivalent in-air one to reach the same confidence, and both indices have to be known precisely. An error of 0.0005 in an assumed index of 1.4100 shifts the reported power of a −6.00 D lens by 0.35 D.

Toric and multifocal soft lenses need more than a single reading in any case: a toric requires power along at least two meridians, and a multifocal is characterised by its power profile across the diameter rather than by one number.

How are diameter and center thickness measured?

Center thickness is read on a dial gauge, and total diameter on a gauge or projection comparator specified by the standard itself.

Ento Key describes the thickness measurement directly: the lens is positioned on the gauge and a plunger lowered onto it with the thumb, and the thickness is read off the dial — a needle on 23 records 0.23 mm. Moving the lens under the plunger changes which part is measured, so the same instrument reads center thickness and edge thickness. That chapter lists the cases in which thickness is worth checking: flexure, oxygen transmission, comfort and lens position.

For diameter, the standard is explicit about apparatus. The published contents of ANSI Z80.20-2010 name a V-groove diameter gauge — with its own specification table and figure — and a projection comparator, alongside a microsphereometer for radius and a dial gauge for center thickness.

What each parameter means as a fitting decision, rather than as a measurement, is on Center Thickness of Contact Lenses and Diameter of Contact Lenses.

How do you judge a bench reading in or out of specification?

A measurement is not a verdict. It becomes one when it is compared with the published tolerance band for that parameter.

Two standards publish those bands: ANSI Z80.20 in the United States and ISO 18369-2 internationally, and Z80.20’s own foreword states it was written to conform to the international standard. The bands differ by lens type — a gas permeable back optic zone radius and a soft back optic zone radius are not toleranced alike — and by power range.

The tables are on Contact Lens Parameter Tolerances Under ANSI Z80.20, which is where the in-or-out judgement is made. Reading a number here and judging it there is the intended order; this page does not restate the bands.

What does a bench check not tell you?

A lens can measure inside every band and still fit badly.

Radius, power, diameter and thickness describe the lens. They do not describe the lens on an eye, and the eye supplies terms no bench measurement contains — corneal and scleral shape beyond the limbus, lid tension, tear film. Three findings in particular are never available on the bench:

• Centration, movement and lag, read at the slit lamp on Contact Lens Fit Assessment
• Rotation and its stability on a toric lens, on Assessing Toric Contact Lens Rotation at the Slit Lamp
• Residual refractive error through the settled lens, which is measured rather than deduced — calculate it on the Over-Refraction Calculator for Contact Lens Parameters

The reverse holds too, and it is the reason to check at all. Blur through a correctly ordered lens is sometimes a radius that arrived wrong, and no amount of over-refraction will find that. Verify power with over-refraction after the trial lens; verify the lens against the order before the trial lens.

Bench checks are performed by a practitioner, not by a wearer

Every measurement on this page is made by a licensed eye-care practitioner, or by a trained technician working under one, on instruments calibrated for the purpose.

There is no domestic equivalent. A lens cannot be checked against its order by looking at it, by comparing it with an older lens, or by wearing it to see what happens. A lens suspected of being out of specification goes back to the supplying laboratory.

Read Starting Contact Lens Parameters Are Not a Prescription for the bound that applies across this site. Convert spectacle prescription to contact lens parameters for power. Check the delivered lens against the order on the bench. Fit it, and verify power with over-refraction after the trial lens. Contact lens parameters at the corneal plane stay unfinished until that sequence is done.

Sources

Clinical claims on this page are attributed to the publications below.

• Ento Key, Verification of Gas-Permeable Lenses(Vinita Allee Henry, Clinical Manual of Contact Lenses) — radiuscope and radiusgauge procedure; real and aerial spoke images; millimeter scale to the nearest hundredth with 8.20 mm as a typical record; re-read after 12 to 24 hours of soaking because gas permeable lenses may flatten on hydration; automated keratometers within tolerance; toricity, warpage and flexure; peripheral curve readable only at 1 mm width or more; back vertex power method, lens stop and contact lens accessory device; up to 1 D front-versus-back difference at high power; front toric recorded prism base down in spherocylindrical form and bitoric recorded per meridian; prism against the 1–5 prism diopter reticule rings; dial gauge thickness with the 0.23 mm example.
• Ento Key, Soft Lens Measurement(Contact Lens Practice, chapter 7) — back vertex power over a 4.5 mm central aperture in air; clean and dab dry; the approximately 10-second dehydration limit; a five-laboratory ring test with 0.10–0.30 D standard deviations on manual measurement of low-powered torics; phosphate-buffered saline conditioning at pH 7.4 ± 0.1 and 310 ± 5 mOsm/kg for at least 30 minutes at 20 °C ± 0.5 °C; wet-cell measurement, the in-saline to in-air conversion and the five-times-accuracy consequence of the refractive index gap; toric and multifocal measurement requirements.
• ANSI Z80.20-2010, Ophthalmics — Contact Lenses — Standard Terminology, Tolerances, Measurements and Physicochemical Properties(published preview) — scope and contents: V-groove diameter gauge, projection comparator, center thickness by dial gauge, microsphereometer, and the annular cylinder and paddle contact lens supports; the foreword statement that the revision was written to conform to the International Standard.

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