To measure the sagittal height (SAG) of a contact lens, use a non-contact optical method such as Low Coherence Interferometry that captures the complete three-dimensional structure of the lens. SAG is the vertical distance from the center of the lens’s back surface to a flat reference plane intersecting the lens at its outermost edges – in effect, how deep the bowl-shaped back surface is. The Rotlex MCT-3000 measures SAG with ±1.0 µm accuracy across a range of 0.5 mm to 5.0 mm, covering all contact lens types from RGP to large-diameter scleral designs. Because SAG determines how a lens fits on the eye, accurate measurement is essential, especially for specialty lenses such as Ortho-K designs.
Sagittal height, commonly known as SAG, is one of the most critical yet challenging contact lens measurements to capture accurately. It determines how a lens fits on the eye, affecting comfort, vision, oxygen transmission, and ocular health. This guide explains what SAG is, how to measure it accurately, and why the measurement method matters so much for specialty lenses.
What Is Sagittal Height (SAG)?
Sagittal height (SAG) is the vertical distance from the center of the lens’s back surface to a flat reference plane that intersects the lens at its outermost edges. In simpler terms, it measures how deep the bowl-shaped back surface of the lens extends from its edge to its center. A deeper bowl has a higher SAG; a shallower bowl has a lower SAG.
SAG is a powerful parameter because it captures the complete curvature characteristics of the lens in a single number. Where base curve describes the central curvature alone, SAG reflects how the entire back surface relates to the eye it will sit on. This is why SAG, rather than base curve alone, often best predicts how a lens will actually fit. A lens with SAG that matches the eye’s sagittal depth centers properly and moves appropriately with each blink; a mismatched SAG causes either excessive movement or insufficient tear exchange.
SAG directly determines several critical performance factors:
- Lens fit on the eye: SAG defines the physical relationship between the lens and the cornea, governing centration and movement.
- Tear layer thickness: the gap between the lens and the eye, which depends on SAG, affects oxygen and tear exchange.
- Comfort and vision: a well-matched SAG produces stable, comfortable, clear vision; a mismatch degrades all three.
Why SAG Is Challenging to Measure
SAG is challenging to measure because it requires capturing the geometry of the entire back surface, not just a single point. Many traditional measurement methods capture only a two-dimensional profile or a single curvature value, which does not fully describe the three-dimensional bowl shape that SAG represents.
Traditional imaging methods that capture only a two-dimensional profile miss information about the complete surface, and they can struggle with the transparent, curved, and sometimes flexible nature of contact lenses. Measuring SAG accurately requires a method that captures the complete three-dimensional structure of the lens and isolates the lens surfaces from any surrounding medium. This is why the measurement technology, more than any other factor, determines whether SAG can be measured accurately.
How to Measure SAG Accurately
The most accurate way to measure SAG is with a non-contact optical method that captures the complete three-dimensional structure of the lens. The Rotlex approach to SAG measurement uses Low Coherence Interferometry (LCI) to measure SAG without contact, in real time, capturing the full three-dimensional lens structure rather than a two-dimensional profile. Unlike traditional imaging methods, LCI measures the complete structure and can detect up to 20 distinct layers in a single sub-second measurement, providing a comprehensive characterization of the lens geometry.
The MCT-3000 measures SAG with ±1.0 µm accuracy across a range of 0.5 mm to 5.0 mm, which covers all contact lens types from rigid gas permeable (RGP) lenses to large-diameter scleral designs. Because the measurement is non-contact, it does not deform soft or flexible lenses, and because it isolates the lens surfaces from any surrounding fluid, it can measure SAG accurately even for lenses measured in a liquid medium.
| Measurement Aspect | Requirement for Accurate SAG | Why |
|---|---|---|
| Dimensionality | Full 3D structure capture | SAG describes the whole bowl shape, not one point |
| Contact | Non-contact | Avoids deforming soft/flexible lenses |
| Range | Cover 0.5–5.0 mm (RGP to scleral) | Different lens types span a wide SAG range |
| Accuracy | ±1.0 µm or better | Specialty lenses need micron-level SAG accuracy |
| Medium handling | Isolate lens from surrounding fluid | Enables measurement in hydrated/liquid conditions |
The practical workflow is efficient: the lens is positioned, the system captures the complete three-dimensional structure optically in a sub-second measurement, and the SAG value is determined from the back surface geometry relative to the edge reference plane. Because the measurement is fast and non-contact, it can be performed at production speeds as well as in the laboratory.
SAG Measurement for Specialty Lenses
Accurate SAG measurement becomes especially critical for specialty contact lenses, where SAG tolerances are far tighter than for conventional lenses. Two categories deserve particular attention: orthokeratology (Ortho-K) lenses and large-diameter scleral lenses.
Orthokeratology lenses reshape the cornea overnight, and their SAG must be extremely precise. For Ortho-K lenses, a SAG error of just 10 micrometers can cause 0.25 diopters of unwanted corneal reshaping, so SAG accuracy of ±10 µm or better is required. A measurement system achieving ±1.0 µm accuracy comfortably exceeds this demanding tolerance, providing the precision that predictable Ortho-K outcomes require. Because Ortho-K is frequently used for myopia control in children, this precision directly affects treatment success during critical developmental years.
Scleral lenses, which vault over the entire cornea and rest on the sclera, also depend on accurate SAG. Their large diameter and deep sagittal depth place them at the high end of the SAG range, and accurate SAG measurement is essential for achieving the correct vault over the cornea. A measurement range extending to 5.0 mm covers even these large-diameter, high-SAG designs, ensuring the full spectrum of specialty lenses can be verified.
Common Mistakes in SAG Measurement
Several common mistakes lead to inaccurate SAG measurement. Avoiding them is part of measuring SAG reliably.
The first mistake is relying on base curve as a proxy for SAG. Base curve describes only the central curvature and does not capture the full bowl shape that determines fit. Two lenses with the same base curve can have different SAG values and therefore fit differently. Measuring SAG directly, rather than inferring it from base curve, avoids this error.
The second mistake is using a two-dimensional profile method that captures only a cross-section rather than the complete three-dimensional structure. SAG reflects the whole back surface, and a method that measures only a profile can miss asymmetries and surface features that affect the true SAG. Full three-dimensional measurement avoids this error.
The third mistake is using a contact method on soft or flexible lenses, which deforms the lens and corrupts the SAG measurement. As with center thickness, non-contact measurement is essential for accurate SAG on flexible materials. The fourth mistake is using a measurement range too narrow to cover the lens type – a system that cannot reach the deep sagittal depth of scleral lenses cannot measure them. Confirming the measurement range covers your lens types avoids this error.
SAG Measurement in Production and QC
In manufacturing, SAG measurement must be fast enough to keep pace with production and reliable enough to support quality decisions. A non-contact optical system measuring in under a second enables SAG verification at production rates, and combined with thickness and other parameters in a single measurement, it provides comprehensive geometric verification. For regulated contact lens manufacturing, the measurement records support a manufacturer’s compliance efforts with electronic records requirements such as FDA 21 CFR Part 11. The measurement system supports the manufacturer’s compliance efforts; compliance itself is established through the complete quality system. Fast, accurate, documented SAG measurement turns a challenging parameter into a routine part of the quality workflow.
Frequently Asked Questions
What is sagittal height (SAG) in a contact lens?
Sagittal height (SAG) is the vertical distance from the center of the lens’s back surface to a flat reference plane intersecting the lens at its outermost edges – in effect, how deep the bowl-shaped back surface is. SAG captures the complete curvature characteristics of the lens and directly determines how the lens fits on the eye.
How do you measure the SAG of a contact lens?
Measure SAG with a non-contact optical method that captures the complete three-dimensional structure of the lens, such as Low Coherence Interferometry. The MCT-3000 measures SAG with ±1.0 µm accuracy across a 0.5–5.0 mm range, covering all lens types from RGP to scleral, without contacting or deforming the lens.
Why is SAG better than base curve for predicting fit?
Base curve describes only the central curvature, while SAG reflects how the entire back surface relates to the eye. Because fit depends on the complete surface relationship rather than central curvature alone, SAG often predicts the actual fit better than base curve. A lens with SAG matching the eye’s sagittal depth centers and moves correctly.
How accurate does SAG measurement need to be for Ortho-K lenses?
Ortho-K lenses require SAG accuracy of ±10 µm or better, because a SAG error of just 10 µm can cause 0.25 diopters of unwanted corneal reshaping. A measurement system achieving ±1.0 µm accuracy exceeds this tolerance, providing the precision that predictable Ortho-K outcomes require.
Can SAG be measured on soft and scleral lenses?
Yes. A non-contact optical method with a measurement range covering 0.5–5.0 mm can measure SAG on all lens types, from small RGP lenses to large-diameter scleral designs. Non-contact measurement avoids deforming soft lenses, and the wide range accommodates the deep sagittal depth of scleral lenses.
Conclusion
Measuring the sagittal height of a contact lens accurately requires a method that captures the complete three-dimensional structure of the lens without contact. Low Coherence Interferometry does exactly this, measuring SAG with micron-level accuracy across the full range of lens types from RGP to scleral. Because SAG determines how a lens fits on the eye – and because specialty lenses such as Ortho-K demand extreme SAG precision – accurate, non-contact SAG measurement is essential to producing lenses that fit, perform, and deliver the clinical outcomes they were designed for.
Disclaimer: This document is intended for informational use only. It does not represent legal, regulatory, or certification advice, and should not be interpreted as a declaration of compliance or approval by Rotlex or any regulatory authority. Product specifications are subject to change; confirm current specifications directly.
