Application: Measurement of base curve and thickness of soft contact lenses.

Background: In the 1970s, we pioneered the adaptation of ultrasonic thickness gages for the testing of soft contact lenses. With ultrasonic gages it is possible to measure both the sagittal height of soft contact lenses for base curve calculation, and the lens thickness as well, without the errors that can be introduced by physical distortion of the lens when using other techniques. Ultrasonic measurement is fast, repeatable, and reliable, and not subject to operator interpretation. The advanced Model 38DL PLUS gage with Multilayer Measurement software incorporates special software for simultaneous measurement of sagittal height and thickness, combined with calculation of the base curve.

Recommended Equipment
Gage: Model 38DL PLUS with Multilayer software option
Transducer: M316-SU F=.75 (20 MHz, 0.125 in. element diameter)
Fixture: B-200 Contact Lens Fixture or equivalent
Note: It is also possible to use the Model 35 or Model 35DL gage for measurement of sagittal height or lens thickness, however these instruments will make only one type of measurement at a time and do not automatically calculate base curve.


Method of Measurement: The B-200 contact lens holding fixture is a small plastic tank containing a tubular pedestal that has an accurately known diameter. The ultrasonic transducer is held in the base of the fixture, aiming a focused sound beam through the center of the pedestal. A very thin plastic membrane is stretched across the top of the pedestal, and the tank is filled with saline solution. The contact lens is placed on top of the pedestal, concave side down. The plastic membrane then becomes the reference chord for the sagittal height measurement.

In the measurement process, the transducer generates a short pulse of sharply focused ultrasonic energy that travels up through the saline solution. Portions of this energy reflect back to the transducer from the membrane, the bottom surface of the lens, the top surface of the lens, and finally from the surface of the saline solution. The gage digitally records this pattern of echoes, and then very precisely measures the time intervals between the first and second echoes, and between the second and third. (The fourth echo, from the surface of the saline solution, is not relevant to this test.)

The time interval between the first and second echoes is proportional to the distance between the membrane and the bottom of the lens, or the sagittal height. Similarly, the time interval between the second and third echoes is proportional to the thickness of the lens. The gage applies the formula

D = VT / 2

where

D = distance or thickness
V = velocity of sound waves in test material
T = measured round trip pulse transit time

Using sound velocity information that has been programmed into the gage. This simple calculation provides the sagittal height and lens thickness information.
The gage calculates base curve radius from the formula

where
R = base curve radius
H = measured sagittal height
r = radius of pedestal

The gage then displays sagittal height (1), lens thickness (2), and base curve information (R) as seen below:


Sagittal height, lens thickness, and base curve display

Calibration: The accuracy of any ultrasonic measurement is only as good as instrument calibration. In the case of contact lens measurements, this means accurate calibration of the sound velocity for the saline solution being used, and for the specific lens material. Note that sound velocity in saline solution will change significantly with temperature and salt concentration, and that lens velocity will also change somewhat with temperature and composition.

The recommended procedure for optimum accuracy is to set up the instrument with the aid of reference standards whose base curve/sagittal height and thickness are precisely known, while maintaining constant temperature. The general procedure for velocity calibration is described in detail in the Model 38DL PLUS operating manual. With proper instrument setup, sagittal height and thickness may be measured to an accuracy of +/- 0.01 mm.