To evaluate the validity of the model, a phantom of the cornea was created to allow the experimental measurement of strain. A soft contact lens (Surevue; Johnson & Johnson, New Brunswick, NJ) and an artificial anterior chamber (Barron; Katena, Denville, NJ) were used to measure hysteresis and internal pressure using the ORA. The soft contact lens was selected because it has a curvature (7.2 mm) similar to that of the cornea and an optical surface. The thickness was 144 μm, as measured by calipers. For the ORA to take reliable measurements, a specular reflection of the infrared light directed at the object surface is necessary. Soft contact lenses are ideal for use with the ORA. They allow for the detection of sharp IR peaks, which in turn allows for the determination of applanation pressure.
The artificial anterior chamber was pressurized to 15 mm Hg to mimic intraocular pressure, and the pressure was monitored (TC Bedside Monitor; Spacelabs, Issaquah, WA). A high-speed camera (Motionscope PCI-500; Redlake, Tucson, AZ) was used to film the surface of the contact lens during ORA measurement. The film was taken at 500 frames per second. Each image was composed of 320 × 280 pixels, and each pixel was approximately 34 μm in length. Both the phantom and the camera were fixed to a metal plate so that the profile of the lens could be seen. The experimental setup is shown in
Figure 5 . The plate ensured that the artificial anterior chamber and the camera remained stationary during the measurement. Vertical displacement measurements were made from the image series. A ruler was imaged to determine the length of a pixel. With the use of commercial software (Photoshop 5.5; Adobe, San Jose, CA), a curve was fitted to the surface of the undisturbed contact lens (
Fig. 6 , right column, at 2 ms). This curve and the measure tool in the software were used to determine the radius of curvature and to measure the maximum vertical displacement of the contact lens. To determine the radius of curvature, the length of a cord (2
c) on the circle and the maximum distance of this cord from the circle (
h 0) were measured. These values and
equation 5were used to solve for the radius of curvature (
R 0). For each subsequent image, white markers were manually placed on the images using a cursor to outline the deformation. To measure the vertical displacement (
h dep) of the contact lens, each of the deformed images was overlaid on the original circle to allow the maximum distance between the markers and the circle to be measured (
Fig. 6 , right column, 10–18 ms).
The ORA uses an infrared light to measure applanation of the cornea. This light is activated for a fixed time before the ORA fires the piston that creates the air pressure stream and begins collecting pressure data, and it allows for the synchronization of the camera images and the pressure data from the ORA.
The radius of applanation was determined by using the timing of the two applanation events recorded by the ORA and the corresponding images taken immediately before and after the applanation. The timing of the applanation events is part of the data that can be downloaded from the ORA with the export data function. With vertical displacement data from the acquired images before and after applanation and the timing of the applanation events, the vertical displacement at applanation was interpolated. The vertical displacement values (
h 0) for the two events were then averaged. We used
equation 5to calculate the radius of applanation area (
c), and we used equations
6 7 8 9 to 10to calculate the strain in the contact lens at each point during the ORA measurement.
Applied air pressure values and timing from the ORA were used to drive the viscoelastic model and generate a strain pattern. To simulate the model passing through applanation and into concavity, the sign of the stress and the action of the tear film were inverted at the applanation times. For validation, the values for springs E1 and E2 were assumed to be equal. Assuming the cornea to be axisymmetric, a single elastic constant should govern corneal behavior. The spring constant and the viscosity value were adjusted until the strain pattern achieved a best fit with the experimentally measured strain. The best fit was determined by minimizing the sum of the squared differences between the experimentally measured strain and the simulated strain at the same point in time.