Combined Laser Ray Tracing and Optical Coherence Tomography system to measure changes in lens shape and on-axis and off-axis focal length with accommodation

Abstract

 

Purpose

 

To measure the changes in lens shape, focal length and aberrations in ex vivo primate lenses on and off-axis during simulated accommodation.

 

Methods

 

Instrumentation: A combined LRT-OCT system was designed and built using a commercial Spectral-Domain OCT system with 16mm depth range and 8µm axial resolution in air (Bioptigen ENVISU R4400). The LRT uses the OCT scanning beam delivery system to deliver input rays to the lens. The beam delivery optics are mounted on a motorized rotation stage that pivots around the lens to allow acquisition of off-axis spot patterns. An imaging sensor was mounted on a two-dimensional (horizontal and vertical) motorized positioning stage below the lens to record the spot positions along each individual ray both on-axis and off-axis. Spot positions are used to reconstruct the ray path and calculate lens power and aberrations. The tissue is mounted in a manual lens stretcher (Bioniko LLC) that we motorized to measure the lens in the unaccommodated and accommodated states. The entire system is controlled via LabView software to allow automated data acquisition.<br /> Measurements: LRT-OCT experiments were performed on 2 cynomolgus monkey lenses from 1 donor (4 years, PMT= 2, 26 hrs) in the unstretched and stretched state. The LRT delivered 51 equally-spaced parallel meridional rays over a 6-mm diameter zone. Experiments were performed at incidence angles from -20° to +20° in 5° increments. Spot positions were recorded at 12 vertical heights along the optical axis. The on and off-axis focal length was calculated from the ray slopes.

 

Results

 

The system can acquire OCT images of the entire lens and focal length measurements at each angle. The on-axis focal length is (OD, OS): 30.2mm, 29.4mm in the unstretched state and 35.5mm, 29.6mm in the stretched state, corresponding to a power of 37.6D, 45.1D, and 44.2D, 45.4D, respectively. The off-axis focal lengths are shown in Figure 2.

 

Conclusions

 

The system is able to measure the off-axis ray-trace of the crystalline lens. The focal length of cynomolgus monkey lenses was found to decrease with increasing incidence angle.  

 

(Left) Combined LRT-OCT system. The lens stretcher and tissue cell not shown. (Right) Lens stretcher, tissue cell and control unit.

 

 

Change in lens focal length versus incidence angle for (Left) OD and (Right) OS in the unstretched and stretched states.