SPIE '98
CWhatUC : A Visual Acuity Simulator


Our paper "CWhatUC : A Visual Acuity Simulator" was accepted to SPIE's Photonics West '98 conference. It was shown in Ophthalmic Technologies VIII. We presented a poster to go along with the paper.

The Paper

The Poster

A Visual Acuity Simulator
Daniel D. Garcia (ddgarcia@cs.berkeley.edu)
Brian A. Barsky (barsky@cs.berkeley.edu)
Stanley A. Klein (klein@adage.berkeley.edu)

The OPTICAL Research Project University of California, Berkeley


The Cornea
The cornea is the transparent tissue covering the front of the eye. It performs 3/4 of the refraction, or bending, of light in the eye, and focuses light towards the lens and the retina. Thus, subtle variations in the shape of the cornea can significantly diminish visual performance.
Recently, instruments to measure corneal topography have become commercially available. These corneal topography devices typically shine rings of light onto the cornea and then capture the reflection pattern with a built-in video camera.
Instead of allowing the instrument to process the pattern, we at the OPTICAL project extract the data and construct a mathematical spline surface representation from these reflection patterns, as is described in our ACM SIGGRAPH '96 paper, "Reconstructing Curved Surfaces From Specular Reflection Patterns Using Spline Surface Fitting of Normals"
Once we have the shape as a mathematical surface, we perform calculations on its shape. The four calculations are:
  1. Axial Refractive Power (Diopters) = n / Distance3D(Corneal_Point, Axis_Intersection)
  2. Instantaneous Refractive Power (Diopters) = n / Distance3D(Corneal_Point, Focus)
  3. Retinal Distance (mm) = Distance2D(Retinal_Intersection, Fovea)
  4. Focusing Distance (mm) = Distance3D(Focus, Fovea)
Simulated Data
We use a simple ellipsoid to simulate the cornea here. The equation for the ellipsoid is (with A=8.7, B=9 and C=10):
(x/A)2 + (y/B)2 + (z/C)2 = 1
  • Axial refractive power is the most informative here, as it shows the inherent astigmatism associated with the asymmetric shape.
  • Retinal distance demonstrates that the upper and lower areas contribute to focus better than the left and right, with good focus in the central circle.
  • Instantaneous refractive power and focusing distance tell us little here other than the focus is worse the further from the center we are.
Real Data
This data is from a patient with keratoconus, a corneal condition, and in the lower left there is an oval region of locally high curvature.
  • Of our measurements, instantaneous refractive power and focusing distance illustrate this best.
  • The axial refractive power map has a crescent shape because the keratoconus is off-center and results in some astigmatism.
  • Retinal distance, in conjunction with focusing distance, gives a measure of which rays contribute to good focus. In this case, only a small central area provides a good focus.
We have presented four metrics for simulating visual acuity based on geometric optics, and shown the results using simulated and real data.
  • Axial refractive power is familiar to clinicians who often use a similar measure for astigmatism.
  • Instantaneous refractive power is useful for describing the corneal shape, but doesn't take the fovea into account.
  • Focusing distance and retinal distance taken together illustrate which regions contribute to a crisp focus onto the fovea.
  • In summary, the four metrics, when used together, provide additional insight into the prediction of a patient's visual acuity.

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