Gonioscopy Lens

Introduction

From non-invasive procedures to ultra-sensitive diagnostic instrumentation, photonic devices play an indispensable role in today’s biomedical industry. Timely design and delivery to market of these new technologies has been possible only with the aid of sophisticated software tools and experienced optical engineers. Photon Engineering firmly believes that its optical engineering product FRED can help accelerate the pace of innovation in the biomedical community. FRED combines an intuitive graphical user interface with a powerful computational engine capable of satisfying the most demanding requirements.

The relevance of FRED to the biomedical industry can best be expressed by presenting several familiar yet innovative applications such as a gonioscope, laser-induced fluorescence in a capillary, and a human skin model.

Gonioscopy lens

The ability to monitor the iridocorneal angle (the angle between the iris and the internal surface of the cornea) is a critical factor in the diagnosis and treatment of glaucoma. A gonioscope can measure this angle by illuminating the eye and collecting reflected light.

To begin simulating the gonioscope, an accurate human eye model is required [1]. A model based on relevant references is available in the Samples folder of FRED. [2],[3] (Figure 1) All essential elements of the eye are included: front and rear surfaces of the cornea, iris, eye lens, and aqueous humor. To modify any aspect of the eye model, simply double-click each element to open a multi-tab dialog box.

Figure 1. FRED model: anterior portion of the human eye.

The next step is to model the gonioscopy lens. FRED allows import of lenses designed in CodeV, Zemax or OSLO. FRED maintains a local coordinate system for each surface, which allows positioning of the goniolens with respect to the cornea. In practice, the lens is coupled to the cornea using index matching fluid. FRED has a unique “gluing” feature which allows for easy insertion of this layer. To contact the surfaces, enter edit mode for the rear surface of the goniolens (Figure 2), click the Glue tab and select the Cornea outer surface as the surface to be glued to. Finally, select the material to use for gluing.

Figure 2. “Gluing” the goniolens to the cornea.

The light source for illumination is created using FRED’s Detailed Optical Source type (Figure 3).

Figure 3. Detailed Optical Source dialog box. The Positions/Directions tab specifies source dimensions, number of rays, and angular emission properties. The Wavelengths tab provides options for spectral content. The Power tab sets total source power and specifies any spatial apodization.

The completed and raytraced model is shown in Figure 4. FRED has the ability to assign a color to rays in four distinct conditions: reflection, transmission, scattering or diffraction. Ray colors can be assigned under the Coating/Ray Control tab of a specific surface. In this example, rays scattering from the cornea rear surface are changed to green and those scattering from the iris to red.

Figure 4. Goniolens model with raytrace paths. Ray colors indicate specific surface intersections.

Figure 5 shows spot diagrams at the lens focus for rays scattered from the iris and the cornea. The left and right plots contrast the difference between a flat and curved irises. As expected, we can see the image shearing with the curved iris in the left chart.

Figure 5. Spot diagrams using planar and curved irises.

[1] The Eye and Visual Optical Instruments, G. Smith & D. Atchison, Cambridge University Press, 1997

[2] Visual Optics Course Notes, Jim Schwiergling, Optical Sciences Center, University of Arizona, 2000.

[3] Tissue Optics; Light Scattering Methods and Instruments for Medical Diagnostics, Valery Tuchin, SPIE Press, 2000.