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Fiona Johnston, Arthur Ho, Minas T Coroneo; Computational model of sclerotic scatter. Invest. Ophthalmol. Vis. Sci. 2019;60(9):589.
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© ARVO (1962-2015); The Authors (2016-present)
A clinical examination technique to detect pathology within the anterior eye is known as ‘sclerotic scatter’ (SS). Its propagation pathway has not been thoroughly investigated. Although currently theorised to occur by ‘total internal reflection’ (TIR) along the corneal surfaces, existing data suggest this may be incomplete.
An anterior eye model representative of a virtual human eye using accepted parameters has been constructed using non-sequential ray-tracing (OpticStudio 18.1). A series of models were developed with different configurations of detectors for internally propagated and emergent light rays to study SS. For verification purposes, the initial model was constructed assuming a monochromatic source (λ= 555 nm) with Lambertian distribution and tracing of 1,000,000 rays. An improved model portraying SS was designed that included a source modeling the spectral characteristics of a tungsten-halogen source typically found in slit-lamps (peak λ= 650 nm) and a detector element on the iris. Analysis for optimal slit-lamp setup parameters for SS was conducted using a model that included bulk scattering within the sclera and detectors placed within regions representative of the clinician’s ‘area of interest’. Three setup parameters were tested including incident beam angle, beam width and position of the incident beam on the sclera.
SS exhibits an inefficient usage of light. Most of the light that is directed into the temporal limbus is lost (52%) and re-emitted back to the clinician creating a glare source. Only 0.006% of light that transits the central cornea undergoes TIR off the anterior cornea and more significantly, a minute 0.000125% off the posterior cornea, challenging previous understanding of light propagation under SS. Slit-lamp set-up parameters to optimise the amount of light reaching the clinician’s area of interest were shown to be at normal incidence to the sclera, directed near to the limbus with a 1 mm beam width.
This study has found that the propagation of light within SS effectively does not occur by TIR off the posterior cornea. SS also represents an extremely inefficient usage of light, with approximately half of the light creating a glare source for the clinician, further hindering visualisation. We have formulated a recommended set of parameters for slit-lamp set-up to reduce scleral re-emittance and to maximise clinical visualisation of opacities.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
Model of SS vs. clinical observation
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