Purchase this article with an account.
Brian Vohnsen, Denise Valente, Salihah Qaysi, Rebecca M McQuaid; How Long Must a Photoreceptor Be to Become Waveguiding?. Invest. Ophthalmol. Vis. Sci. 2016;57(12):4643.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
Photoreceptors are endowed with directional properties and the common consensus is that cones and rods are biological waveguides that direct light to the outer segments whereby the degradation caused by intraocular scattering is dampened. The purpose of this study is to analyze the role of photoreceptor inner and outer segment length for efficient forward propagation of light and thus to establish minimum requirements on photoreceptor length for efficient waveguiding to be established.
A numerical analysis of forward light propagation across an ideal elongated cylindrical waveguide having parameters similar to those of individual cones and rods in the human retina (inner and outer segment lengths both analyzed in the range of 20 – 40 μm) is performed with a finite-element method that predicts the electromagnetic propagation of light across the photoreceptor model (ComsolTM). The model includes both guided and radiative modes represented by the light concentrated predominantly within and outside of the waveguide, respectively. This model expands on the layered outer-segment model recently presented by one of the authors (Vohnsen, Biomed. Opt. Express 5, 2014).
The model excludes any irregularities by assuming a perfect cylindrical cell body and thus it is at best representative of the average geometry. Retinal photoreceptors show more variability that will cause increased leakage by scattering of light. It is found that effective waveguiding requires a length one-order-of-magnitude above the waveguide diameter in order to effectively decouple the radiative modes. For on axis incidence about 20% of the light is represented by radiative modes and for oblique incidence of light this fraction increases. Thus the elongated structure of the photoreceptors is not sufficiently long to guarantee that only light coupled to guided modes will contribute to vision under normal illumination conditions.
The guided modes are caused by multiple reflections of light along the waveguide body where for natural pupil sizes only one or two reflections are likely to occur before the light reaches the end of the cell. This prevents effective coupling of light to guided modes. A large fraction of the incident light will not be well confined along the cell body but leak into the intracellular matrix and neighboring photoreceptors. This needs to be taken into account in more refined optical models of the retina.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
This PDF is available to Subscribers Only