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Fig. 2. Simulated enhancement factor curves for three model photoreceptors. Based on the dimensions of cones in (A) birds (lOS=30 μm, dOS=1.5 μm, dOD=3 μm), (B) frogs (lOS=12 μm, tapering dOS=4.5–0 μm, dOD=6.2 μm) and (C) turtles (lOS=10 μm, tapering dOS=3–1 μm, dOD=5 μm), where lOS is the outer segment length, and dOS and dOD are the diameters of the outer segment and the oil droplet, respectively. Families of curves were calculated for a wavelengthinvariant value of the refractive index of the oil droplet, nOD, increasing in steps of 0.05 from 1.45 to 1.8. Grey regions show enhancement factors <1, corresponding to loss of light because of the oil droplet. Thick light blue lines show DG. In all three cases, the higher the refractive index of the oil droplet, the lower the enhancement factor. In no case did the enhancement factor approach DG. Image published in: Wilby D and Roberts NW (2017) © 2017. This image is reproduced with permission of the journal and the copyright holder. This is an open-access article distributed under the terms of the Creative Commons Attribution license Larger Image Printer Friendly View

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