Fig. 1

Types of eyeshine in vertebrate eyes. a ICR eyeshine constitutes a case of iridescence produced by arrays of regularly arranged collagen fibres in the cornea. Light reflection is only partial, specular, and both wavelength- and angle-dependent. b Retinal-tapetum-reflected (RTR) eyeshine is caused by an intracellular tapetum lucidum in the inner part of the RPE. Tapetal reflection can be specular and/or diffuse. The eyes’ optics project the reflected light back to its source, an effect known as retroreflection. c In CTR eyeshine, the tapetum lucidum lies within the inner portion of the choroid. For light to reach the tapetum, the RPE must transmit light at least partially (e.g., through melanosome motility). The choroidal tapetum can be cellular (specialised, reflective cells) or fibrous (regularly arranged, extracellular collagen fibres), both of which can generate strong retroreflection. d SAR eyeshine has similar properties and requirements as CTR, but is caused by the stratum argenteum, a distinct, cellular, reflective layer in the outer choroid. e PET eyeshine occurs when the RPE and choroid are (partially) unpigmented, and the eyes are in an exposed, dorsal position. Ambient light shines through the fundus and is seen as PET eyeshine from a wide range of directions. PET eyeshine regularly co-occurs with SAR eyeshine. f In optic-nerve-transmitted (ONT) eyeshine, light enters the eye through the ON. Pigmentation of the RPE and choroid are irrelevant for ONT eyeshine, but pigment sheaths around the intraocular ON protect the photoreceptors from transmitted light. ONT eyeshine can co-occur with any other type of eyeshine. Since the light is emitted from the optic disc, its projection and visibility are restricted to a narrow beam of the same shape as the disc. g Cross-section through an eye for a general overview of the involved structures. D dorsal, M medial