1 Invertebrate photoreceptor optics

Animals sustain their life by accumulating the necessary energy from their environment, requiring the continuous acquisition of information with their sensory systems. The prime sensory instrument of many animals is the visual system, with the photoreceptors in the retina as the main elements sampling the visual information. To gain insight into the spatial and spectral properties of the photoreceptors, it is essential to understand the optics of the eyes. The optics of invertebrate eyes and their photoreceptors, the theme of this chapter, has been described in many excellent reviews published in the recent decades (Snyder and Menzel, 1975; Miller, 1979; Snyder, 1979; Land, 1981; Nilsson, 1989; Warrant and McIntyre, 1993; Land and Nilsson, 2002). Here, we discuss a number of recent developments. Vision starts when a visual pigment molecule absorbs a photon. The absorbed light energy excites the molecule, which then goes through a series of photochemical steps and ultimately causes phototransduction, that is, triggers the chain of molecular processes that results in a neural signal (see Chapter 2). The visual pigment molecules are embedded in a specialised organelle of the photoreceptors that is constructed from an intricately folded part of the cell membrane. Accordingly two main types of photoreceptors can be distinguished, ciliary and rhabdomeric photoreceptors, depending on the cellular origin of the visual pigment-bearing organelle: a cilium or a stack of microvilli, respectively (Eakin, 1972; Arendt et al., 2004). This structure often adopts a cylindrical shape and thus acts as an optical waveguide, since the material within the cylinder, with a high concentration of cell membranes composed of phospholipids