Photoreceptor, interface

FIGURE 16.4 Precursors of RPE cell lipofuscin form in the outer segments of photoreceptor cells. The retina of normal rat (A, B) and the Royal College of Surgeons (RCS) rat (C, D) viewed under the phase contrast (A, C) and the epifluorescence microscopy (B, D). In the normal rat, autofluorescent material accumulates as lipofuscin in RPE cells (arrows). In the RCS, due to a defect in RPE cell phagocytosis, shed outer segment membrane builds up at the photoreceptor-RPE interface the autofluorescence in this debris is attributable to lipofuscin precursors that form in photoreceptor outer segments. 

Osawa and Doi (1992) described a surface layer of plasma-polymerized amorphous C over the transport layer to retard ozone interactions with the transport material along with increased transport material at the interface to compensate for any material that may be destroyed. The reasoning is that without a protective surface layer, the transport layer is destroyed by ozone but the surface is continually abraded so the cycling characteristics are relatively unaffected. However, in an overcoated photoreceptor the surface layer cannot be worn away and destruction of transport material by ozone results in a rising residual potential with cycling. This effect can be reduced by having a higher concentration of transport material at the interface. 

Experiments using a two-layer heterostructure in which the photocurrent action spectra are observed both for front and rear (symbatic and antibatic) illumination of the interfaee between a photosensitizer and a hole transport layer have shown that the surface enhaneement of bound pair generation is due to a layer typieally 300-500 nm thiek [13]. Within this distance of the interface, excitons generated by the optieal absorption may diffuse toward the interface and initiate bound pair generation. The importanee of these excitons for a specific photoreceptor can be iden-... 

In azo-dye-based laminar photoreceptor systems, neither the dissociation efficiency nor the primary yield of bound pairs is dependent on excitation wavelength [11 13]. There is a strong positive linear correlation between the dissociation efficiency and the peak value of the dipole moment associated with the transition between the ground and first excited states in the azo photosensitizer (obtained from electroabsorption data) [14]. It is likely that in these systems the efficiency of dissociation is constrained by the electron transport process away from the interface into the bulk of the sensitizer material, because transport of holes through the donor matrix is much more rapid. 

Some of the interfaces meeting the photoreceptor are obvious its surface encounters ionized gas which serves to deposit or remove corona charge, the developer mass, the transfer sheet, and cleaning devices. Other interfaces with essential functions are found in the internal structure of the photoreceptor itself. Let us now look at these in some detail. 

Materials flexibility has thus been achieved— albeit at the cost of introducing an internal interface within the photoreceptor structure—an interface which must be carefully controlled in the coating process to prevent undesirable intermixing of layers, delamination, and the formation of barriers impeding charge carrier injection. 

The first layer of the visual cortex is the striate cortex (VI). About half of its cells are devoted to the central 600 arcmin of the visual field (equals 1 % of the entire visual field [2, p. 201]). This is consistent with the higher amount of ganglion cells devoted to the central receptive field, interfacing the photoreceptors in the fovea centralis. Thus, spatial and color resolution for this part of the visual field are at its highest. The striate cortex has a high density of interconnections to the cortex... 

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