Game Development Reference
In-Depth Information
Optic nerve
Ganglion cell
Amacrine cell
Bipolar cell
Horizontal cell
Figure 3.5 Anatomic structure of the retina
Photoreceptors: First retinal level
There are two types of photoreceptors - cones and rods; their distribution on the retina
is very different (Buser & Imbert, 1987):
The cones, concentrated at the fovea, capture the wavelength. They are of three
types, sensitive to wavelengths centred around 560, 530 and 420 nm respec-
tively. Information of colour is produced at the cortical level by comparing the
information received by various receptors containing different photopigments;
The rods, on the other hand, are a lot less sensitive to colour. They are almost
everywhere in the retina, but absent at the centre of the fovea and very dense at
approximately 20 of visual angle.
The density of photoreceptors and the bandwidth of spatial frequency are interrelated.
In humans, the cut-off frequency is approximately 60 cpd (cycle per degree) and the
maximum linear density is close to 120 units per degree in optimal conditions. This
ratio is consistent with Shannon's sampling theorem. Though the number of rods is
much more than that of cones (120
10 6 ), the cones have a major
contribution in the information transmitted to the deeper structures of the visual sys-
tem. It is thanks to these cones and rods that this system can adapt to the ambient light
intensity and cover a range of intensities close to 7 logarithmic units. Only the rods can
operate when the illumination is very low. In such conditions, the cones are positively
coupled to their neighbours to increase the surface of the photon detector. Sensitivity is
increased at the expense of the resolution of the system. On the other hand, when the
10 6 versus 6
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