Large-scale Integration–Based Stimulus Electrodes for Retinal Prosthesis - Artificial Sight: Basic Research, Biomedical Engineering, and Clinical Advances - page 156

Figure 8.4. Output pulse frequency of the PFM photosensor with variable photosensitivity
as a function of input light intensity. Reprinted from Ref. [4] with permission from IEEE.

(6th-order range of input light intensity), but is reduced to around 2-Log to
be limited at 250 Hz when the low pass filter is turned on. By introducing the
variable sensitivity, the total coverage of input light intensity becomes 5-Log
between n

= =

0 and n 7. The other functions in Table 8.1 have also been

demonstrated.

Image Preprocessing Using the PFM Photosensor
When the PFM-based stimulator device is applied to a retinal prosthesis, the
resolution is less than approximately 30

30. This limitation arises because
the electrode pitch is larger than 100m, according to the electrophysiological
experiments, and the width of the chip is less than approximately 3mm according
to the implantation operation. In order to obtain a rough but clear-shaped image
with such a low resolution, image processing, such as edge enhancement, is
preferable.
In order to implement image processing, we have proposed a new principle
of spatial filtering in the pulse frequency domain [7]. The spatial filtering is
generally based on the spatial correlation operation using a kernel h as

×

=
x

hx y fx x y y

gxy + + (2)

y