A Real-Time and Portable Bionic Eye Simulator - Biomedical Engineering Systems and Technologies - page 36

Fig. 5. Averaging sampler implementation

Fig. 6. Thresholding: full resolution image (left), 4-level image (middle), binary image (right)

count values are compared against the mapping lookup table. This lookup table stores
the corresponding phosphene index number for each pixel within the central 480 ×

480 window of the full camera view. Pixels not belonging to a phosphene are assigned
number zero. Once the phosphene index number is determined, the pixel is sampled by
adding to a storage register that corresponds to that particular phosphene index number.
This process repeats until all pixels have been sampled. Finally, an average is performed
on all of the storage registers according to the number of pixels that are within each
phosphene, and the results are stored in a separate set of storage registers.

3.3 Thresholding

Various studies [4,8,20] have shown that the modulation of phosphene brightness is
possible using a number of different techniques. However, there is some ambiguity in
the possible number distinguishable brightness levels.
Our system takes an optimistic approach at simulation of this property, having the
option to display at 2, 4 or 8 levels of intensity or greyscale. Since our system uses 10-
bit storage registers for pixels, the full greyscale intensity range is 0 to 1023. This range
is divided evenly in order to create bands of intensity for 2, 4 and 8 level modes. Results
of 2 and 4-level thresholding are shown in Figure 6. It is often difficult to perceive the
results of the system in a static image form, therefore we encourage you to view the
videos we have listed in the Appendix.
To avoid high frequency oscillation between intensity bands, a hysteresis feature was
included. Two threshold values are used to define changes between intensity bands,
instead of one value. When a phosphene's intensity is between the two thresholds, no
change occurs. Figure 7 shows how hysteresis reduces the oscillation problem.

3.4 Phosphene Modelling

Stimulation of each electrode on the implant will produce a phenomenon in the patient's
visual field known as a phosphene, whose appearance is somewhat similar to a bright