A Real-Time and Portable Bionic Eye Simulator - Biomedical Engineering Systems and Technologies - page 37

Fig. 7. Binary thresholding with hysteresis

Fig. 8. Phosphene modelling: without Gaussian function (left), with Gaussian function (right)

spot of light [4]. Rather than simply using square pixels that perfectly line up with each
other, we attempted to model the output visualisation based on what phosphenes would
approximately look like.
In the literature, one common approach is to model the phosphene using a 2D Gaus-
sian mask [7]. The 2D Gaussian function is based on the standard distribution curve,
except in two dimensions instead of one. This creates the appearance of a round 'spot'
where the centre of the spot has the highest intensity value with the intensity values
decreasing radially towards the outside edge of the spot, following the standard dis-
tribution curve. A comparison between a phosphene with and without the Gaussian
function applied is shown in Figure 8.

3.5 Frame Rate Reduction

The ability of a person to detect motion is very important when it comes to mobility
exercises in low resolution vision. A key factor that would limit one's ability to detect
motion in the immediate environment is the lack of temporal resolution. It is expected
that the temporal resolution of electrode stimulation achievable by the Monash Bionic
Eye may be in the range of 5-15 frames per second. In order to simulate this temporal
resolution and investigate the possible implications it may have on a patient's ability to
move around, we have implemented a frame rate reduction function. The output frame
rate of our system can be changed in real-time. Our system has 8 different discrete
frame rates available for selection (1, 2, 4, 8, 10, 15, 30 and 60 frames per second).
Variable frame rate is achieved by holding the stored frame output data for the specific
period of the chosen frame rate.

3.6 Threshold Ramping

It became apparent, through general use of the system, that variations of lighting in the
environment resulted in poor scene conversion and representation. This was due to the
use of fixed intensity values for greyscale thresholding.