Biomedical Engineering Reference
In-Depth Information
a normally illuminated room condition is about 3 to 4mm; we chose the
dimension of the micro-camera as 5mm long and 4mm wide as demonstrated
in Figure 10.16. The micro-lens should be of wide angle and short focal length,
such that the visual field could be extended.
The necessary highest resolution of the CMOS could be calculated based on
visual discrimination. For example, for a given focal length of 4mm of the
micro-lens, if the best discrimination ability was described with visual angle
of 10 minutes of arc or more, the maximum single pixel should not be larger
than 5 m. Much higher resolution could consume more power, hence more heat
would be created, which was a considerable problem since it was to be implanted
into the eye. In addition, more time was needed for the data processing which
also should be taken into account with real time visual purpose.
Evaluation of the Micro-Camera in the Model Eye
In order to evaluate the imaging quality of the micro-camera in different environ-
ments, we put the micro-camera in the position of the crystalline lens of a model
test eye (Figure 10.17). The model test eye was full of 0.9% NaCl fluid when
the pictures were to be taken in order to simulate the internal circumstance of
the anterior chamber of the eye. The images were recorded with the target at
different distances from the model test eye. It was demonstrated that the images
became only a little blurred when recorded with the micro-camera in 0.9% NaCl
fluid (Figure 10.18). It also could be seen that the depth of field was large
enough for the visual prosthesis to “see” clearly in the near and middle distance.
In addition, it might have enough resolution for the image information extraction
to correctly stimulate the optic nerve or neurons.
Figure 10.16. The implantable micro-camera including micro-lens, CMOS and cable
(Top). The sketches of the implantable micro-camera (Bottom).
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