Biomedical Engineering Reference
In-Depth Information
- The IR video display on the goggles can emit as much power as the eye can
thermally tolerate thus providing a robust signal to each pixel at any level of
ambient illumination.
- The implant-stimulated vision is provided simultaneously with the residual
natural vision in the areas outside the retinal implant. The infrared projected
image is not detected by photoreceptors. Conversely, the implant's response
to natural visible light in the eye is negligible compared to the bright and
pulsed infrared image.
- Intensity, duration, and repetition rate of the stimulating signal produced by
the retinal chip can be controlled by the intensity, duration, and repetition
rate of the light-emitting pixels in the LCD screen. These parameters can
be adjusted without need for any changes in the retinal chip itself. This
feature provides flexibility in optimization of the stimulation parameters
and image-processing algorithm, which might have to be adjusted for each
patient.
- This projection system can be used for both epiretinal and subretinal implants.
As described above, the stimulation current for an electrode of 10m in diameter
is on the order of 2A. The photodiode converts photons into electric current
with efficiency of up to 0.6 A/W, thus 34W of light power will be required
for activation of one pixel. If light pulses are applied for 0.5ms at 25 Hz, the
average power will be 42 nW/pixel. With 18,000 pixels on the chip, the total
light power irradiating an implant will be 0.75mW.
The LCD screens used in video goggles emit light into a wide angle, and
only a small fraction of it (typically <1%) reaches the retina, while most of it
is absorbed by the sclera and iris. In addition, only a small part of the retina
(about 5%) is covered by an implant. To provide 1mW of light on a 3mm retinal
implant, the LCD goggles should in total emit about 2W of light power! This is
certainly not practical.
This problem can be resolved by addressing both aspects of the loss of light:
(1) providing a collimated illumination, and (2) activating only a small part of
the screen - that which is projected onto the implant, position of which will
be constantly monitored with a tracking system. The pulsed illumination for the
implant will be provided by the near-IR LED array or laser diode illuminating the
LCD with collimated beam. A condenser lens directs the main axis of the diodes
into the center of the eyeball. Assuming no magnification between the screen
and the retina, the diameter of the light spot on the pupil will be D
=
d
chip
+
L,
where d
chip
is the implant size,
=
8
is the divergence of the LED beam,
and L
17mm is the distance between the implant and the pupil. With these
assumptions the spot of light on iris can be as small as 5.3mm in diameter. To
provide for a eye scanning range of 30
, a spot size on the iris should be actually
a little larger - about 9mm in diameter, thus with the pupil of 3mm in diameter,
10% of light will be transmitted into the eye, thus only 10mW of power would
be required from the LCD screen.
=
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