Biomedical Engineering Reference
In-Depth Information
The wild-type BR-PVA elements in the gray-scale array are 144
m thick on an average
(standard deviation 26
m).
According to the displacement current model (33,34), the PER linearly depends on the
number of BR molecules per unit distance in the direction perpendicular to the electrodes.
This states that the variation in the film thickness is the dominant factor of quality, assum-
ing that the BR suspension is homogeneous.
16.5.2
Color-Sensitive Imaging Array
Color sensitivity can be achieved in an imaging device by capturing the radiant energy
from a color scene by at least two sensor elements having different spectral sensitivi-
ties. Three color components are sufficient for general use if the image is viewed by a
human being. However, if accuracy is wanted, a three-component color representation
is not enough for all purposes (35). The spectral properties of the sensor elements are
important, since discrimination of colors is expected. Depending on the application, a
sound color-sensitive imaging device should capture the information of the energy
spectrum, and offer a signal-to-noise ratio (SNR) good enough for discrimination of
different colors.
To modify the spectral properties of BR, retinal substitution was used. The absorption
and action spectra of the single-pixel photosensors based on BR are shown in Figures 16.5
and 16.6. All figures show that there is solid support to achieve color sensitivity.
The color-sensitive BR imaging array was prepared in the same procedure as the gray-
scale array, except that individual array elements were prepared one by one using the
three types of BR. Layout of the array elements and the complete color-sensitive BR imag-
ing array are shown in Figure 16.7. Average thickness of the elements in the color-sensi-
tive array with different types of BR is 101-107
m).
Other geometries for the array elements, for example hexagons, could be employed with
ease due to the flexible preparation process. This would enable the use of interpolation
methods to enhance the spatial resolution when captured data is converted into a rectan-
gular grid. It would be possible to prepare the film on top of very diverse substrates too, as
long as the gravity coating method can be used.
m (standard deviation 16-20
16.5.3
Electronics
The electronics for an element in the imaging array consist of an instrumentation ampli-
fier, signal amplifier, peak-hold circuit, and buffer circuit. The peak photovoltages from
each element are digitized and processed by a microcontroller (36). The electronics
based on the microcontroller was used for demonstration purposes but the photoelec-
tric measurements were performed using the electronics developed for the single-pixel
photosensors.
16.5.3.1 Photosensor Electronics
The relatively large size of the photosensor causes it to have small source capacitance
(31). This needs to be compensated by proper impedance matching between the sensor
and signal-conditioning electronics. Therefore, the photosensor has been connected as
closely as possible to the voltage follower shown in Figure 16.8a. According to the man-
ufacturer of the operational amplifier (TL082MJG), the input resistance is 10
12
, rise time
is 50 ns, and unity gain bandwidth is 3 MHz.
