Biomedical Engineering Reference
In-Depth Information
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6
5
4
3
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1
0
FIGURE 16.15
Time course of the PER for an element in
the gray-scale imaging array when puls-
ing at 100 Hz with high intensity of light.
0
500
1000
1500
Time (ms)
are similar to the responses from the single-pixel photosensors. Uniformity of the
responses has been shown to be moderate but the preparation procedure for the BR-thick
films is robust. The differences in the responses could be easily compensated by calibrat-
ing the sensor in the application. The elements in the color-sensitive imaging array gen-
erate detectable responses but the amplitudes are small. This is because of the
concentrations of BR in the elements, and low conductivity of the gold layer. The sensi-
tivity of the array elements to radiant energy and spectrum of light have not been thor-
oughly measured, since elementary experiments have showed them to function like the
photosensors. The pulsing experiment has not been performed to 4-keto and 3,4-didehy-
dro BR because of the limited responses from the color-sensitive array. The experiment
with the gray-scale array shows that wild-type BR could be pulsed even at 100 Hz when
the intensity of light is limited. This indicated that at least the gray-scale array can be
used in principle for video capturing. The characteristics of the imaging arrays based on
BR are summarized in Table 16.1.
16.5.6
SOM
The color space formed by training SOM with responses of BR elements to the
collection of 84-color lights can be visualized. Figure 16.16 shows learned color space.
Each group of color patches are the colors assigned to one neuron in the map after the
training is completed. Figure 16.17 shows in three-dimensional space the self-organiz-
ing map embedded into the training data set; the weights corresponding to the neurons
are the vertices of the grid. It can be seen from Figure 16.16 that the colors have been
sensibly organized; similar colors have been clustered to occupy close regions of the
color space. Analogically, Figure 16.17 shows the color space formed by training the
SOM with the simulated responses of human cones to the same collection of 84-color
lights.
The sensible organization of colors may also be observed by looking at how individual
neurons in the trained map respond to monochromatic stimuli. Each neuron in the map has
a distinct response curve to colors throughout the color space. Figure 16.18 shows the
response curves of neurons in the map and the wavelength regions for which each particular
