Biomedical Engineering Reference
In-Depth Information
0.2
2
0.15
1.5
0.1
1
0.05
0.5
0
0
350
400
450
500
550
600
650
700
750
Wavelength (nm)
FIGURE 16.2
Wavelength discrimination capability of CIEXYZ tristimulus values. Lower value means better discriminability.
The CIE 2° color matching functions
x
,
y
, and
z
are plotted for comparison as dashed lines on the background.
integrate both photosensors and parallel-processing elements on a single chip (26). The
vision chips provide many advantages over conventional vision-processing systems con-
sisting of a camera and a digital processor. Among the advantages are speed and size. The
speed of many vision chips exceeds that of the conventional systems mainly due to the
tight coupling of image sensing and processing in a single chip, which allows for a high
degree of parallelism. Also, the data transfer bottleneck between sensor and processor is
alleviated to a significant extent.
The preprocessing capability is also included in our model of a color-vision system out-
lined in Figure 16.3. The system consists of two layers: a photosensitive layer and a pre-
processing layer. The photosensitive layer converts the image of the scene into electrical
signals, which are passed on to the preprocessing layer. The output of the sensor is com-
municated to a processor. The preprocessing can include some image processing, for
example, edge detection or color-coordinate transformations and color grouping.
16.4.1
Photosensitive Layer
The photosensitive layer consists of one or more arrays of photosensitive elements. In gen-
eral, the elements can be charge-coupled device (CCD) elements, cones in the retina, patches
of BR film, or any other suitable photosensitive material. The geometrical arrangement of the
elements is not restricted. The output of the preprocessing layer is an array of
n
-tuples.
Each photosensitive element is characterized by its relative spectral response function
r
(
), which expresses the relative strength of response to equal-energy monochromatic
stimuli. The response of an element to a stimulus with spectral energy
P
(
) is
∫
RPr
()()d
(16.2)
This can be seen as an inner product of
P
(
) and
r
(
).
