Biomedical Engineering Reference
In-Depth Information
25
Signal at 10 Hz
20
15
10
FIGURE 17.14
Amplitude spectrum of the signal gen-
erated by the bR photoreceptor in
response to a 10-Hz input signal. This
amplitude spectrum in volts RMS is cal-
culated by using FFT function in
Matlab.
5
0
0
50
100
150
200
Frequency (Hz)
40
35
30
FIGURE 17.15
Signal-to-noise ratio as a function of
light power for a single bR photore-
ceptor. A laser light source operating
at a wavelength of 568 nm, with an
output power ranging between 1.45
and 22 mW, is used to acquire the data
for this experiment.
25
20
15
0
5
10
15
20
25
Illumination power (mW)
17.4.2
Linearity and Dynamic Range
Bacteriorhodopsin photoreceptors exhibit differential response when illuminated by a step
light signal. Analysis of the equivalent circuit developed in Section 17.3.1 reveals the origin
of this differential photosensitivity. The photoreceptor model consists of a serially con-
nected resistor and capacitor, as well as a parallel connected resistor and capacitor. When a
switched integrator is used as the front-end amplifier, the membrane capacitance
C
m
and
resistance
R
m
are effectively shorted out, allowing the bR photoreceptor to function as a
high-pass filter. Polarity of the photoinduced current changes at the rising and falling edges
of the input-step signal, which corresponds to the charge displacement inside the bR film.
Photodetection systems can be characterized by their linearity over a wide dynamic
range so that the data can be reproduced with accuracy. A photodetector is considered lin-
ear if the generated photocurrent increases proportionally to the incident light power. The
maximum signal level that can be detected by a photodetector is its saturation level. The
lowest level of the detectable light is determined by noise in the system.
The peak of the differential response produced by a bR photodetector varies directly with
changes in light intensity (provided that saturation does not occur). It has been reported that
a bR detector can maintain its linearity even when exposed directly to sunlight (4). This
material also responds to very small change of light intensity, demonstrating high sensitivity.
This property is investigated experimentally, where a tunable argon/krypton laser system
with nine selectable wavelengths ranging from 476 nm (blue) to 676 nm (red) (Melles Griot,
35KAP431-220) is used as the light source. Compared with other sources of light, laser can
produce an intense beam of light with an extremely pure wavelength (monochromatic), a
fixed phase relationship (coherence), and a very low rate of expansion (highly collimated).
