Biomedical Engineering Reference
In-Depth Information
These properties make laser a good tool for high-precision measurements. Linearity of a sin-
gle bR photoreceptor pixel is tested for three wavelengths (488, 568, and 647 nm, respec-
tively), where the laser beam is adjusted using a beam expander to cover the entire pixel
area. The illumination power level is plotted against the pixel photoresponse as shown in
Figure 17.16. It shows that the peak signal amplitudes for three wavelengths keep a good lin-
earity over the light power range between 200
W and 12 mW.
17.4.3
Spectral Response
The magnitude of the photocurrent that is generated by a bR photoreceptor is depend-
ent upon the wavelength of the incident light. bR responds mostly to visible light, where
its responsivity peak is located around 570 nm. Wavelength-dependent response is
measured using the same tunable argon/krypton laser system, and seven wavelengths
covering the whole visible range from blue, green, yellow to red are selected. The out-
put beam power for each wavelength is also tunable over the range between 200
W and
12 mW. Experimental data are plotted in a 3D, where the photoresponse is a function of
both illumination power and wavelength (Figure 17.17). It can be seen that the photore-
ceptor responds with higher sensitivity to some wavelengths than to others. Therefore,
the spectral response is also expressed in a plot of responsivity versus wavelength
(Figure 17.18). Compared with the absorption spectrum, Figure 17.5c, the maximum
photoelectric response occurs in the vicinity of the optical absorption peak of approxi-
mately 568 nm, indicating good agreement between the two experiments.
17.4.4
Response Time
Another beneficial feature of the bR-based photoreceptor is its fast response time. Unlike
many bioelectric signals that are generated by slow ionic diffusion process, the fast
response of the bR photoreceptor is caused by light-induced charge displacement. Fast sig-
nal rise time, on the order of picoseconds, can be obtained using ultrafast laser pulses (83).
The overall response time of a bR photoreceptor is also dependent on the amplification
7
568 nm
647 nm
488 nm
6
5
4
3
2
1
0
0
2
4
6
8
10
12
Illumination power (mW)
FIGURE 17.16
Linearity of photoelectric signals generated by a bR photoreceptor in response to the laser beam at wavelengths
of 568, 647, and 488 nm, respectively. Each data point corresponds to the peak photoreceptor response to a step
signal, showing that response is proportional to the input light power.
