Biomedical Engineering Reference
In-Depth Information
circuitry. To illustrate such effects, the response of bR photoreceptor to step light signals is
tested using durations of 2 s and 33 ms. A fast camera shutter is used to modulate a 568-nm
laser beam, and is capable of producing rise times that are less than 2 ms. In Figure 17.19a,
the top trace represents the output from a silicon photodetector operating in photovoltaic
mode and the bottoms trace is the output of a bR photoreceptor. It can be seen that the bR
photoreceptor reaches maximum response in approximately 40 ms. A step light signal with
33-ms time duration is chosen to better observe the response lag produced by the bR pho-
toreceptor. The comparison of the two traces in Figure 17.19b shows that the delayed
response is not an intrinsic characteristic of the bR protein, but a limitation of the front-end
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FIGURE 17.17
Measured response of a bR photoreceptor over varying wavelength and light power.
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Wavelength (nm)
FIGURE 17.18
Responsivity of a bR photoreceptor as a function of incident light wavelength.
