Biomedical Engineering Reference
In-Depth Information
2.3.4.2 Sensitive Area
As previously described, the optical detection of SHG signal can be performed in de-scan and non-
de-scan mode. In de-scan mode, the stability of the focused SHG light allows the use of a detector with
a very small sensitive area, such as an APD. On the other hand, in non-de-scan mode, it is necessary to
use a detector with larger sensitive area, such as a PMT. The dimension of the scanned area depends on
the magnification obtained with the objective and the focusing optics. Therefore, the choice of the sen-
sitive area of the detector must be operated taking into account the maximum scanning area with the
properties of the optical detection system. A small sensitive area can cause a limited field of view, due to
dependence of the detected intensity on the position of the scanned beam. This effect is translated in a
bright central region and darker sides in the images.
2.3.4.3 electrical Detection Modality
The most important issue to be considered when choosing a detector for an SHG microscope is the
modality in which the detector is going to be used: proportional mode or photon-counting regime.
Each of these two modalities requires a detector with particular features in terms of temporal
response and dynamic range. In an ideal detector (with quantum efficiency equal to 1), an output
pulse is provided every time a photon interacts with the photocathode. In the real scenario, the quan-
tum efficiency is <1, but the output pulse rate is still proportional to the rate of incoming photons
(Figure 2.16).
•
Proportional regime.
If the pulse rate is equal or greater than the inverse of the pulse width, the
detector pulses are superimposed resulting in a current signal, which is proportional to the inten-
sity of the detected light (Figure 2.16a). The detection is obtained by integrating the output signal
with a current integrator. The time constant of the circuit should be chosen according to the
sampling rate. The proportional regime has to be used if a high light level is expected and a fast
detector is not required.
Integrator
(a)
C
Arrival of photons
R
Photoelectron emission
Output
(voltage)
Output pulses
Output signal
(overlapped pulses)
(b)
Arrival of photons
Pulse
counter
Photoelectron emission
Output
(number of pulses)
Output signal
(separated pulses)
FIgurE 2.16
Detection modality with the corresponding acquisition principle. (a) Proportional regime: Output
pulses are overlapped in time. The output voltage is proportional to the detected light intensity over the integration
time. (b) Photon-counting regime: Pulses are well separated in time. The output is the number of pulses detected
in the pixel dwell time.
