# Optoelectronic Instrumentation and Measurement Strategies for Optical Chemical (Bio)Sensing
## 3 Spectroscopic Techniques
### Measurement of Luminescence Intensity  
The measurement of luminescence (fluorescence and phosphorescence) intensity is a simple technique that can be used with all of the optical phenomena we have mentioned. It consists of measuring the light signal that is produced in a permanent regime. For weakly absorbent samples, the intensity of luminescence, \(I\), is a fraction of the intensity of the excitation light, \(I_{ex}\), and obeys [PERSON]'s law [13]:  
\[I=(2.3)\ k\ I_{ex}\ \phi\ \epsilon_{\lambda}\ l\ C \tag{6}\]  
where \(\phi\) is the quantum yield of the luminophore, \(\epsilon_{\lambda}\) is the molar absorbance at \(\lambda_{ex}\), \(l\) is the length of penetration into the sample, \(C\) is the concentration of the luminophore, and \(k\) a geometric factor, which depends on the configuration of the measurement system.  
Figure 8 shows the instrumentation setup of a fiber optic sensor based on the measurement of luminescence intensity. An LED is commonly used as the light source if the chosen LED is capable of emitting at the appropriate wavelengths to excite the optical sensor. It is also common to modulate the light intensity at a certain frequency to reduce the effect of external interference. Another option is to use a laser diode, which can also be modulated, although the particularities of the characteristic curve do not make laser diodes the most suitable for this purpose. The optical filter between the fiber end and the light detector can improve the S/N ratio by reducing the presence of external light sources. The light detector must respond to the wavelengths selected by the optical filter. The choice depends mainly on the light levels. If these are very low, a multiplier tube should be used if price is not a limiting factor. As an alternative at lower cost, avalanche photodiodes can be used, which have internal gain, although are dependent on temperature.  
The light intensity measurements can be disturbed by external factors such as fluctuations in the light source, coupling of optical fibers, changes in fiber attenuation due to curvature, and the length of the optical path [37; 38]. Despite these drawbacks, intensity measurements are still used successfully to study the structure and molecular dynamics of biological and environmental sensors due to their high sensitivity and selectivity [39]. In addition, according to the application, this technique can be more versatile than other techniques, such as in the measurement of lifetimes, because sensors that exhibit changes in intensity against such parameters do not induce changes in lifetimes.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Feature** & & **Fluorescence** & & **Phosphorescence** \\ \hline Probability of happening & \(\bullet\) & High & \(\bullet\) & Low \\ Emission wavelength & \(\bullet\) & Close to the excitation & \(\bullet\) & Separate from excitation \\ Time in the excited state & \(\bullet\) & Short (2–20 ns) & \(\bullet\) & Long (1 \(\upmu\)s–10 s) \\ Light intensity & \(\bullet\) & Relatively high & \(\bullet\) & Low \\ \hline \hline \end{tabular}
\end{table}
Table 5: Comparison of fluorescence and phosphorescence phenomena.