# Optoelectronic Instrumentation and Measurement Strategies for Optical Chemical (Bio)Sensing
## 2 Optical Devices
### Light Detectors  
A light detector (photodetector) transforms the optical signal into an electrical signal, preserving the original chemical information. The spectral characteristics of the photodetector must be adapted to the emission spectrum of the optical sensor to avoid a loss of information at critical wavelengths. In addition, the photodetector system must have a high sensitivity to ensure a high signal-to-noise ratio (SNR). The most used photodetectors in instrumentation are photodiodes and photomultiplier tubes (PMTs) [22]. Photodiodes (PDs) are the preferred detectors for low-cost instrumentation. They can operate at high levels of light without degradation. Depending on the semiconductor material, their spectral responses vary between 180 and 2600 nm. PDs are fast (depending on the internal capacitance, the bandwidth in some cases is up to 1 GHz), robust, and cheap, and in photovoltaic mode they do not require power. Their main drawbacks are their relatively high levels of noise and their lack of internal amplification. As their output signal is usually small, they require an additional amplifier, which also introduces noise. PDs can be used with low light intensities, limiting the signal bandwidth, and with noise cancellation techniques. Silicon PDs are appropriate for high-speed applications, such as for spectrophotometry, optical measurement equipment, analytical instrument, and radiation detection. InGaAs PDs used for near-infrared light detection are suitable for a wide range of applications, including optical communication, analysis, and measurement. UV LEDs are well suited for spectroscopic applications in instrumentation used for analytical and life sciences. Avalanche photodiodes (APDs) are devices that require a high reverse voltage (between 100 V and several kV), resulting in internal amplification due to the phenomenon of avalanche multiplication. APDs have a higher SNR than silicon photodiodes, as well as a fast response rate, low dark current, and high sensitivity. Their spectral range is between 200 and 1150 nm. In the non-avalanche mode, one can achieve internal amplification rates of between 10 and 100 times those of a PD. APDs are fast; they can be used at frequencies up to 10 GHz. Although their internal noise is higher than normal and more expensive \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Gas Discharge Lamps** & & **LEDs** & & **Laser Diodes** \\ \hline \(\bullet\) & Spectrum: UV–IR & \(\bullet\) & Spectrum: UV–IR & \(\bullet\) & Spectrum: Visible \\ \(\bullet\) & Low mechanical stability & \(\bullet\) & High energy efficiency & \(\bullet\) & Bandwidth: 0.5–3 nm \\ \(\bullet\) & External device to & \(\bullet\) & Bandwidth: 10–40 nm & \(\bullet\) & Optical power: 1.5–3 W \\  & modulate light & \(\bullet\) & Optical power: 0.5–20 & \(\bullet\) & Long life span \\ \(\bullet\) & Specially designed & & mW & \(\bullet\) & Direct modulation \\  & power supplies and & \(\bullet\) & Long life span & \(\bullet\) & Current) \\  & trigger sockets & \(\bullet\) & Direct electronic & \(\bullet\) & Specially designed \\ \(\bullet\) & Photometric instruments & & modulation & power supplies \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The main characteristics of light sources used for optical sensing. PDs, they are becoming increasingly popular because they can tolerate intense lighting (optical power meters) and their sensitivity is comparable to some PMTs. PMTs are the best choice when radiant sensitivity, speed, and minimum noise are the main requirements of the system. The internal amplification of a PMT is up to \(10^{6}\) or more. Another advantage is the possibility of larger areas of detection. The noise is significantly lower compared to solid-state devices. The drawbacks of PMTs are the need for a high-voltage power supply (600-1200 V) and the possibility of destruction by overexposure.