312 CHEMICAL SENSORS. VOLUME 5: ELECTROCHEMICAL AND OPTICAL SENSORS Optical chemical sensors offer several advantages in various fields of application. These sensors are capable of observing a sample in its dynamic environment, no matter how distant, diffi cult to reach, or hostile this environment is. These devices are intrinsically safe, involving a low optical power, and are nonelectrical at the sensing point. These sensors are electrically passive and immune to electromagnetic disturbances, are geometrically flexible and corrosion-resistant, are capable of being miniaturized, and are compatible with telemetry. Furthermore, the possibility of multiplexing several sensors to a single instrumentation unit can afford substantial economic advantages to this type of chemical sensor. These features impart to the sensor immense potential importance in biomedical, process, and environmental monitoring applications. Optical sensors also possess a few limitations which may need to be overcome if they are to replace existing sensor devices such as electrochemical sensors. The equilibrium involved in the chemical trans- duction restricts the concentration range over which optical sensors are effectively useful. The dynamic range of optical sensors that have been studied to date extend over one or two orders of magnitude and, thus, a series of chemical transducers is required in order to sense a single analyte at widely different levels of concentration. In some chemical transduction systems, the equilibrium involved limits the reversibility characteristics of optical sensors. Ideally, these sensors should be capable of continuous use. However, in systems where a reaction is involved, the equilibrium state favors the formation of products and this results in the consumption of the reagent by its conversion to some other form; in some cases the reagent can be regenerated using another chemical reaction. Another limitation is the response time