Fundamentals of Instrumentation and Measurement

Chapter 3¹

Physical Principles of Optical, Thermal and Mechanical Sensors

There are now so many sensors available [NOR 89] that it would be impossible to discuss the principles of all of them in a single chapter. We have therefore limited ourselves to three classes of measurands: optical, thermal and mechanical. However, even with this restriction, we still must limit our scope, and will only present the most frequently used laws for these types of physical sensors.

3.1. Optical sensors

One important class of sensors detects electromagnetic beams. Within this group, we will restrict our discussion to those optic sensors that are sensitive only to beams with wavelengths of 10 nm − 1 mm, that is, frequencies of between 10¹⁶ and 10¹¹ Hz.

In the specific case of light sensors, for reasons relating to the sensitivity of the human eye, it is necessary to introduce specific concepts when discussing visibility (0.4 μm to 0.8 μm). After a brief recapitulation of the variables that act as measurands for optical sensors, we will define the reference light source used in making calibrations. We will then discuss the principles of sensors that are used in constructing semiconductors.

3.1.1. Energetic flux

An electromagnetic beam transmits energy. One way to see this is to place a thermometer with a darkened reservoir in an electromagnetic beam and check that the temperature increases. We know that the flux of Poynting’s vector, the vectorial product of electrical and magnetic fields, across ...