Sensors used in optical applications

Chapter 1: Introduction

The rapid development of electronic devices used for measurement in the last 30 years has led to a replacement of classic measuring instruments with optical sensors in human activities. This was possible due to the increase in the degree of accuracy and optical sensors presenting a series of properties superior to classic measuring devices. Among them we mention the much smaller dimensions and the elimination of some vulnerabilities that can lead to inherent accidents.

While sensors have progressed rapidly in the last years, they had been around for over 150 years with the first sensor being used to record temperatures. In 1860 Wilhelm von Siemens used the knowledge that different materials have different electrical resistances and temperature sensitivities and developed a temperature sensor based on a copper resistor.

In 1980, sensors became of vital importante for the development of computers and communication technologies, so much so that the US established in the same decade the National Technology Group (or the BGT) to better organize and develop sensors in state owned companies.

In the following years, sensors became more and more used in everyday objects such as elevator buttons (which are tactile sensors) and lamps which increase or decrease light by touching the base. While in th past, sensors were only used for the measurement of temperature, pressure and flow measurement, with the advances in the fields of micromachinery and easy to use microcontroller platforms, their uses have expande togheter with their performances and with that, their sizes have decreased.

The decrease in size is important for sensors, since they are usually designed to avoid interference with the measured data, making the sensor smaller will improve accuracy and may offer other advantages like more convinience and better space management.

It should be noted however that high quality sensors obeys the following rules:

˗ high degree of sensitivity to the measured property

˗ very insensitive to any other property likely to be encountered during the measurements

˗ it has a very low or null influence on the measured property

The present work is structured in 5 distinc chapters, through which it is desired to demonstrate the large scale use of optical sensors.

In the first chapter called introduction, it was presented in general terms what optical sensors are, a short history, their qualities and how to properly use them.

In chapter 2, the main categories of optical sensors, their properties, their use in various fields and their operating principles were reviewed.

In chapter 3, 2 optical sensors were chosen, which have become common in human activity, with wide use, respectively sensors for measuring temperature and sensors for measuring the percentage of oxygen in the blood. Sensors were chosen, the electronic circuit was built, after which the 2 sensors were calibrated experimentally. Finally, some conclusions will be drawn based on the results.

In chapter 4, the practical applications and fields of use of optical sensors were reviewed, together with the performances of sensors used.

In chapter 5, a series of final conclusions were formulated, regarding the use of optical sensors.

Chapter 2 : Sensors. Definition, classification

An optical sensor is a device that converts light rays into electronic signals, as such they are highly influented by the changes in wavelengths of the enviroment. The sensors can detect frequency, wavelength or polarization of light and convert these into electric signals with the help of the photoelectric effect.

There are multiple ways to catalogue the types of sensors, such as:

- Temperature Sensor

- Proximity Sensor

- Accelerometer

- Infrared Sensor

- Fiber optics Sensors

- Pressure Sensor

- Light Sensor

- Smoke, Gas and Alcohol Sensor

Temperature sensors:

Temperature sensors are devices that provide readings via electrical signals, with the sensor being made up of two metals that generate an electrical voltage or resistance once it has noticed a change in temperature. This type of sensor is vital in maintaining a constant temperature.

There are two main types of temperature sensors:

˗ contact temperature sensors

˗ non-contact temperature sensors