THERMOGRAPHY (THERMAL IMAGING) AN INTRODUCTION

In this section we will discuss about thermal imaging, another powerful tool in machine diagnostics & predictive maintenance arena. Thermography is an art & science to detect & measure radiation using electro-optical device & correlating to surface temperature of the subject. The history of thermal imaging can be understood by looking at the certain chronological developments in past. In 1672 Newton passed a beam of light though a prism & observed a coloured strip of light containing the colours purple, blue, green, yellow, orange and red. Later on Sir Frederick William Herschel in 1800 made an important discovery by moving Newton’s experiment one step ahead. He placed glass thermometers on different spectral colour patterns & observed a rising trend in temperature from blue to red colour spectrum. He also observed that the temperature beyond the visible colour spectrum was even higher. He concluded from this experiment that an invisible form of energy must be at work in that range, and that the sun emits invisible radiation beyond the visible light range. He called this radiation as ultra-red, now known as infrared radiation.

The laws of radiation that revolutionized the field of thermal imaging-

·      Kirchhoff’s Radiation Law: This states that every type of matter continuously radiates energy. The radiant energy is visible or invisible, depends on the temperature. Later on Kirchhoff coined the term Black Body which means that a body always emits precisely as much heat as it absorbs.

·       Hertz oscillator & Stefan-Boltzmann-law: In 1865 James C. Maxwell first predicted the assumption that light consists of electromagnetic waves which was later confirmed experimentally by Heinrich Rudolf Hertz. Josef Stefan and Ludwig Boltzmann refined Gustav Kirchhoff's theory in 1884 and established the findings of their research in the Stefan-Boltzmann law which states that the total thermal energy emitted by a black body depends on that body's intrinsic temperature.

·       Planck’s radiation law: In 1900 Max Planck did a breakthrough discovery which laid foundation of the present day understanding of electromagnetic radiation & gave birth to quantum physics, & still regarded as physical basis for thermography. It defines the intensity distribution of the electromagnetic energy emitted by a black body as a function of temperature, wavelength and frequency.

In principle thermograph is device to detect temperature pattern in the infrared wavelength of the subject. It started with the Herchel’s experiment & later on after discovery of seeback effect, which led to invention of the thermomultiplier, an early version of a thermocouple by Leopoldo Nobili. Samuel Langley used bolometer to detect body heat from a cow from a distance of 304 m. Sir William Herschel’s son Sir John Herschel, used a device called an evaporograph and produced the first infrared image in 1840. From photo-conducting detectors in early phase, the thermal imaging technology has advanced a lot till date. Unlike earlier cumbersome cooled detectors, now a days much sophisticated, uncooled simple detector technology is available at a reasonable price in the market. Till 1960’s the thermography technology was basically used in military applications but after that, non-military applications like medical, industrial & building maintenance stated taking prominance. Today thermography has emerged as a proven predictive maintenance technology in the industrial domain.