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.
