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Partial History of Thermal Testing
The detection of thermal energy is not a problem
for the human body. Some sources say that the nerve endings in
human skin respond to temperature changes as small as 0.009oC
(0.0162oF). While humans have always had the ability
to detect thermal energy, they have not had a way to quantify
temperature until a few hundred years ago. A few of the more significant
thermal measurement advances are discussed in the following paragraphs.
The Thermometer
 Ancient
Greeks knew that air was expanded by heat. This knowledge was
eventually used to develop the thermoscope, which traps air in
a bulb so that the size of the bulb changes as the air expands
or contracts in response to a temperature increase or decrease.
The image on the right shows the first published sketch of a thermoscope,
which was published by Italian inventor Santorio Santorii. The
next step in making a thermometer was to apply a scale to measure
the expansion and relate this to heat. Some references say that
Galileo Galilei invented a rudimentary water thermometer in 1593
but there is no surviving documentation to support his work on
this. Therefore, Santorii is regarded as the inventor of the thermometer,
for he published the earliest account of it in 1612. Gabriel Fahrenheit
invented the first mercury thermometer in 1714.
Infrared Energy
Sir William Herschel, an astronomer, is credited with the discovery
of infrared energy in 1800. Knowing that sunlight was made up
of all the colors of the spectrum, Herschel wanted to explore
the colors and their relationship to heat. He devised an experiment
using a prism to spread the light into the color spectrum and
thermometers with blackened bulbs to measure the temperatures
of the different colors. Herschel observed an increase in temperature
from violet to red and observed that the hottest temperature was
actually beyond red light. Herschel termed the radiation causing
the heating beyond the visible red range "calorific rays."
Today, it is called "infrared" energy.
The Seebeck Effect (Thermocouples)
 In
1821, Thomas Johann Seebeck found that a circuit made from two
dissimilar metals, with junctions at different temperatures, would
deflect a compass needle. He initially believed this was due to
magnetism induced by a temperature difference, but soon realized
that it was an electrical current that was induced. More specifically,
the temperature difference produces an electric potential (voltage)
which can drive electric current in a closed circuit. Today, this
is known as the Seebeck effect.
The voltage difference, DV, produced across the
terminals of an open circuit made from a pair of dissimilar metals,
A and B, whose two junctions are held at different temperatures,
is directly proportional to the difference between the hot and
cold junction temperatures, Th - Tc. The
Seebeck voltage does not depend on the distribution of temperature
along the metals between the junctions. This is the physical basis
for a thermocouple, which was invented by Nobili in 1829.
Noncontact Thermal Detectors
Melloni soon used the thermocouple technology to produce a device
called the thermopile. A thermopile is made of thermocouple junction
pairs connected electrically in series. The absorption of thermal
radiation by one of the thermocouple junctions, called the active
junction, increases its temperature. The differential temperature
between the active junction and a reference junction kept at a
fixed temperature produces an electromotive force directly proportional
to the differential temperature created. This effect is called
a thermoelectric effect. Melloni was able to show that a person 30
feet away could be detected by focusing his or her thermal energy on the
thermopile. Thermopile detectors are used today for spectrometers,
process temperature monitoring, fire and flame detection, presence
monitor, and a number of other non-contact temperature measurement
devices. A device similar to the thermopile measured a change
in electrical resistance rather than a voltage change. This device
was named the bolometer, and in 1880 it was shown that it could
detect a cow over 1000 feet away.
During World War I, Case became the first to experiment
with photoconducting detectors. These thallium sulfide detectors
produced signals due to the direct interaction of infrared photons
and were faster and much more sensitive than other thermal detectors
that functioned from being heated. During World War II, photoconductive
or quantum detectors were further refined and this resulted in
a number of military applications, such as target locating, tracking,
weapons guiding and intelligence gathering.
Imaging Systems
Application areas expanded to surveillance and intrusion during
the Vietnam era. Shortly thereafter space-based applications
for natural resource and pollution monitoring and astronomy were
developed. IR imaging technology developed for the military spilled
over into commercial markets in the 1960s. Initial applications
were in laboratory level R&D, preventative maintenance applications,
and surveillance. The first portable systems suitable for NDT
applications were produced in the 1970s. These systems utilized
a cooled scanned detector and the image quality was poor by today's
standards. However, infrared imaging systems were soon being widely
used for a variety of industrial and medical applications.
In the late 1980s, the US military released the
focal plane array (FPA) technology into the commercial marketplace.
The FPA uses a large array of tiny IR sensitive semiconductor
detectors, similar to those used in charge couple device (CCD)
cameras. This resulted in a dramatic increase in image quality.
Concurrently, the advances in computer technology and image processing
programs helped to simplify data collection and to improve data
interpretation.
Current State
In 1992, the American Society for Nondestructive Testing officially
adopted infrared testing as a standard test method. Today, a wide
variety of thermal measurement equipment is commercially available
and the technology is heavily used by industry. Researchers continue
to improve systems and explore new applications.
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