Introduction
Thermal Infrared Testing is
also known as (AKA Thermal Inspection, Thermography, Thermal
Imaging, Thermal Wave Imaging and Infrared Testing)
(Photo
courtesy of NASA/JPL-Caltech/IPAC)
Thermal NDT methods involve
the measurement or mapping of surface temperatures as heat flows to, from
and/or through an object. The simplest thermal measurements involve making
point measurements with a thermocouple. This type of measurement might be
useful in locating hot spots, such as a bearing that is wearing out and
starting to heat up due to an increase in friction.
In its more advanced form,
the use of thermal imaging systems allow thermal information to be very rapidly
collected over a wide area and in a non-contact mode. Thermal imaging systems
are instruments that create pictures of heat flow rather than of light. Thermal
imaging is a fast, cost effective way to perform detailed thermal analysis. The
image above is a heat map of the space shuttle as it lands.
Thermal measurement methods
have a wide range of uses. They are used by the police and military for night
vision, surveillance, and navigation aid; by firemen and emergency rescue
personnel for fire assessment, and for search and rescue; by the medical
profession as a diagnostic tool; and by industry for energy audits,
preventative maintenance, processes control andnondestructive testing.
The basic premise of thermographic NDT is that the flow of heat from the
surface of a solid is affected by internal flaws such asdisbonds,
voids or inclusions. The use of thermal imaging systems for industrial NDT
applications will be the focus of this material.
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 thethermoscope, 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 athermoscope, which was published by Italian
inventorSantorio 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
noncontact 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 sulfidedetectors 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.