Flir ThermovisionPathFindIR LE User Manual

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true only until 1830, when the Italian investigator, Melloni, made his great 
discovery that naturally occurring rock salt (NaCl)—which was available in 
large enough natural crystals to be made into lenses and prisms—is 
remarkably transparent to the infrared. The result was that rock salt 
became the principal infrared optical material, and remained so for the 
next hundred years, until the art of synthetic crystal growing was mastered 
in the 1930’s.
Figure 8-3: Macedonio Melloni (1798–1854)
Thermometers, as radiation detectors, remained unchallenged until 1829, 
the year Nobili invented the thermocouple. (Herschel’s own thermometer 
could be read to 0.2 °C (0.036 °F), and later models were able to be read 
to 0.05 °C (0.09 °F)). Then a breakthrough occurred; Melloni connected a 
number of thermocouples in series to form the first thermopile. The new 
device was at least 40 times as sensitive as the best thermometer of the 
day for detecting heat radiation—capable of detecting the heat from a 
person standing three meters away.
The first so-called ‘heat-picture’ became possible in 1840, the result of 
work by Sir John Herschel, son of the discoverer of the infrared and a 
famous astronomer in his own right. Based upon the differential 
evaporation of a thin film of oil when exposed to a heat pattern focused 
upon it, the thermal image could be seen by reflected light where the 
interference effects of the oil film made the image visible to the eye. Sir 
John also managed to obtain a primitive record of the thermal image on 
paper, which he called a “thermograph”. 
						

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Figure 8-4: Samuel P. Langley (1834–1906)
The improvement of infrared-detector sensitivity progressed slowly. 
Another major breakthrough, made by Langley in 1880, was the invention 
of the bolometer. This consisted of a thin blackened strip of platinum 
connected in one arm of a Wheatstone bridge circuit upon which the 
infrared radiation was focused and to which a sensitive galvanometer 
responded. This instrument is said to have been able to detect the heat 
from a cow at a distance of 400 meters. 
An English scientist, Sir James Dewar, first introduced the use of liquefied 
gases as cooling agents (such as liquid nitrogen with a temperature of -
196 °C (-320.8 °F)) in low temperature research. In 1892 he invented a 
unique vacuum insulating container in which it is possible to store 
liquefied gases for entire days. The common ‘thermos bottle’, used for 
storing hot and cold drinks, is based upon his invention. 
Between the years 1900 and 1920, the inventors of the world ‘discovered’ 
the infrared. Many patents were issued for devices to detect personnel, 
artillery, aircraft, ships—and even icebergs. The first operating systems, in 
the modern sense, began to be developed during the 1914–18 war, when 
both sides had research programs devoted to the military exploitation of 
the infrared. These programs included experimental systems for enemy 
intrusion/detection, remote temperature sensing, secure communications, 
and ‘flying torpedo’ guidance. An infrared search system tested during this 
period was able to detect an approaching airplane at a distance of 1.5 km 
(0.94 miles), or a person more than 300 meters (984 ft.) away. 
The most sensitive systems up to this time were all based upon variations 
of the bolometer idea, but the period between the two wars saw the 
development of two revolutionary new infrared detectors: the image  
						

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converter and the photon detector. At first, the image converter received 
the greatest attention by the military, because it enabled an observer for 
the first time in history to literally ‘see in the dark’. However, the 
sensitivity of the image converter was limited to the near infrared 
wavelengths, and the most interesting military targets (i.e. enemy soldiers) 
had to be illuminated by infrared search beams. Since this involved the 
risk of giving away the observer’s position to a similarly-equipped enemy 
observer, it is understandable that military interest in the image converter 
eventually faded. 
The tactical military disadvantages of so-called 'active’ (i.e. search beam-
equipped) thermal imaging systems provided impetus following the 1939–
45 war for extensive secret military infrared-research programs into the 
possibilities of developing ‘passive’ (no search beam) systems around the 
extremely sensitive photon detector. During this period, military secrecy 
regulations completely prevented disclosure of the status of infrared-
imaging technology. This secrecy only began to be lifted in the middle of 
the 1950’s, and from that time adequate thermal-imaging devices finally 
began to be available to civilian science and industry.
8.2 How do Infrared Cameras Work?
Infrared energy is part of a complete range of radiation called the 
electromagnetic spectrum. The electromagnetic spectrum includes gamma 
rays, X-rays, ultraviolet, visible, infrared, microwaves (RADAR), and radio 
waves. The only difference between these different types of radiation is 
their wavelength or frequency. All of these forms of radiation travel at the 
speed of light (186,000 miles or 300,000,000 meters per second in a 
vacuum). Infrared radiation lies between the visible and RADAR portions 
of the electromagnetic spectrum. Thus infrared waves have wavelengths 
longer than visible and shorter than RADAR. 
						

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The primary source of infrared radiation is heat or thermal radiation. Any 
object which has a temperature radiates in the infrared portion of the 
electromagnetic spectrum. Even objects that are very cold, such as an ice 
cube, emit infrared. When an object is not quite hot enough to radiate 
visible light, it will emit most of its energy in the infrared. For example, hot 
charcoal may not give off light, but it does emit infrared radiation which 
we feel as heat. The warmer the object, the more infrared radiation it 
emits. 
Infrared cameras produce an image of invisible infrared or “heat” radiation 
that is unseen by the human eye. There are no colors or “shades” of gray 
in infrared, only varying intensities of radiated energy. The infrared imager 
converts this energy into an image that we can interpret. Several detector 
technologies exist; the sensor in the PathFindIR LE is of the latest solid 
state design, offering long life and fully automatic image optimization 
(contrast and gain). True thermal imagers should not be confused with 
infrared illuminator cameras that are often presented as simply “infrared 
cameras.” There are hundreds of low cost infrared illuminated cameras on 
the market at prices below $100. These cameras do not produce the same 
image because they do not detect heat. They operate in wavelengths near 
visible, and require an IR illuminator to provide an image. IR illuminators 
have very short range, and require a lot of power to see beyond 5 meters.
Figure 8-5: Electromagnetic Spectrum 
						

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Document History
Table -1: Revision History
Revision Date Comment
100 May 15, 2008 Initial release 
						

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