30-04-2012, 04:20 PM
INFRA RED DETECTORS
[attachment=20974]
INTRODUCTION
Infrared (IR) detectors have been called the eyes of the digital battlefield. Military applications
in Western countries have spearheaded and dominated the requirements in this field akin to many
other emerging fields. In addition to many military applications for IR systems such as target
acquisition, search and track, missile seeker guidance, there is a great potential for IR systems in the
commercial market. IR systems enhance automobile and aircraft safety, medical diagnosis, and
manufacturing quality and control. Industry is looking to expand into the commercial market
because the military market is decreasing and concurrently becoming more specialized.
MODERN HISTORY
Modern history of IR detectors commenced with development of first IR detector by Case in
1917. He discovered that a substance made of thallium and sulphur exhibited photoconductivity.
Later he found that addition of oxygen enhanced the response. However, it had problems of noise
and stability. World War II stirred further research in detectors. In 1940 s Photon detectors were
developed to improve sensitivity and response time. Lead sulfide (PbS) was the first practical IR
detector. PbS is sensitive to infrared wavelengths up to 3μm. IR frequencies are in millions of
Hertzs; hence, it is easier to express waves in microns.
IR FUNDAMENTALS
IR radiations are electromagnetic (EM) waves where wavelengths are larger than those of red
light. IR radiation occurs between 0.75μm to 1000μm. Various authors have published different
proposals of division of IR range. The division shown below (Table1) is based on Hudson [9]. 1μm
is sensitivity limit of popular Si detectors. Similarly, wavelength 3 μm is sensitivity of PbS and
InGaAs detectors. 6 μm is sensitivity limit of InSb, PbSe, PtSi detectors and MCT detectors are
optimised for 3 5 μm atmospheric window; and finally wavelength 15 μm is a long wavelength
sensitivity limit of HgCdTe detectors optimised for 8 14 μm atmospheric window.
Atmospheric transmission
Atmospheric transmission is a must for all IR applications on earth.. IR applications require
radiation transmission through air, but the processes of scattering and absorption attenuates the
radiation. Scattering causes a change in the direction of a radiation beam; it is caused by absorption
and subsequent reradiation of energy by suspended particles. For larger particles, scattering is
independent of wavelength. Scattering by gas molecules is negligibly small for wavelengths longer
than 2 μm. Also, smoke and light mist particles are usually small with respect to IR wavelengths,
and IR radiation can therefore penetrate further through smoke and mists than visible radiation.
[attachment=20974]
INTRODUCTION
Infrared (IR) detectors have been called the eyes of the digital battlefield. Military applications
in Western countries have spearheaded and dominated the requirements in this field akin to many
other emerging fields. In addition to many military applications for IR systems such as target
acquisition, search and track, missile seeker guidance, there is a great potential for IR systems in the
commercial market. IR systems enhance automobile and aircraft safety, medical diagnosis, and
manufacturing quality and control. Industry is looking to expand into the commercial market
because the military market is decreasing and concurrently becoming more specialized.
MODERN HISTORY
Modern history of IR detectors commenced with development of first IR detector by Case in
1917. He discovered that a substance made of thallium and sulphur exhibited photoconductivity.
Later he found that addition of oxygen enhanced the response. However, it had problems of noise
and stability. World War II stirred further research in detectors. In 1940 s Photon detectors were
developed to improve sensitivity and response time. Lead sulfide (PbS) was the first practical IR
detector. PbS is sensitive to infrared wavelengths up to 3μm. IR frequencies are in millions of
Hertzs; hence, it is easier to express waves in microns.
IR FUNDAMENTALS
IR radiations are electromagnetic (EM) waves where wavelengths are larger than those of red
light. IR radiation occurs between 0.75μm to 1000μm. Various authors have published different
proposals of division of IR range. The division shown below (Table1) is based on Hudson [9]. 1μm
is sensitivity limit of popular Si detectors. Similarly, wavelength 3 μm is sensitivity of PbS and
InGaAs detectors. 6 μm is sensitivity limit of InSb, PbSe, PtSi detectors and MCT detectors are
optimised for 3 5 μm atmospheric window; and finally wavelength 15 μm is a long wavelength
sensitivity limit of HgCdTe detectors optimised for 8 14 μm atmospheric window.
Atmospheric transmission
Atmospheric transmission is a must for all IR applications on earth.. IR applications require
radiation transmission through air, but the processes of scattering and absorption attenuates the
radiation. Scattering causes a change in the direction of a radiation beam; it is caused by absorption
and subsequent reradiation of energy by suspended particles. For larger particles, scattering is
independent of wavelength. Scattering by gas molecules is negligibly small for wavelengths longer
than 2 μm. Also, smoke and light mist particles are usually small with respect to IR wavelengths,
and IR radiation can therefore penetrate further through smoke and mists than visible radiation.