7.17 Infrared Instruments
235
7.17.5 IR Motion Detection
By detecting the shift in frequency of emitted infrared radiation, one can detect the
motion of the reflecting objects. This is possible because the returned waves are
Doppler shifted in frequency by f = f/(1 − v cos θ/v s ), where f is the frequency
of the emitted wave, v is the speed of the object, v s is the speed of light in the
medium in which the light is propagating, and θ is the angle between the direction
of motion of the object and the direction from the object to the detector.
7.17.6 Infrared Communications
Infrared light can carry information, as any wave can. But infrared light is easily
generated and detected by solid-state devices. The ubiquity of remote control
devices shows how useful this form of communication is. Being limited in air
to ‘line-of-sight’ is an advantage when the range of the device is intended to
be relatively short. IR light absorption in the air is strongly dependent on the
IR resonances in carbon dioxide, water, ozone, methane, nitrous oxide, and other
molecules in the air, and on any dust or aerosols present.
7.17.7 IR Solid-State (LED) Emitters
A light-emitting diode (LED) uses an electric field across an n-p junction 29 in a
solid-state diode to cause some electrons in the n-type semiconductor to gain enough
energy to surmount the n-p junction potential difference. When those electrons
drop in energy on entering the p-type semiconductor, the energy they lose can go
into light. If the n-p juncture is close to the surface of the p-type material of the
semiconductor, and the conductor on the surface is transparent, striated, or only
partially covers the crystal surface, then the light can escape. The process is called
‘electroluminescence’, and typically produces infrared or visible light, depending
on the crystal material and its doping. The first LEDs were made from galliumarsenide-phosphide semiconductors, and emitted in the infrared (about 900 nm).
29 If the semiconductor silicon or germanium, which have four valence electrons in each atom, is
doped with arsenic, phosphorous, or antimony (all with five valence electrons) it becomes an n-type
semiconductor, i.e. electrons available for conduction is higher than for pure silicon or germanium.
If silicon or germanium is doped with boron, aluminum, gallium, or indium which have only 3
valence electrons in each atom, the material becomes a ‘p-type’ semiconductor, i.e. it has fewer
available electrons for conduction than the pure silicon or germanium.
235
7.17.5 IR Motion Detection
By detecting the shift in frequency of emitted infrared radiation, one can detect the
motion of the reflecting objects. This is possible because the returned waves are
Doppler shifted in frequency by f = f/(1 − v cos θ/v s ), where f is the frequency
of the emitted wave, v is the speed of the object, v s is the speed of light in the
medium in which the light is propagating, and θ is the angle between the direction
of motion of the object and the direction from the object to the detector.
7.17.6 Infrared Communications
Infrared light can carry information, as any wave can. But infrared light is easily
generated and detected by solid-state devices. The ubiquity of remote control
devices shows how useful this form of communication is. Being limited in air
to ‘line-of-sight’ is an advantage when the range of the device is intended to
be relatively short. IR light absorption in the air is strongly dependent on the
IR resonances in carbon dioxide, water, ozone, methane, nitrous oxide, and other
molecules in the air, and on any dust or aerosols present.
7.17.7 IR Solid-State (LED) Emitters
A light-emitting diode (LED) uses an electric field across an n-p junction 29 in a
solid-state diode to cause some electrons in the n-type semiconductor to gain enough
energy to surmount the n-p junction potential difference. When those electrons
drop in energy on entering the p-type semiconductor, the energy they lose can go
into light. If the n-p juncture is close to the surface of the p-type material of the
semiconductor, and the conductor on the surface is transparent, striated, or only
partially covers the crystal surface, then the light can escape. The process is called
‘electroluminescence’, and typically produces infrared or visible light, depending
on the crystal material and its doping. The first LEDs were made from galliumarsenide-phosphide semiconductors, and emitted in the infrared (about 900 nm).
29 If the semiconductor silicon or germanium, which have four valence electrons in each atom, is
doped with arsenic, phosphorous, or antimony (all with five valence electrons) it becomes an n-type
semiconductor, i.e. electrons available for conduction is higher than for pure silicon or germanium.
If silicon or germanium is doped with boron, aluminum, gallium, or indium which have only 3
valence electrons in each atom, the material becomes a ‘p-type’ semiconductor, i.e. it has fewer
available electrons for conduction than the pure silicon or germanium.