| Atmospheric Transmission |
| Absorption of the infrared energy by the atmosphere.
High transmission ranges are known as “atmospheric
windows” through which infrared imaging over very
long distances can be performed. |
| |
| Electromagnetic Spectrum |
|
The electromagnetic spectrum divides up the regions
of electromagnetic radiation into different ranges
having unique characteristics. This radiation is divided
up rather arbitrarily into a number of regions based
on wavelength: Gamma <10 nanometers, Ultraviolet
radiation, Visible light 0.4 to 0.7 micrometers, Infrared
Radiation, Microwaves, Radio waves. The following
is a sub-categorization for the infrared range relevant
for night vision:
 |
Short wave infrared range (SWIR): Also known
as the Near infrared range, that portion of the
infrared spectrum from 750nm to 2500nm. |
|
Mid wave infrared range (MWIR): That portion
of the infrared spectrum from about 3 microns
to 5 microns. |
|
Long wave infrared range (LWIR): That portion
of the infrared spectrum from about 8 microns
to 12 microns. |
|
| |
| Generations
of Image Intensifiers |
| The different paradigms of image intensifier technology
have been identified by
“generations” of technology (also known as “Gen”).
Generation 0 technology first developed in the 1950s depended
on near infrared illumination to produce reasonable night
vision images. After the light was converted to electrons,
these electrons were accelerated so they hit a phosphor screen
with greater energy, creating a visible image. Unfortunately,
the accelerated electrons were somewhat distorted and vision
with this method was impaired. Generation 1 image intensifiers
were then developed that used a photocathode material that
was better than Gen 0 in converting light to electrons. These
units were able to operate at lower light levels than the
Gen 0 and, became known as "starlight scopes" since
near infrared illumination was not required. When three tubes
were cascaded together, the sensitivity was sufficient for
most night vision applications, but distortion existed. Generation
2 image intensifiers marked the development of a microchannel
plate which multiplies the number of electrons by the thousands.
A single unit of a Generation 2 image intensifier produced
the same sensitivity as a 3-tube cascaded Generation 1 device
but in a much small package and without distortion. Generation
3 is the most sophisticated night vision technology available
today. The image intensifier's photocathode is coated with
sensitive gallium arsenide, which allows for a more efficient
conversion of light to electrical energy at extremely low
levels of light. Generation 3 provides the clearest, sharpest
night vision image available. |
| |
| Image Intensifier
Tube |
| An electro-optical device which converts photons to
electrons, amplifies them, then converts them back to
photons so the user can see at light levels that are
normally too low. |
| |
| Infrared |
| The range of electromagnetic radiation having a wavelength
longer than that of visible light and shorter than that
of microwave radiation. The name “infrared”
translates to "below red", where red is the color of visible
light of longest wavelength. Infrared radiation spans the
wavelengths between approximately 750nm (0.75 microns) and
1mm (1000 microns). |
| |
| Microbolometer |
| An infrared detector that absorbs the IR radiation
and warms slightly; the electrical resistance across
the bolometer changes as a function of temperature,
which can be measured and made into a thermal image.
See also our White
Paper about Uncooled Microbolometers. |
