Thermal vision devices and riflescopes
What is a Thermal vision?
Every object having a temperature above the absolute zero (- 273.2°C) emits electromagnetic waves in infrared range.
According to the laws of physics, intensity of thermal emission is proportional to the fourth power of temperature of the heated object.
Consequently, detection ability of heated objects’emission by receivers sensitive in the infrared range depends mainly on temperature of the object and its surrounding background.
It does not depend practically on the illumination level in the visible range.
Infrared emission occupies an extensive part of the spectrum, which is commonly divided into several ranges:
Fig. 4. Spectral ranges of observation devices and atmospheric transparency windows vs. emission wavelength.
NV –Night vision device range (from 0.4 to 1 microns);
SWIR – Short wave Infrared range (from 0,76 to 3 microns);
MWIR – Mid wave Infrared range (from 3 to 6 microns);
LWIR – Long wave Infrared range (from 8 to 14 microns).
A commonly accepted division of infrared emission into ranges is connected with both sensitivity ranges of existing receivers and atmospheric transparency windows.
A night vision device works in visible and close infrared ranges (wavelengths from 0.4 to 1.0 microns) thanks to detection of natural and artifi cial light refl ected from observed objects.
A modern compact thermal vision device works in the range from 8 to 14 microns that corresponds to one of several atmospheric transparency windows.
The action principle of thermal vision devices is based on the ability of certain materials to register object images formed by infrared emissions and transform them into electrical signals.
Received electrical signals that are determined by the thermal scene of observed terrain are transferred onto built-in micro display after amplifi cation and software processing.
The display transforms these signals into a picture of observed objects that is visible by human eye.
The thermal rifl escopes produced by Dedal-NV Company are built with an uncooled microbolometer core, which consists of an array of thermosensitive elements, and an electronic circuit of preliminary signal processing.
A bolometric element is made of bi-metal thermosensitive structures that change their resistance depending on the incident infrared emission within selected wavelength range.
With the same lens focal distance, a smaller pixel (element) provides better image sharpness (17 microns or 25 microns of element diameter) and a larger array dimension with the same pixel size (384x288 or 640x480 pixels)
provides wider field of view.
Optical parts of the objective lenses in thermal vision devices are made of Germanium (Ge), which is transparent in the selected wavelength range.
When choosing a rifl escope, pay attention to the focusing principle. Internal movement of objective lenses combined with re-focusing (internal focusing mechanism) provides optimal objective focus adjustment at different observation
distances and excludes shift of the mean point of impact as a result of focus adjusting. Large objective lens focal distance increases detection, recognition and identifi cation ranges.
The software processing of the emission receiver signal may have frame frequency of 9 Hz or 25-60 Hz. 9 Hz frame frequency is enough for observing of relatively static scenes with slowly moving objects.
Professional rifl escopes shall utilize cores with frame frequency of 25 Hz and higher.
The Dedal’s thermal vision rifl escopes of the Pro series utilize original software, which, opposite to many competing devices, provide:
• completely automatic shutterless array calibration, which constantly optimizes rifl escope operation in different conditions without any manual adjustments;
• dynamic contrast system allowing a better image: more robust outline and more detailed elaboration of the target and its background, which leads to better and faster identifi cation of target and its position;
• frame frequency of 25-60 Hz for precise acquisition of moving targets.