This page describes the fundamental technological and operating differences between thermal cameras (infrared cameras) and microwave radars.
Compared to microwave radar, thermal imaging has fewer ways to mitigate nuisance alarms, and has degraded performance in adverse weather.
Thermal Camera Overview
Thermal cameras are passive sensors that measure temperature (infrared radiation). In effect the thermal camera has a “flat” view of the world with no depth perception, much like a conventional camera but instead of seeing visible light reflected from objects it sees infrared (heat) that is emitted instead.
Thermal camera resolution is substantially lower than conventional cameras, for example a resolution of only 320×240 (76,800 pixels) is typical. Compared to an optical camera with “4k” resolution of 3840×2160 (8,294,400 pixels) the thermal camera will have far less detail in the captured image.
While it is possible to increase thermal camera resolution, the smaller pixels are less sensitive to infrared, which degrades image quality. This sensitivity is referred to as Noise Equivalent Temperature Difference (NETD), which is measured in milliKelvin (mK). NETD less than 25 mK is excellent, but above 80 mK is poor.
Thermal cameras typically utilise “un-cooled” microbolometer sensors that are much less sensitive than the larger, and much more expensive “cooled” thermal imaging semiconductor sensors.
Exactly like optical cameras, larger objective lenses increase the operating range but there is a corresponding decrease in field of view.
Colour is used to represent temperature.
Microwave Radar Overview
Microwave radar is an active sensor system that emits microwaves that reflect from objects; the reflections are subsequently detected and measured by the radar. Since it is an active system it emits a signal and uses the round-trip time to directly measure range to object. Moving objects cause a Doppler frequency shift to appear on the received signal which is measured directly and correlates to speed of the object toward or away from the radar.
The resolution of the radar is set by the beam width (defined by antenna and operating frequency) and the radar pulse or chirp duration. The maximum operating range is determined by a number of factors including transmitted power and receiver sensitivity. Radar has lots of flexibility to optimise for certain requirements such as cost, resolution, operating range, equipment size, etc.
A variety of methods are available to sweep the radar beam over a given area, such as mechanical rotation and electronic beam steering. This defines the radar field of view which can be as large as the full 360 degrees.
Colour is used to represent the reflectivity of each object.
Performance in Adverse Weather
Fog, rain, and high humidity can severely limit the range of thermal imaging systems due to scattering from droplets of water. The higher the density of droplets, the more the infrared (thermal) signal is diminished, hence performance suffers in rain and fog. Thermal camera performance is also dependant on the humidity and ambient temperature. Furthermore higher ambient temperatures increase the temperature of the un-cooled microbolometer sensors, leading to reduced image fidelity (increased sensor noise).
Microwave radar detection range is not noticeably affected by fog, rain, humidity, ambient temperature or the target proximity to any heat source. Higher ambient temperatures typically have a negligible impact on ground surveillance radar sensitivity.
Target Masking
Thermal imaging relies on heat measurements, therefore intruders can be partially or fully masked by walking near to walls or stone structures that retain heat, or sources of heat, such as vehicles, generators, air conditioning or exhaust vents, furnaces, etc.
Intruders may quickly and easily mask themselves by placing a thick blanket over their body to reduce thermal emissions.
Radar uses microwave radio signals that “see through” thick fabrics to see the underlying object. Radar will easily detect an intruder wearing thick clothing, covered in a blanket, wrapped in heat blocking foil sheet, or close to any heat source.
Unlike passive thermal cameras, microwave radars actively illuminate the search area with invisible microwave signals not infrared, so hot objects do not affect radar, mask intruders or have any effect on radar operation.
Direct Measurement of Target Range
Thermal imaging systems only measure temperature, not distance to the object being measured and can therefore suffer from false alarms. Thermal cameras have no depth perception so cannot scale object size based on distance, so very small, but very close objects (such as insects or birds) can cover a similar number of pixels as a distant larger object such as a human. Because thermal camera analytics analyses blocks of pixels that do not necessarily correlate to any particular topographical area this can lead to false detections and nuisance alarms.
Radar accurately discriminates between small/close and big/distant targets because of the inherent range measurements that underpin the operating concept. Because radar directly measures range it is automatically desensitised to certain locations to reduce the false alarm rate. For example if a tree is blowing in the wind, only the immediate vicinity of the tree is made less sensitive, not the whole area.
Rotating thermal cameras can cover up to 360°, but these suffer from the same fundamental inability to measure range. Some systems attempt to estimate range based on mathematical assumptions, but not direct measurement. These assumptions are often incorrect in complex natural environments, resulting in nuisance alarms being generated by the thermal camera.
Due to the large unwieldy size of cooled thermal sensors, it is impractical for 360-degree thermal cameras to utilise anything other than small microbolometers with the corresponding inferior sensitivity (NETD).
Summary
- Microwave radar target detection probability is high, even in challenging weather conditions where thermal cameras may struggle due to scattering.
- Microwave radar can detect intruders regardless of the ambient temperature/humidity, or how much heat the intruder emits or tries to mask.
- Microwave radar directly measures multiple parameters, such as: bearing, range, speed and amplitude. Even rotating thermal cameras can only directly measure bearing and amplitude so have far less situational awareness to mitigate nuisance alarms.
- Microwave radar is an active system (like a searchlight), detecting targets in any environmental conditions, whereas thermal imaging is a passive technique that is completely reliant on the uncontrolled temperature difference between target and environment to operate effectively.