The question the most frequently asked to thermal imager manufacturers is certainly: how far can the IR camera detect a target?
It is indeed a good criterion to qualify one sensor from another considering the final application. And the answer given to this question will probably include the “DRI ranges” expression.
DRI stands for Detection, Recognition and Identification. DRI ranges, expressed in km (or miles), can be found in the specification table of infrared camera brochures. In order to select the right sensor meeting the application requirements, these DRI ranges have to be, first, perfectly defined, but also assessed with regards to globally adopted industrial standards.
John Johnson, a scientist from the Army Night Vision Laboratory, was a pioneer in 1958, conducting experiments to test observers’ ability to identify targets through analog sensors. The terms “Detection”, “Recognition” and “Identification” were defined as follow:
The following pictures illustrate these definitions:
| Detection: At several kms, two targets are detected out of the background | Recognition: A human is walking along the fence | Identification: Two males with trousers and jackets are identified, one is smoking. |
Johnson defined thresholds, known as the Johnson criteria, as the minimum number of line pairs to detect, recognize or identify targets captured by scene imagers. The DRI lower limits, according to the Johnson criteria are typically in the following range:
A line pair (lp) is defined as one white line adjacent to a black line.
With Johnson criteria in mind, it is easy to understand that the spatial resolution of a camera is an important parameter determining the quality of the security system. The spatial resolution is the camera’s ability to capture very small details at great distances. It is closely related to the number of camera pixels. Clearly, the more pixels there are, the better the spatial resolution will be and the greater the detection range will be.
However, the thermal sensitivity has also a high impact on detection performance. The thermal sensitivity is the ability to distinguish very small differences in temperature. The “cooled” IR cameras offer better thermal sensitivity than “uncooled” IR cameras. For the same number of pixels, the image of a “cooled” thermal camera is of better quality (i.e. less “noisy”) than an “uncooled” camera’s image.
Another important characteristic of an infrared camera is the field of view (FOV). A large panoramic FOV is necessary in order to enhance the probability to detect a single event into a complete 360-degree scene. See in the picture below the image captured by a thermal sensor with a wide panoramic horizontal FOV, and, in green in the center, the area screened by a PTZ camera zoomed to a 2° FOV. It is clear that this PTZ camera has no chance at all to detect any event in this scenery.
So, the Johnson criteria is not sufficient to set up reliable, repeatable, specification table, as several factors influence the performance ranges: the field of view, the spatial resolution and thermal sensitivity of the infrared camera, the spectral band of detection, the shape and camouflage of the target, the experience of the operator …
In order to compare the performance of systems from different manufacturers, it is necessary to establish a common test frame, listing the conditions influencing the DRI range values. STANAG 4347 standards, developed by NATO, is widely adopted in the thermal imaging industry. This standard defines the target parameters (resulting from size, shape, temperature, material properties, emissivity…) and the conditions of use (resulting from ambient temperature, type of background scene…) to take into account in the MRTD (Minimum resolvable temperature difference) computation giving the values of the DRI ranges. See in the table below examples of target definition and the corresponding detection range of the SPYNEL-X, the IRST (Infrared Search&Track) with the best image quality and the longest detection range on the market:
TARGET
HEIGHT (m)
WIDTH (m)
SPYNEL-X 8000 DETECTION RANGE
Human (1m2)
1.65
0.60
Up to 9 km
NATO Target (5m2)
2.30
2.30
Up to 12.5 km
HGH provides different models of SPYNEL cameras, varying in DRI ranges, vertical field of views and detector types (cooled/uncooled, MWIR/LWIR). Depending on the application requirements (type of targets to be tracked, lower bounds of detection, recognition, identification) and on the critical site configuration (perimeter size, free zones, building location, obstacles), our engineers will guide you in finding the right SPYNEL model fitting your need.
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