The core of thermal imaging
The core of Seantinel Thermal Imaging System is an advanced, versatile, modular 17μm platform that has been specially designed for open sea applications.
Developed to meet extremely harsh environmental conditions, this platform gives excellent high end performance and has miniature dimensions especially designed for mid-range surveillance. It's the state of the art image processing, working without image degradation.
The terms “Detection”, “Recognition” and “Identification” ( DRI ), used with thermal imaging cameras, are defined by the Johnson Criteria (1958). Assuming that a small inflatable boat is 9 m x 1 m, the following would apply:
4,5 pixel x 1 pixel
Ability to distinguish an object from the background:
you can see something is there.
18 pixel x 2 pixel
Ability to classify the object class:
you can see that a boat of some kind is there.
36 pixel x 4 pixel
Ability to describe the object:
you can see that the boat is a small inflatable boat.
Thermal cameras provide images based on the heat energy (Infra-Red) emitted by the object.
The infra-red spectrum includes Near infrared, Mid and Far infrared frequencies.
Thermal cameras operate in the Mid to Far infrared frequencies.
There are “Uncooled” and “Cooled” types of thermal imaging cameras. The uncooled thermal cameras operate in Long-Wave Infrared (LWIR) band from 7-14UM (7000nm-14000nm, while the cooled thermal cameras operate in Mid-Wave Infrared (MWIR) and typically use wavelengths of 3-5UM (3000nm ~5000nm).
In an uncooled thermal imaging camera, the infrared-detecting elements are contained in a unit that operates at room temperature. The more common type of thermal imaging devices, uncooled systems operate quietly and can be activated immediately. Cooled thermal imaging devices have their detectors stored in a unit that cools them to -32 degrees F (0 degrees C) or lower. While these cryogenically-cooled systems have incredibly high resolution and sensitivity as a result of their elements being cooled, they are more expensive and are more susceptible to wear and tear from use than uncooled cameras.
Johnson’s Criteria: detection, recognition, identification.
When customers are considering which thermal security camera or system to buy, one of the first questions asked of thermal imager manufacturers is usually: “At what distance can the IR camera detect a target?”. In other words, what is the camera’s ability to capture very small details at great distances? When thinking about effective surveillance, it is indeed a good criterion to differentiate one sensor from another.
No matter which manufacturer you are buying from, the answer given to this question will almost always include the “DRI ranges” expression. DRI refers to the distance at which a target can be Detected, Recognized, or Identified, based on certain universally accepted parameters. In order to select the right sensor for your defense, security, or surveillance needs, these DRI ranges have to be, first, perfectly defined, but also assessed with regards to globally adopted industrial standards. Enter Johnson’s criteria.
The Origin of Johnson’s Criteria
In 1958, at the first ever “Night Vision Image Intensifier Symposium”, John Johnson, a night vision scientist at the U.S. Army’s “Night Vision and Electronic Sensors Directorate” (NVESD), presented a paper named the “Analysis of Image Forming Systems”. Johnson’s paper defined a clear system with criteria and methodology for predicting an observer’s ability to find and assess targets using image intensifying equipment (such as thermal cameras), under various conditions. It worked well, and it was the first of its kind.
Johnson’s Criteria Definitions
Johnson’s model provided definitive criteria for calculating the maximum range at which “Detection, Recognition, and Identification (D, R, I)” could take place, with a 50% probability of success. (Orientation was also discussed, but this parameter is not used or recognized today). Although newer methodologies for D,R,I exist today, such as NVESD’s “Night Vision Image Performance Model” (NV-IPM), the “Johnson’s Criteria” system was groundbreaking for its time, was the accepted standard in the defense industry for many years, and is still widely used in the security industry today.
Johnson defined “Detection” as the ability to subtend 1 TV line pair (+/- 0.25 line pairs) across the critical dimension of the subject (this translates to 2 pixels when using an LCD monitor). At the range that this occurs, regardless of target type, the observer could detect that a subject was in the field of view, 50% of the time. Today, many security camera companies loosely follow Johnson’s Criteria and define their camera’s “Detection” performance range as the ability to subtend either 1.5 or 2 pixels on the target, using various target sizes.
Johnson defined “Recognition” as the ability to subtend 4 TV line pairs (+/- 0.8 line pairs) across the critical dimension of the subject (this translates to 8 +/- 1 pixels when using an LCD monitor). At the range that this occurs, regardless of target type, the observer determines the type of subject, a human or a car for example, 50% of the time. Today many security camera companies typically define their cameras “Recognition” performance range as the ability to subtend 6 pixels on the target, using various target sizes.
Johnson defined “Identification” as the ability to subtend 6.4 TV line pairs (+/- 1.5 line pairs) across the critical dimension of the subject (this translates to 12 +/- 3 pixels when using an LCD monitor). At the range that this occurs, regardless of target type, the observer could detect the subject. Today many security camera companies loosely follow Johnson’s Criteria and define their cameras “Identification” performance range as the ability to subtend 12 pixels on the target, using various target sizes.