When one is in the market for a thermal imaging system, they will have plenty of manufacturers to talk to who will offer a wide variety of systems. It is prudent to study your application and try and define it as best you can. After you have developed your list of specific requirements, manufacturers and knowledgeable distributors can make clear recommendations based on your needs.
Here are some specific requirements that you might consider:
- What is the distance between where my camera system will be mounted and the outer perimeter of my protection zone?
- At the outer perimeter of the protection zone, what might attempt to enter? A man, a vehicle, animals? What size will my target be?
- Should the camera be capable of panning and tilting? Or should it be fixed in one position looking at the same field of view continuously?
- Thermal camera resolution? 320 x 240 and 640 x 480 are most common but 1920x1280 is available.
- Should the system have a visible camera to compliment the infrared imaging for day time use?
- How will I control the system? How will I see the video? Using analog video and serial communications or using a network?
- Where will I mount the system? On a pole? On a wall? On a tower?
When you know the answers to these questions, the manufacturer or distributor can make calculations and provide a system that is the best value for the application.
Some of those calculations are based on the system’s ability to see the target at the outer perimeter of your protection zone. The infrared industry often refers to this ability as detection, recognition, or identification.
- Detection is the ability to distinguish an object from the background.
- Recognition is the ability to classify an object. (man, vehicle, animal, boat)
- Identification is the ability to make out details on the object (man with hat on, type of vehicle, species of animal)
There are a number of different criteria that are used to determine if detection of an object at a distance is probable. The oldest and most common is the Johnson Criteria. The U.S. Army updated the Johnson criteria and they call their model NVThermIP. NATO also developed a criteria that they use and it is called STANAG 4347. Essentially, these criteria all define when something can be predictably detected. It is a generally accepted rule of thumb, that if your system can put three pixels on the target, then detection is probable.
There are research papers devoted to this subject and include all the mathematics and conditions for every possibility. Assumptions are made about the strength of the signal, the atmosphere, the optics and detector, image processing, the quality of the display, and lastly the human observer.
The easiest way to insure you are going to get a product that will meet your requirements is to work with an infrared imaging professional. They can take your requirement and apply a specific detector and optic combination that will produce the best possible results.
- Protection zone is 2 kilometers;
- Target size is a man; 0.75 meters x 2 meters (1.5 meters total)
- PT is required; zoom is preferable over a fixed field of view
- Thermal camera resolution required to be 640 x 480
- A visible camera for daytime use is required
- The camera will be mounted on a pole and connected to a network. IP video and control are required.
Using a spreadsheet and simple calculations, the infrared imaging professional can present a few different options.
An uncooled vanadium oxide detector with 640 x 480, 17 micron pixels combined with an optic that is 105mm at its narrow position will provide the following results:
- Horizontal Field of View (HFOV): 5.9 degrees
- Vertical Field of View (VFOV): 4.4 degree
- Horizontal Field Size: 207 meters displayed from the left of the monitor to the right
- Vertical Field Size: 155 meters displayed from the top of the monitor to the bottom
- Number of pixels per meter displayed: 3.08
This means that each meter of the man’s height will have three pixels on it. In good conditions, this might be enough to detect a man.
An uncooled vanadium oxide detector with 640 x 480, 17 micron pixels combined with an optic that is 150mm at its narrow position will provide the following results:
- Horizontal Field of View (HFOV): 4.1 degrees
- Vertical Field of View (VFOV): 3.1 degree
- Horizontal Field Size: 145 meters displayed from the left of the monitor to the right
- Vertical Field Size: 108 meters displayed from the top of the monitor to the bottom
- Number of pixels per meter displayed: 4.4
The detector resolution and optic are major drivers of the cost of a system. The more resolution, the higher the price; Larger optics with longer focal lengths are usually more expensive than smaller ones. Ideally, you want the least expensive system that meets your requirements. That is why it is important to define your requirements and talk to your infrared imaging professional. They may have ideas how you can accomplish your goal and save money.