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How to Obtain Long-distance High-resolution Infrared Images?

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How to Obtain Long-distance High-resolution Infrared Images?

Issue Time:2021-08-20
Like a visible light camera, the optical system in an infrared thermal imaging camera focuses infrared radiation of a specific wavelength on the infrared detector, through photoelectric conversion and image processing, to form a visual image. Because of the limitation of infrared detectors, the detailed resolution capability of infrared thermal imaging cameras may be worse than that of visible light cameras.

In fact, the infrared continuous zoom lens provides good functions for high-performance thermal imaging cameras, which is convenient for users to track or observe targets at a long distance, or observe close targets in detail. Through continuous zooming, the thermal imaging camera can analyze the characteristics of the moving target based on the temporal and spatial position information (TSPI). 

For example, If infrared thermal imaging cameras used in shooting range measurement want to see clearly the aircraft several kilometers away, but also want to see the short-range and small-sized shells. Both of these measurements require high-resolution systems.

To achieve this capability, the target needs to occupy more pixels on the detector, and a large-scale area array infrared detector is essential. In addition, the small pixel pitch of the detector and the long focal length optical system is also very helpful. The following are the factors that affect the resolution of long-distance infrared images and the method of adjustment.

Instantaneous field of view

The spatial resolution of an infrared camera is determined by its instantaneous field of view (IFOV). The instantaneous field of view is the light-receiving angle of the observation field of a single detection pixel in the detector. It determines the instantaneous area of the target observed at a given distance. This area is the smallest unit that the sensor can distinguish.
We can imagine that a cone is projected from a pixel on the detector onto the imaged object. The area at the end of the cone is the instantaneous field of view, that is, the size of the area that can be seen by each pixel. The smaller the IFOV, the smaller the minimum resolvable unit, and the higher the image spatial resolution. IFOV depends on the sensor optical system and the size of the detector. The smaller the pixel size and the longer the focal length, the smaller the area, so the observed target can occupy more pixel units on the detector.
 
Instantaneous field of view (red area) relative to the target size
The instantaneous field of view is determined by the pixel size of the lens and the detector
With the development of technology, the pixel pitch of the detector is getting smaller and smaller, and the pixel pitch has been reduced from the original 30 μm to 15 μm, or even 10 μm. The combination of a small pixel detector and a long focal length lens provides hardware guarantee for obtaining high-resolution images from a long distance.

Small pixel detectors are difficult to achieve high-speed imaging of targets while providing higher spatial resolution. In high-speed imaging, the integration time is very short, so the camera needs to capture as much light as possible as quickly as possible, which requires larger pixels. However, most applications for long-distance imaging do not require high speed, and objects moving at a distance will not move on the detector as quickly as near objects. Therefore, a smaller pixel size is generally acceptable.

Temporal and Spatial Position Information (TSPI)

The focal length of the lens is a key factor for long-distance imaging. When detecting and identifying targets of different types and different distances, the focal length of the equipment required is also different. If the size of the target has been determined, the user can calculate the distance of the target through the known focal length. When an object is moving, such as an airplane moving toward or away from the observer, the advantage of using a continuous zoom lens with excellent parfocality is very obvious. 

In this way, the target can be tracked at every point in time, and this information can be saved as part of the source data of the image, thereby providing a key TSPI for the target. The effective focus output can tell the user the distance of the target, and the user can even judge the size of the target according to the percentage of the field of view it occupies.

For TSPI time information, users can connect the camera to a device that provides IRIG-B time code, and the IRIG device provides a reference for time synchronization. For example, IRIG-B provides signals at a rate of 100 pulses per second. By bundling IRIG-B with a camera with a continuous zoom lens, users can know the distance of their target at any given time.
Note: IRIG is the organization responsible for the formulation of shooting range standards and other tasks under the Range Commanders Council of the United States. IRIG-B time code (abbreviated as B code) is a serial time code formulated by the Telecommunication Group to which it belongs.
This information enables users to analyze data more effectively. For example, by knowing the distance of the target, they can calculate its radiance brightness. The calculation of radiance brightness is affected by the amount of atmosphere between the object and the detector. Through the above tests, when the researchers have determined the distance, they can use the atmospheric transmission software to simulate the atmospheric environment and compensate for the distance information obtained.

Stable platform

When the camera uses an ultra-long focal length to track the target, the stability of the platform is essential. The slight movement of the camera means that the area being photographed will undergo drastic changes, which may result in the complete loss of the target. The high-precision stable tracking platform can provide precise indication accuracy while keeping the camera stable.

In fact, cameras used in tracking systems generally use long focal length lenses. The tracking processing software uses the target recognition mode and predicts the degree of displacement of the target from one frame to the next. This information is then fed back to the tracking platform to correct the pointing of the camera in real-time and lock the moving target.

Spatial resolution is an important issue that needs to be considered when testing infrared cameras in the shooting range. It is related to the size of the pixels (the smaller the better) and the focal length of the zoom lens (the focal length should be as long as possible). With the timing input, the camera can record the Spatio-temporal position information in the image source data, allowing the user to accurately calculate the important features related to the target. As part of the system that provides stabilization and tracking information, infrared cameras with high spatial resolution have gradually become the key equipment for testing shooting ranges.

The LWIR lens produced by Quanhom supports a resolution of up to 1280x1024 (SXGA) and can still operate stably in a variety of complex environments. If you want to know more about infrared thermal imaging lenses after reading the above content, you can get a comprehensive solution by contacting us.

With excellent technology and high-quality products, Quanhom has become one of the leading manufacturers of Opto-electromechanical components. We focus on the production of various thermal infrared lenses (including LWIR, MWIR, and SWIR). We have a professional production team and a strict quality inspection system, and we carry out all aspects of quality control from product design to export. And we will also provide thoughtful one-stop service and effective solution technology according to the needs of customers. If you are interested in our infrared thermal imaging lens, please contact us immediately!