Low-light camera classification and technical analysis

In the "Safe City" project, finance, cultural and cultural resources, hotels, office buildings, residential areas, safe villages, campuses, ports, highways, streets and other security monitoring applications, the performance requirements of the camera are getting higher and higher, due to the difficulty of conventional cameras. To meet the needs of 24-hour continuous monitoring, low-light cameras became the first choice for 24-hour monitoring.

What is a low light camera

The low-light camera can be seen from the name and refers to a camera that can still obtain a relatively clear image when the light is dark (low illumination). In the security industry, it is generally considered that cameras with an illuminance index lower than 0.1 Lux (F1.2, AGCON, 30 IRE) can be referred to as low illuminance cameras.

According to the traditional name, low-light cameras can be divided into two categories of “starlight level” and “moonlight level”. Roughly speaking, cameras with illuminance lower than 0.01Lux (F1.2, AGCON, 30IRE) are called “starlight”. "Low-level camera", with an illuminance of between 0.1Lux and 0.01Lux (F1.2, AGCON, 30IRE), is called "moonlight-level low-illuminance camera." Of course, there is no clear definition in the above-mentioned division, or there will be differences, but it will be a consensus reached by the industry.

The low-illuminance camera called in the industry, in addition to the mechanical color to black (also known as ICR), now also includes the hemisphere, one machine, dome and conventional bolt.

The camera's low illuminance indicator is a parameter to evaluate the camera's sensitivity. The smaller the number, the higher the camera's sensitivity and the lower the illuminance performance. The above mentioned 0.1Lux (F1.2, AGCON, 30IRE) is not only 0.1 Lux. "This illuminance index, but also has additional restrictions: F1.2, AGCON, 30IRE, why have these restrictions? It is to ensure the fairness, consistency and effectiveness of the camera imaging evaluation.

F1.2 indicates that when the camera reaches low illumination, it requires the size of the lens flux (F value, luminous flux, measured by the ratio of the lens focal length f and the aperture D, the smaller the F value, the larger the aperture, the luminous flux to the CCD The bigger the value is, the smaller the F value is, the better the lens is when the focal length f is the same.

AGCON indicates the state when the camera's automatic gain is turned on.

30IRE indicates that the camera's video output reaches the lowest level of picture quality on the monitor (standard images are 100 IRE or 700 mV; 30 IRE is 210 mV, which is the lowest level, which is no more than 30 IRE). In addition to the restrictions explicitly written here, in fact, there is an implied condition that the camera's electronic shutter is the default 1/50s (under PAL) because only under these conditions can the judgement be made. Comparability and fairness.

Often on the technical indicators of some cameras, only one “0.0001 lux” word is seen, the low illumination value is written very low, but the following additional conditions are not, so it is not really clear whether the camera is really low illumination. Maybe it's just fake propaganda.

Why do I need a camera with low illumination?

In the "Safe City" project, finance, cultural and cultural resources, hotels, office buildings, residential areas, safe village settlements, campuses, ports, highways, streets and other occasions, the camera performance requirements are relatively high, because the conventional camera is difficult to meet the 24-hour continuous monitoring The demand for low-light cameras became the primary choice. In these applications, such as the "Safe City" project, there is insufficient lighting at night, and it is impossible to install large-scale lighting facilities on a large scale. Under such circumstances, to obtain better monitoring results, it is necessary to install high-quality low-light cameras. It can not only guarantee the monitoring effect, but also simplify the system structure and achieve better reliability and lower cost.

In addition to these traditional monitoring areas, forests, hospitals, etc., are generally equipped with low-light cameras. In these cases, ambient lighting is not ideal, but it is not appropriate to add light sources, such as hospitals. In order to ensure the rest of the patients, it is not convenient to fill light. Ways to improve the ambient illumination can only be found in the camera performance, so you need to install low-light camera.

There are also some places where there is no condition to add light sources and low-light cameras are also needed. For example, in some outdoor sites such as construction sites, oil fields, factories, and freight yards, the monitoring area is large, and there are often no street lights. In this case, the use of ordinary cameras cannot guarantee the monitoring effect. It is also necessary to use low-light cameras to ensure effective monitoring results.

In short, low-light cameras rely mainly on their own performance to achieve better monitoring results under weak light, making them less dependent on the external environment, for special occasions, can achieve covert surveillance at night. Since the reliability and performance requirements for low-light cameras are relatively high, the price is also higher than that of ordinary cameras. The user budget should also be higher.

Low-light camera classification and technical principles

With the continuous expansion of application requirements, low-light camera technology is also continuously improving. There have been three types of low-illuminance cameras: electronic day/night cameras, slow-shutter cameras, and professional ultra-low-light cameras (usually ICR mechanical color to black Types of).

Electronic day and night type camera, also called electronic color to black camera, it is the use of electronic circuit switching to convert color images to black and white, in order to enhance the photographic effect under low light conditions.

Slow-shutter cameras, also known as frame-accumulating cameras, increase the brightness of an image by extending the exposure time. When such a camera turns on the slow-shutter function, the output image is not a real-time image, as long as the camera's shutter is slowed by more than 4 times on the monitor. You can see more obvious image smearing. Slow shutter camera does not enhance the image through the hardware performance of the machine, but enhances the image clarity in low light situations by sacrificing the real-time performance of the image. But for monitoring, it is important to have clear, consistent, real-time images, so frame-accumulating slow-shutter cameras are often used for monitoring static things, not for non-static applications.

Professional ultra-low illumination camera (usually ICR mechanical color to black type), through the use of high-performance CCD, professional low-light circuit design, ICR filter switching mechanism and other hardware methods, to enhance the camera under weak light imaging quality , to ensure that the screen real-time, clear output. The CCD of this type of camera adopts advanced technology, and the sensitometric performance is about 1.5 times better than that of the previous generation CCD. With the performance improvement of the DSP, the overall performance far exceeds that of the previous two types of cameras. It is called true color to black. Camera.

The future development of low-light cameras

The development of low-light cameras focuses on product performance and product functions.

The improvement of product performance mainly relies on better photosensitive devices, such as SONY's SuperHADII CCD, followed by the use of better DSP technology to achieve camera performance through DSP signal optimization, image noise reduction, and overall machine sensitivity enhancement.

Product features are more to be combined with actual needs:

For example, in actual practice, it is easy to get virtual focus after the color is turned black. Therefore, automatic back focus adjustment (ABF technology) and stepper motor-driven ICR double filter technology are introduced (one filter is responsible for filtering infrared light, and the other is responsible for optical make up).

In addition, when a low-light camera and an infrared lamp work together, problems such as frequent day and night switching are prone to occur, so AMPON has developed an infrared detection and locking technology, and some manufacturers have developed a photosensitive switch metering technology, etc. to protect the color-to-black camera. Conversion performance is stable.

The most reasonable way to improve the effect of low-light camera is to use hardware to improve product performance, for example, through the efforts of CCD, DSP, to really enhance the low-light effect. The method of extending the exposure time (slow shutter speed) or frame accumulation technique to enhance the effect of low illumination often brings undesirable effects. If a slow shutter is used, the brightness of the screen is increased due to the prolonged exposure time, and it seems that the effect of low illumination is improved. In reality, the details of the captured image are blurred, so it is not desirable to prolong the exposure time. Some products increase the factory default brightness to 120 IRE (more than the standard 100 IRE, 700 mV), often with good nighttime brightness, excessive daytime exposure, and poor quality monitoring.

The last thing to say is digital noise reduction. This is a very effective way to suppress noise. Through digital processing, clutter can be reduced and the desired surveillance information subject can be highlighted. The performance of the camera will be greatly improved.

Low-illuminance cameras have high requirements for their important supporting devices, such as high-quality lenses, and their F-value can be made smaller, that is, the amount of light passing through is larger, and if the color-to-black camera needs to use infrared sensing Lens, its biggest advantage is that the day and night focus the same, there will be no color to black defocus situation. This is because it uses aspherical lenses and special glass materials, so it can eliminate focus errors and achieve day and night focus, and this performance is critical for color to black cameras.

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