Analysis on the development trend of high-definition lens technology and application

    Abstract: HD lenses, also known as megapixel lenses, have advantages over ordinary industrial lenses in terms of high definition, strong spectral transmission, and high spectral correction. As the development of HD in the industry becomes increasingly fierce, the application of various HD lenses and camera products is becoming more and more extensive, and megapixel lenses have become one of the focuses of industry applications.
 

    As the concept of high-definition becomes more and more obvious, the technology of the security industry is gradually maturing. For the field of monitoring, the development of megapixel high-definition is in full swing. The emergence of high-definition cameras has greatly improved the image quality of security systems, such as 720P and 1080P IP high-definition cameras, 3M and 5M resolution industrial high-definition cameras, etc., and the matching high-definition lenses have also sprung up in the market.

　　HD lenses, also known as megapixel lenses, have performance advantages over ordinary industrial lenses, such as high definition, strong spectral transmission, and high spectral correction. As the development of HD becomes increasingly fierce, the application of various HD lenses and camera products is becoming more and more extensive, and megapixel lenses have become one of the focuses of industry applications.

　　The camera lens is one of the key devices of the video surveillance system. Its quality (indicators) will directly affect the overall indicators of the camera. Whether the camera lens is properly selected is related to both the system quality and the project cost. However, in the face of various high-definition surveillance products and ever-changing application scenarios, the million-dollar high-definition surveillance lens must provide the camera with excellent imaging effects in the complex light environment of various scenes, which requires the high-definition surveillance lens to have a certain technical content.

　　Aspherical lens technology is a very important technology for megapixel HD lenses and an important guarantee for the lens to achieve HD effects. Traditional spherical lenses have the inherent defect of spherical aberration, which brings about an insurmountable light spot phenomenon, greatly affecting the imaging quality of the camera.

　　Aspherical lenses have good light transmittance and low refractive index. Using one aspherical lens can replace a group of spherical lenses, thereby simplifying the optical structure of the camera lens, making it easier to obtain a larger aperture in the optical path and mechanical structure. In addition, aspherical lenses can make the light from the edge and the light from the center pass through the lens and focus on the same plane, so that the entire lens surface can be clearly and effectively used, so that the entire lens can be correctly focused and the effective aperture becomes larger.

　　The increase in the effective aperture of the aspherical lens allows more light to be projected onto the photosensitive surface of the CCD or CMOS, which increases the sensitivity of the camera. In addition, because one aspherical lens can replace several or a group of spherical lenses, the size and weight of the camera lens can be reduced. Since the light passes through fewer lenses, the light transmittance is greatly enhanced, and the image becomes detailed and bright.

　　The important feature of aspherical lenses is that they can improve the refractive index of the edge of the lens to light, making the edge imaging of the lens better. Spherical lenses, especially high-magnification spherical lenses, are more prone to spherical aberration and image distortion. In particular, the light refracted at the edge of the spherical lens will cause barrel distortion. When aspherical lenses are used for high-magnification zoom lenses, the aspherical lenses correct the incident light at the edge in the opposite direction of the spherical lens distortion, thus effectively eliminating the barrel distortion. The aspherical design corrects the undesirable phenomena such as unclear images, distorted horizons, and narrow fields of view, making the image clearer and more delicate. It is usually used in wide-angle and ultra-wide-angle lenses, and is suitable for large-scale monitoring places such as squares and parking lots.

　　The "transmittance" in megapixel lens technology directly affects the clarity of the lens and is an indispensable condition for high-definition image quality. "Transmittance" refers to the light transmittance of the lens, and the main determining factor is the coating technology. The so-called lens coating is to use vacuum vapor deposition technology to coat an extremely thin transparent film on the surface of the lens. The anti-reflective film can produce an anti-reflection effect. The higher the transmittance, the greater the light transmittance and the lower the illumination requirement. When shooting against the light, due to the influence of the film layer, the phantoms and halos that are easy to produce can be reduced to a minimum. Especially under night monitoring conditions, the level of illumination is a determining factor in the quality of night lenses.

　　With the advancement of processing technology and the continuous expansion of its scope of use, highly technical multi-layer coating technology has emerged. The use of multi-layer coating technology can prevent light reflection, promote light transmission, increase the overall light throughput of the lens, and ultimately improve the image quality. It can be said that without the leapfrog development of multi-layer coating technology, high-magnification and large-aperture zoom lenses would not be as developed as they are today.

　　Recently, zoom lenses composed of multiple lens groups have become the mainstream of lens development in the world. With multi-layer coating, the stray light derived from the light reaching the film surface is greatly reduced, so that the lens identification rate (resolution) is not reduced and the color reproduction is not damaged. Re-evaluating the improved film layer and developing the most suitable coating technology for various lenses are important topics that manufacturers have been studying.

　　Lens coating seems to be a "post-processing" step. The coating coating varies according to the technology of each manufacturer. All surveillance lens coatings cannot be guaranteed to be completely consistent. Therefore, understanding the coating will directly affect the performance of the lens. The main types of lens coatings include nano coatings, integrated coatings, composite coatings and transparent coatings. There are also some high-performance coatings BBAR and high-fidelity coatings. Currently, surveillance lenses are mostly BBAR and nano coatings, and the rest are mostly used for DSC or SLR camera lenses.

　　As a lens that plays a key role in the imaging process, the following issues should be paid attention to in its application.

　　Using ED lenses is the key

　　In the commonly used security monitoring market, fixed-focus or manual zoom lenses are more often used. Because the monitoring distance is relatively close, the focal length of the lens generally used is within 50mm. This scenario has almost no requirements for the anti-dispersion performance of the lens. However, once an electric zoom lens is used to monitor a distant target, the dispersion problem becomes prominent (dispersion is the focus shift of various wavelengths of light in visible light after refraction through the lens, which is manifested in the image as blue or red color bars on the edge of the object). Originally, in the analog monitoring system, due to the large pixel size, the dispersion problem was not very prominent, and it would only be manifested when the focal length was configured above 300mm. In the era of high-definition, this problem can be clearly seen in the commonly used electric zoom lens of the 120mm focal length, which has higher requirements for the lens in terms of material and coating accuracy. The higher the clarity of the camera, the more obvious the impact of the lens material on the imaging effect. The high-definition electric zoom lens uses ED lenses, which can provide a clearer picture effect for the camera when monitoring at a long focal length and long distance.

　　Choose a large aperture design lens

　　Lenses using ED lenses cannot completely solve the problem of long-distance monitoring. Images with a resolution of 720P or above are usually called high-definition images. According to the free choice of the market, most of them use the output format of 1080P. In the field of long-distance monitoring, users hope to use high-definition monitoring systems to increase the pixels of the picture, so as to see the distant target more clearly, but the actual results are far from satisfactory, and the reasons are not difficult to guess. Most of the high-definition cameras we use now use CMOS chips, and the illumination performance can only reach 0.5Lux. As the focal length of the telephoto lens changes, the luminous flux will also change. The longer the focal length, the smaller the luminous flux. When the focal length is extended to the maximum value, the luminous flux is reduced to the weakest. Although it is daytime, for the camera, it is equivalent to being blocked in front, leaving only a small hole to transmit light. In this case, the picture quality of ordinary high-definition cameras will be attenuated due to insufficient illumination. There are two ways to solve this problem.

　　1. Use high-definition cameras designed for long-distance monitoring, such as high-definition SDI cameras. In addition to the minimum illumination of 0.3 Lux, this type of camera also adds color restoration and low-illumination image enhancement technology for the long-distance monitoring field, so that clear images can be obtained even when the light flux decreases.

　　2. Increase the luminous flux of the lens when the focal length is extended. Once the lens is produced, the range of luminous flux is determined and cannot be changed. Therefore, when choosing a lens, you must choose products with better luminous flux performance at long focal lengths. According to the optical relationship of the lens: F (inverse ratio of luminous flux) = f (focal length) / D (effective aperture in front of the lens). The F value is inversely proportional to the luminous flux. The smaller this value is, the better the luminous flux performance is. So when f is the same, choose a lens with a larger effective aperture, and the better the picture effect will be.

　　IR function cannot be ignored

　　When users pay attention to HD systems, they are not only concerned about whether they can get clear images during the day, but also the monitoring performance at night. In HD systems, the IR function is generally forgotten. However, with the improvement of the photosensitivity of HD systems, the IR function is gradually recognized by users in all-weather monitoring systems. Especially in electric zoom systems, the IR function is crucial. For systems that use auxiliary light sources such as laser illuminators, whether the IR function is available will directly affect the ease of use and usability of the system.

　　Compared with mature analog monitoring systems, HD imaging systems still have many technical details that are not taken seriously. However, as HD systems become more popular, the choice of HD lenses will become more rational, and with the strong demand, it will in turn promote the continuous upgrading and development of products.

　　As the trend of high definition in the field of video surveillance intensifies, it has become the hottest topic in the surveillance industry. Major mainstream manufacturers in the industry have launched their own high-definition products, especially high-definition network cameras, which are rich in models and have relatively complete peripheral supporting equipment. Various IP high-definition system solutions have emerged, which greatly improve the video data carrying and application capabilities, greatly improve the video image resolution, and have the advantages of high scalability and centralized management, becoming the mainstream of current high-definition video surveillance application solutions.

　　Network cameras are usually divided into two types: one is based on an analog camera with a video and network processing module embedded in the front end, so it has the same disadvantages as analog cameras: the image resolution is not high, and the highest image quality does not exceed D1; the other is a digital high-definition network camera, which is characterized by directly processing digital video and transmitting it through the network, avoiding the problem of ordinary analog cameras being susceptible to interference, and can achieve a clean picture, clear and delicate image quality, and a high signal-to-noise ratio.

　　In order to get the best performance from a megapixel camera, it is very important to use a high-quality megapixel lens. The quality of the optical image is a key factor in the overall image quality. As the "eyeball" of a megapixel network camera, a megapixel lens can provide high contrast, high sharpness and clarity for the entire image, while a non-megapixel lens cannot fully display the high resolution of a megapixel sensor, especially at the edge of the picture.

　　According to some experts, the two important indicators of clarity include resolution and contrast. Resolution refers to the ability of the lens to reproduce the details of the scene being photographed. The higher the resolution, the clearer and more delicate the picture; contrast refers to the ability to reproduce the contrast details and light and dark levels of the scene being photographed. The higher the contrast, the clearer the outline of the scene, the sharper the edges, the richer the levels, the stronger the texture, and the brighter the tone. Lenses with high resolution and low contrast have unclear image outlines, dark contrast, and dull tones, which makes people feel unclear. This is also a major reason for the sluggish market for domestic megapixel HD lenses. Therefore, only when the contrast and resolution reach a certain level can they be truly ideal megapixel HD lenses.

　　In addition, megapixel high-definition camera lenses also have the following development trends.

　　Large diameter miniaturization

　　In order to increase the amount of light transmitted by the lens, the lens aperture must be increased. The brightness of the image in dim environments and night mode is better than that of small-aperture lenses. Miniaturization means that the lens structure is compact, which can be installed on different products and reduce the size of the product. Compared with foreign megapixel surveillance lenses, the application of aspherical technology is mature. Often, the same lens has a sufficient focal length, a large aperture, a small structure, and high clarity. Most major domestic lens manufacturers are slowly exploring and striving to maturely apply it to the development of new megapixel surveillance lens products as soon as possible to compete with foreign manufacturers.

　　Focus day and night

　　In the intelligent transportation industry, the imager target area is getting larger and larger, and more and more products are using 1" CMOS. Compared with 2/3" CMOS, such products have better low-light effects. Currently, many products do not have the function of night vision parfocus and need to rely on flash to fill in the light. However, many countries do not allow the use of flash, and China has also begun to explore solutions without flash.

　　At the same time, under the premise of ensuring the image quality during the day, users have higher and higher requirements for the image quality at night. Under the premise of ensuring the image quality during the day, the infrared filter function can be used, and ED (low refractive index and high dispersion coefficient) glass can better correct the chromatic aberration and improve the confocal ability and image quality of day and night imaging. In recent years, major glass manufacturers have also been working hard to promote ED glass.

　　Noise Light Elimination

　　Noise light mainly includes stray light and ghost images. Stray light refers to rainbow-shaped stray light caused by the reflection of mechanical parts. Stray light can be perfectly eliminated by extinction treatment of the inner wall of the lens and the spacer. Ghost images refer to the secondary imaging of the screen caused by multiple reflections of light between the lenses inside the lens. Ghost images are similar to the outline of strong light objects and cannot be completely eliminated. They can only be reduced as much as possible by coating multiple layers of anti-reflection film. A high-definition megapixel surveillance lens with superior overall performance should have excellent performance in eliminating noise light in order to perform well in strong light environments and match its reputation as a megapixel high-definition surveillance lens.

　　Nowadays, the trend of high definition has become a consensus in the development of the surveillance industry. As high definition develops, the megapixel high-definition lens, as one of the core components, has also shown a rapid market growth. With the development of technology and the market, higher requirements will be placed on the lens. Market competition forces companies to enhance their sense of innovation. Domestic lens manufacturers must attach great importance to technological innovation and product innovation in order to achieve breakthrough improvements in product quality and stability.