【英文原文】查看这里

　　有一个简单的规则，它决定了任何CCTV工程的成效-没有光就没有图像。在本文中，Ian Crosby(博世安防系统照明 欧洲产品营销经理)探讨了在现代视频监控系统中红外(IR)照明在带宽上的效果和如何使用IR照明能够减少存储的成本。

夜间监控要采集到高质量的图像有许多要克服的挑战

　　在低照度条件下管理带宽

　　无论是模拟还是数字(如编码算法像H.264压缩模拟视频以用于IP系统)，几乎所有的CCTV摄像机都可以在白天光线充足情况下捕捉到有用的图像。但是，却是你的安防系统平台在夜间工作的怎么样决定了其整体功效，因为大多数犯罪在黑暗中的脆弱时间里发生，今天的安防系统需要配备二十四小时监控设备。　　

　　大多数犯罪正是利用监控系统的弱点在晚上作案

　　绝大多数现代安防摄像机规格看起来能在非常低的光照条件下有效运作，但他们标注的照度率非常低，通常在0.1lux以内。但是人们普遍的认识是如果没有专业的CCTV照明—无论是白光还是红外，大多数摄像机在黑暗坏境和夜间只能捕捉到低质量的“嘈杂”图像。

　　改善图像质量和可用性对特殊安全照明有明显好处。然而，对CCTV用户来说另一个关键的好处到现在才逐渐显现。当光线水平下降的时候，来自网络的每个摄像机的压缩视频流比特率有显著增加。 这种增加的比特率，如果不加以解决，将导致系统的总存储成本大幅增加。

　　了解夜视安防摄像机的自动增益控制(ACC)

　　为了理解为什么低照度监控对带宽有如此高的要求，以及要以更高的比特率来传输视频，我们需要考虑自动增益控制(AGC)。

　　AGC是提高在低照度条件下信号强度的摄像机技术。它的工作原理很简单，放大图像;这种放大的效果不过是增加了视频信号和。

　　来看一个典型的安防工程：现场监控摄像机在白天传递良好的可用的图像，但是在黑暗降临使相机的AGC功能开始生效了。越是黑暗它的AGC增加幅度越大，摄像机抓取的图像也变得颗粒化和“嘈杂”。最终图像被“雪花”完全遮蔽了，几乎毫无用处。

　　那么，为什么“嘈杂”的夜间监控视频影响比特率呢?[nextpage]

　　视频压缩算法的影响

　　要理解为什么比特率会上升，重要的是要对视频压缩算法如何工作有一个基本的理解。压缩的基本原理是消除所有非本质的信息以及减小文件的大小。所有的压缩均需要在图像质量和文件大小之间进行一个折中。更高的压缩率可以是文件大小更小但是图像质量会降低;而较低的压缩率可以产生高质量的图像但是需要更大的文件尺寸。

低照条件下产生的问题将大大较少和限制监控摄像机技术

　　对于大多数设备，摄像机的帧率和分辨率需要进行调整以适应应用的需要;这些参数通常是前期指定的。这样有一个明显的好处是降低帧率和减少分辨率(更低的比特率)，同时也有一个主要的有点。牺牲帧率和分辨率的结果是低质量的看起来“波涛汹涌”的监控视频画面，这可能错过所记录的时间中的关键时刻。

　　当今最流行的视频压缩引起结合了JPEG、MPEG或者M-JPEG。最近的是新的H.264算法，它比MPEG4压缩技术少约30%的带宽，而比M-JPEG的效率高80%。然而所有算法都有共同的的减少信息的原则;减少不相干信息，消除不被人眼注意的视频信号(比如细微的颜色变化)或从相同帧或者中间帧删除重复的冗余信息，比如大的色彩区块或者固定物体。

　　低照度监控、压缩品质和比特率之间的关系

　　在晚上相机通常镜头视野里的活动和运动较少，因而夜间视频应该以较低的比特率来存储视频。然而，摄像机采集增加的的图像噪点引起会干扰视频编码器的压缩算法。准确的说，压缩算法将噪点通过AGC增强图片看作是现场的运动，因此有用的信息就不能减小不相干或冗余的体积。由于这样夜间监控视频压缩得较少，体积也就比它应有的大小更大。现在清楚了，低照度录监控、压缩品质和比特率三者之间有直接的关系。

　　比特率对安防系统成本的影响

　　不断增长的存储空间是昂贵的，并且是现代数字监控系统的主要开支之一

　　比特率是摄像机或编码器每秒传输的大量数据。也是每秒钟必须被保存并写入存储介质的大量数据。摄像机或编码器的每个通道的比特率越高，系统整体存储的要求就越高。逐渐增加的存储空间是昂贵的，并且是现代数字监控系统的主要开支之一。

　　红外与编码相结合(IP和混合)的好处

　　与其不断的给一个安防系统增加存储，不如解决问题的根本所在—夜间的高比特率。首先看起来一个快速的解决方法是简单的禁用摄像机的AGC功能。这或许可以减少比特率，但代价是牺牲所有重要的图像细节。禁用使相机的AGC功能会导致监控画面的画质就算不是没有用，也会让画质变的糟糕。一个更好解决方案是在镜头上安装节能红外照明。使用红外时，摄像机就没必要采集那么多画面，编码器算法也不会那么多，DVR和其他存储设备能以最佳效果工作。[nextpage]

　　使用红外照明时安防摄像机传送高分辨率、高质量的图像噪点几乎为零的夜间监控视频证据，现在我们知道，更少的视频噪点等于更佳的压缩品质、更少的比特率和存储需求。

　　使用红外照明能使安防摄像机传送更分辨率的也将爱你监控视频

　　安防摄像机照明和图像的质量

　　最基本的，红外是光线。光线是人眼看不见的，但它被单色(黑白)和双模式监控摄像机所使用来揭示可能对人眼来说完全黑暗的夜间场景。记住我们的金科玉律，没有光就没有画面。

　　红外防止嘈杂的夜间图像和导致高比特率的一连串事件，过多的存储要求和不必要的开支。此外，某些红外照明器功能创新了3D扩散(黑钻石)技术，以引导光线给摄像机视野的前景和背景，以产生完美均匀照亮的夜间图像，避免因其他一些红外照明器造成的热点和曝光过度。要记住我们的宗旨是，图像的质量要高，而存储需求和系统整体的成本要低。

　　在数字视频监控应用中利用IR减少存储需求

　　虽然使用红外照明来提高图像的质量和可用性广受安防产业的认可，但它作为一个带宽管理工具来使用可能会令人感到惊讶。由于存储是运行一个安防系统导致的最大的开支之一，因而把IR来作为一个减少数字视频应用中的存储需求有效的工具，欢迎并要鼓励这样做将带来惊喜。

        作者：Ian Crosby　　博世安防系统EMEA地区照明产品营销经理

【英文原文】下一页[nextpage]

Benefits of using infrared (IR) illumination in modern surveillance environments

　　There is a simple rule which determines the effectiveness of any CCTV installation - without light there can be no picture. In this article, Ian Crosby (EMEA Product Marketing Manager - Illumination of Bosch Security Systems) looks into the effects of infrared (IR) illumination on bandwidth and how the use of IR lighting can contribute to reduced cost of storage in a modern video surveillance system.

　　Managing bandwidth in low light conditions

　　Whether analogue or digital (where encoding algorithms like H.264 encode analogue video for use with IP systems), virtually all CCTV cameras can capture good, useable images in bright, daytime conditions. However, it is how well your security system performs at night that determines its overall effectiveness, and since most crime takes place during the vulnerable hours of darkness, todays security systems need to be equipped for round-the-clock surveillance.

　　It is precisely at night that most criminals will try to take advantage of weaknesses in surveillance systems

　　The specifications of most modern security cameras state extremely low lux ratings - often in the range of 0.1 lux - seeming to suggest effective operation in very low light conditions. It is generally accepted however that without specialist CCTV illumination - be it visible white light or infrared - most cameras will only capture low quality, "noisy" images in dark, night-time conditions.

　　Improving image quality and usability is one obvious benefit of specifying security illumination. However, another key benefit for CCTV users is only now coming to light. When light levels drop there is a significant increase in the bit rate of the encoded video stream from each camera in a network. This increased bit rate, if left unaddressed, will cause a substantial increase in the total storage costs of the system.

　　Understanding automatic gain control (ACC) in night vision security cameras

　　To understand why low light surveillance has such a higher bandwidth requirement and transmits video at a higher bit rate, we need to consider automatic gain control (AGC).

　　AGC is camera technology, which increases signal strength in low light conditions. It works simply by amplifying the image; the effect of this amplification however is increased video signal and in turn increased image noise.

　   Take a typical security installation: on-site surveillance cameras deliver good, useable images during the day but as darkness approaches the cameras AGC function kicks in. The darker it gets the more AGC increases in magnitude and the images the cameras capture become grainy and "noisy". Eventually the image is completely obscured by "snow" and becomes practically useless.

　　So why does "noisy" night-time surveillance video affect bit rate?

　　Impact of video compression algorithms

　　To understand why there is a rise in bit rate, it is important to have a basic understanding of how video compression algorithms work. The basic principle of compression is to eliminate all unessential information and reduce file size. All compression requires a compromise between image quality and file size. Higher compression ratios enable smaller file sizes but low quality images; lower compression ratios produce high quality images but need larger file sizes.

　　Problems of low light conditions can be greatly reduced or eliminated with the proper surveillance camera technology

　　With most installations, camera frame rate and resolution are adjusted to suit the applications needs; these parameters are generally specified up-front. While there is an obvious benefit to lowering frame rate and reducing resolution (lower bit rate) there is also a major disadvantage to doing this. Sacrificing frame rate and resolution results in low quality "choppy" surveillance video that can miss critical moments in a recorded event.[nextpage]

　　Todays most popular video compression engines incorporate JPEG, MPEG or M-JPEG. Most recent is the new H.264 algorithm that uses approximately 30% less bandwidth than MPEG4 compression technology, which is itself 80% more efficient than M-JPEG. All however share common reduction principles; irrelevancy reduction, which removes sections of the video signal not noticeable by the human eye (like subtle colour changes) or redundancy reduction which removes duplicated information from either the same frame or between frames, large block areas of colour or stationary objects for example.

　　Relationship between low light surveillance, compression and bit rate

　　At night there is generally less activity on scene and less movement in the cameras field of view, therefore night-time video should be able to be stored at a lower bit rate. However, image noise caused by increased camera gain can interfere with compression algorithms used by video encoders. To be precise, compression algorithms interpret noisy AGC enhanced images as movement on-scene and therefore useful information that cannot be reduced in size by irrelevancy or redundancy. Because of this night-time surveillance video is less compressed and larger in file size than it should be. It is now clear that there is a direct relationship between low light surveillance, compression and bit rate.

　　The effect of bit rate on cost of a security system

　　Increasing storage space is a costly exercise and one of the principal expenses of a modern digital security system

　　Bit rate is the amount of data transmitted per second by a camera or encoder. This is also the amount of data per second, which must be retained and written to the storage medium. The higher the bit rate of each channel (camera or encoder) the greater the overall storage requirement of the system. Increasing storage space is a costly exercise and one of the principal expenses of a modern digital security system.

　　The benefits of combining infrared and encoding (IP and Hybrid)

　　Rather than continually adding storage to a security system it is best to address the root cause of the problem - high bit rate at night. At first it seems a quick fix would be to simply disable the cameras AGC function. While this would reduce bit rate, it would be at the expense of all important image detail. Disabling the cameras AGC function would result in poor quality, if not useless, surveillance images. A better solution is to install energy efficient infrared illumination on scene. When used with infrared, high camera gain becomes unnecessary and compression algorithms in encoders, DVRs and other recording equipment work to optimum effect.

　　Using infrared illumination enables security cameras to deliver high resolution, evidentiary quality night-time surveillance video with virtually zero image noise, and as we now know, less video noise equals better compression, reduced bit rate and reduced storage requirements.

　　Using infrared illumination enables security cameras to deliver high resolution night-time surveillance video

　　Security camera lighting and picture quality

　　At the most basic level, infrared is light. Light that is invisible to the human eye but which is used by monochrome and dual mode surveillance cameras to reveal night-time scenes which would appear completely dark to the human eye - remember our golden rule, without light there can be no picture.

　　Infrared prevents noisy night-time images and the subsequent chain of events that causes high bit rates, excessive storage requirements and unnecessary expense. In addition, certain infrared illuminators feature innovative 3D diffuser (Black Diamond) technology which directs light to the foreground and background of the cameras field of view, for perfectly evenly illuminated night-time images, free from hotspots and overexposure caused by some other IR illuminators. Remember the higher the quality of the image, the lower the storage requirement and overall cost of the system.[nextpage]

　　Using IR to reduce storage needs in digital video applications

　　Although the use of infrared lighting to improve image quality and usability is widely understood by the security industry, its use as a bandwidth management tool may come as a surprise. Given that storage is one of the biggest expenses incurred when operating a security system, that surprise is a welcome one and should encourage the use of IR as an effective tool for reducing storage needs in digital video applications.