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عبده الشاعر 04-05-2011 10:53 AM

اختيار العدسات والكابلات فى نظام c.c.t.v
 
اختيار العدسات والكابلات فى نظام C.C.T.V
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[SIZE=3][COLOR=#1f497d][F**T=Arial]Please find attached the comparis** between [U][B]RG6 and RG59 Cables[/B][/U]. I can summarize it by:[/F**T][/COLOR][F**T=Times New Roman][/F**T][/SIZE]
[COLOR=#1f497d][F**T=Arial][SIZE=3]1.[/SIZE][/F**T][/COLOR][COLOR=#1f497d][F**T=Times New Roman] [/F**T][/COLOR][SIZE=3][COLOR=#1f497d][F**T=Arial]RG6 cable has a thicker c**ductor than the RG59[/F**T][/COLOR][F**T=Times New Roman][/F**T][/SIZE]
[COLOR=#1f497d][F**T=Arial][SIZE=3]2.[/SIZE][/F**T][/COLOR][COLOR=#1f497d][F**T=Times New Roman] [/F**T][/COLOR][SIZE=3][COLOR=#1f497d][F**T=Arial] [/F**T][/COLOR][COLOR=#366388][F**T=Arial]RG6 cable[/F**T][/COLOR][COLOR=#1f497d][F**T=Arial] has better insulati** than the RG59[/F**T][/COLOR][F**T=Times New Roman][/F**T][/SIZE]
[COLOR=#1f497d][F**T=Arial][SIZE=3]3.[/SIZE][/F**T][/COLOR][COLOR=#1f497d][F**T=Times New Roman] [/F**T][/COLOR][SIZE=3][COLOR=#1f497d][F**T=Arial] RG6 can carry signals of much higher frequencies than the RG59[/F**T][/COLOR][F**T=Times New Roman][/F**T][/SIZE]
[COLOR=#1f497d][F**T=Arial][SIZE=3]4.[/SIZE][/F**T][/COLOR][COLOR=#1f497d][F**T=Times New Roman] [/F**T][/COLOR][SIZE=3][COLOR=#1f497d][F**T=Arial] RG6 has much lower signal losses compared to the RG59[/F**T][/COLOR][F**T=Times New Roman][/F**T][/SIZE]
[COLOR=#1f497d][F**T=Arial][SIZE=3]5.[/SIZE][/F**T][/COLOR][COLOR=#1f497d][F**T=Times New Roman] [/F**T][/COLOR][SIZE=3][COLOR=#1f497d][F**T=Arial] RG6 cable may not operate below 50Mhz while the RG59 can[/F**T][/COLOR][F**T=Times New Roman][/F**T][/SIZE]
[COLOR=#1f497d][F**T=Arial][SIZE=3]6.[/SIZE][/F**T][/COLOR][COLOR=#1f497d][F**T=Times New Roman] [/F**T][/COLOR][SIZE=3][COLOR=#1f497d][F**T=Arial] RG6 is better for satellite signals while RG59 is better for video signals[/F**T][/COLOR][F**T=Times New Roman][/F**T][/SIZE]
[COLOR=#1f497d][F**T=Arial][SIZE=3]7.[/SIZE][/F**T][/COLOR][COLOR=#1f497d][F**T=Times New Roman] [/F**T][/COLOR][SIZE=3][COLOR=#1f497d][F**T=Arial] RG6 cables cost **** than RG59[/F**T][/COLOR][F**T=Times New Roman][/F**T][/SIZE]
[SIZE=3][COLOR=#1f497d][F**T=Arial]Distance of RG6 cables are 2000 ft (606 meters)[/F**T][/COLOR][F**T=Times New Roman][COLOR=#000000] [/COLOR][/F**T][/SIZE]
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[COLOR=#666666][F**T=Arial][U]Lens elements[/U][/F**T][/COLOR]
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[COLOR=black][F**T=Verdana]A lens or lens assembly ** a network camera performs several functi**s. They include:[/F**T][/COLOR]
[COLOR=black][F**T=Symbol]· [/F**T][/COLOR][COLOR=black][F**T=Verdana]Defining the field of view; that is, defining how much of a scene and level of detail are to be captured. [/F**T][/COLOR]
[COLOR=black][F**T=Symbol]· [/F**T][/COLOR][COLOR=black][F**T=Verdana]C**trolling the amount of light passing through to the image sensor so that an image is correctly exposed. [/F**T][/COLOR]
[COLOR=black][F**T=Symbol]· [/F**T][/COLOR][COLOR=black][F**T=Verdana]Focusing by adjusting either elements ****** the lens assembly or the distance between the lens assembly and the image sensor. [/F**T][/COLOR]
[B][COLOR=#444444][F**T=Arial]Field of view[/F**T][/COLOR][/B]
[COLOR=black][F**T=Verdana]A c**siderati** to take into account when selecting a camera is the field of view required; that is, the area of coverage and the degree of detail to be viewed. The field of view is determined by the focal length of the lens and the size of the image sensor; both are specified in a network camera’s datasheet. [/F**T][/COLOR]
[COLOR=black][F**T=Verdana]A lens' focal length is defined as the distance between the entrance lens (or a specific point in a complicated lens assembly) and the point where all the light rays c**verge to a point (normally the camera’s image sensor). The l**ger the focal length, the narrower the field of view. [/F**T][/COLOR]
[COLOR=black][F**T=Verdana]The fastest way to find out what focal length lens is required for a desired field of view is to use a rotating lens calculator or an **line [/F**T][/COLOR][URL="http://www.axis.com/techsup/cam_servers/lens_calculators/index.htm"][COLOR=#3366cc][F**T=Verdana]lens calculator[/F**T][/COLOR][/URL][COLOR=black][F**T=Verdana], both of which are available from Axis. The size of a network camera’s image sensor, typically 1/4”, 1/3”, 1/2” and 2/3”, must also be used in the calculati**. (The drawback of using a lens calculator is that it does not take into account any possible geometrical distorti** of a lens.) [/F**T][/COLOR]
[COLOR=black][F**T=Verdana]The field of view can be classified into three types:[/F**T][/COLOR]
[COLOR=black][F**T=Symbol]· [/F**T][/COLOR][B][COLOR=black][F**T=Verdana]Normal view:[/F**T][/COLOR][/B][COLOR=black][F**T=Verdana] offering the same field of view as the human eye. [/F**T][/COLOR]
[COLOR=black][F**T=Symbol]· [/F**T][/COLOR][B][COLOR=black][F**T=Verdana]Telephoto:[/F**T][/COLOR][/B][COLOR=black][F**T=Verdana] a narrower field of view, providing, in general, finer details than a human eye can deliver. A telephoto lens is used when the surveillance object is either small or located far away from the camera. A telephoto lens generally has less light gathering capability than a normal lens. [/F**T][/COLOR]
[COLOR=black][F**T=Symbol]· [/F**T][/COLOR][B][COLOR=black][F**T=Verdana]Wide angle:[/F**T][/COLOR][/B][COLOR=black][F**T=Verdana] a larger field of view with less detail than in normal view. A wide-angle lens generally provides good depth of field and fair, low-light performance. Wide-angle lenses sometimes produce geometrical distorti**s such as the “fish-eye” effect. [/F**T][/COLOR]
[LEFT][I][COLOR=black][F**T=Verdana]Different fields of view: wide-angle view (at left); normal view (middle); telephoto (at right). [/F**T][/COLOR][/I]
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[LEFT][I][COLOR=black][F**T=Verdana]Network camera lenses with different focal lengths: wide-angle (at left); normal (middle); telephoto (at right). [/F**T][/COLOR][/I]
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[COLOR=black][F**T=Verdana]There are three main types of lenses:[/F**T][/COLOR]
[COLOR=black][F**T=Symbol]· [/F**T][/COLOR][B][COLOR=black][F**T=Verdana]Fixed lens:[/F**T][/COLOR][/B][COLOR=black][F**T=Verdana] Such a lens offers a focal length that is fixed; that is, **ly **e field of view (either normal, telephoto or wide angle). A comm** focal length of a fixed network camera lens is 4 mm. [/F**T][/COLOR]
[COLOR=black][F**T=Symbol]· [/F**T][/COLOR][B][COLOR=black][F**T=Verdana]Varifocal lens:[/F**T][/COLOR][/B][COLOR=black][F**T=Verdana] This type of lens offers a range of focal lengths, and hence, different fields of view. The field of view can be manually adjusted. Whenever the field of view is changed, the user has to manually refocus the lens. Varifocal lenses for network cameras often provide focal lengths that range from 3 mm to 8 mm. [/F**T][/COLOR]
[COLOR=black][F**T=Symbol]· [/F**T][/COLOR][B][COLOR=black][F**T=Verdana]Zoom lens:[/F**T][/COLOR][/B][COLOR=black][F**T=Verdana] Zoom lenses are like varifocal lenses in that they enable the user to select different fields of view. However, with zoom lenses, there is no need to refocus the lens if the field of view is changed. Focus can be maintained ****** a range of focal lengths, for example, 6 mm to 48 mm. Lens adjustments can be either manual or motorized for remote c**trol. When a lens states, for example, 3x-zoom capability, it is referring to the ratio between the lens’ l**gest and shortest focal length. [/F**T][/COLOR]
[B][COLOR=#444444][F**T=Arial]Matching lens and sensor[/F**T][/COLOR][/B]
[COLOR=black][F**T=Verdana]If a network camera offers an exchangeable lens, it is important to select a lens suitable for the camera. A lens made for a 1/2-inch image sensor will work with 1/2-inch, 1/3-inch and 1/4-inch image sensors, but not with a 2/3-inch image sensor. [/F**T][/COLOR]
[COLOR=black][F**T=Verdana]If a lens is made for a smaller image sensor than the **e that is actually fitted inside the camera, the image will have black corners (see left-hand illustrati** below). If a lens is made for a larger image sensor than the **e that is actually fitted inside the camera, the field of view will be smaller than the lens’ capability since part of the informati** will be “lost” outside the image sensor (see right-hand illustrati**). This situati** creates a telephoto effect as it makes everything look zoomed in.[/F**T][/COLOR]
[LEFT][I][COLOR=black][F**T=Verdana]Examples of different lenses mounted **to a 1/3-inch image sensor. [/F**T][/COLOR][/I]
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[COLOR=black][F**T=Verdana]When replacing a lens ** a megapixel camera, a high quality lens is required since megapixel sensors have pixels that are much smaller than those ** a VGA sensor (640x480 pixels). It is best to match the lens resoluti** to the camera resoluti** in order to fully use the camera’s capability. [/F**T][/COLOR]
[B][COLOR=#444444][F**T=Arial]Lens mount standards[/F**T][/COLOR][/B]
[COLOR=black][F**T=Verdana]When changing a lens, it is also important to know what type of lens mount the network camera has. There are two main standards used ** network cameras: CS-mount and C-mount. They both have a 1-inch thread and they look the same. What differs is the distance from the lenses to the sensor when fitted ** the camera:[/F**T][/COLOR]
[COLOR=black][F**T=Symbol]· [/F**T][/COLOR][COLOR=black][F**T=Verdana]CS-mount. The distance between the sensor and the lens should be 12.5 mm. [/F**T][/COLOR]
[COLOR=black][F**T=Symbol]· [/F**T][/COLOR][COLOR=black][F**T=Verdana]C-mount. The distance between the sensor and the lens should be 17.526 mm. [/F**T][/COLOR]
[COLOR=black][F**T=Verdana]It is possible to mount a C-mount lens to a CS-mount camera body by using a 5 mm spacer (C/CS adapter ring). If it is impossible to focus a camera, it is likely that the wr**g type of lens is used. [/F**T][/COLOR]
[COLOR=black][F**T=Verdana]See also: [/F**T][/COLOR][URL="http://www.axis.com/products/video/camera/about_cameras/lens_ta.htm"][COLOR=#3366cc][F**T=Verdana]Lenses for thermal cameras[/F**T][/COLOR][/URL][COLOR=black][F**T=Verdana] using TA mounts [/F**T][/COLOR]
[B][COLOR=#444444][F**T=Arial]F-number and exposure[/F**T][/COLOR][/B]
[COLOR=black][F**T=Verdana]In low-light situati**s, particularly in indoor envir**ments, an important factor to look for in a network camera is the lens’ light-gathering ability. This can be determined by the lens’ f-number, also known as f-stop. An f-number defines how much light can pass through a lens. [/F**T][/COLOR]
[COLOR=black][F**T=Verdana]An f-number is the ratio of the lens’ focal length to the diameter of the aperture or iris diameter; that is, f-number = focal length/aperture. [/F**T][/COLOR]
[COLOR=black][F**T=Verdana]The smaller the f-number (either short focal length relative to the aperture, or large aperture relative to the focal length), the better the lens’ light gathering ability; i.e. **** light can pass through the lens to the image sensor. In low-light situati**s, a smaller f-number generally produces a better image quality. (There may be some sensors, however, that may not be able to take advantage of a lower f-number in low-light situati**s due to the way they are designed.) A higher f-number, ** the other hand, increases the depth of field, which is explained below. A lens with a lower f-number is normally **** expensive than a lens with a higher f-number. [/F**T][/COLOR]
[COLOR=black][F**T=Verdana]F-numbers are often written as F/x. The slash indicates divisi**. An F/4 means the iris diameter is equal to the focal length divided by 4; so if a camera has an 8 mm lens, light must pass through an iris opening that is 2 mm in diameter. [/F**T][/COLOR]
[COLOR=black][F**T=Verdana]While lenses with automatically adjustable iris (DC-iris) have a range of f-numbers, often **ly the maximum light gathering end of the range (smallest f-number) is specified. [/F**T][/COLOR]
[COLOR=black][F**T=Verdana]A lens’ light-gathering ability or f-number, and the exposure time (i.e., the length of time an image sensor is exposed to light) are the two main elements that c**trol how much light an image sensor receives. A third element, the gain, is an amplifier that is used to make the image brighter. However, increasing the gain also increases the level of noise (graininess) in an image, so adjusting the exposure time or iris opening is preferred. [/F**T][/COLOR]
[COLOR=black][F**T=Verdana]Limits to the exposure time and gain can be set in some Axis cameras. The l**ger the exposure time, the **** light an image sensor receives. Bright envir**ments require shorter exposure time, while low-light c**diti**s require l**ger exposure time. It is important to be aware that increasing the exposure time also increases moti** blur, while increasing the iris opening has the downside of reducing the depth of field, which is explained in the secti** below. [/F**T][/COLOR]
[COLOR=black][F**T=Verdana]When deciding up** the exposure, a shorter exposure time is recommended when rapid movement or when a high frame rate is required. A l**ger exposure time will improve image quality in poor lighting c**diti**s, but it may increase moti** blur and lower the total frame rate since a l**ger time is required to expose each frame. In some network cameras, an automatic exposure setting means the frame rate will increase or decrease with the amount of available light. It is **ly as the light level decreases that artificial light or prioritized frame rate or image quality is important to c**sider.[/F**T][/COLOR]
[LEFT][I][COLOR=black][F**T=Verdana]A camera user interface with opti**s for setting, am**g other things, exposure in low-light c**diti**s. [/F**T][/COLOR][/I]
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[B][COLOR=#444444][F**T=Arial]Fixed or adjustable iris[/F**T][/COLOR][/B]
[COLOR=black][F**T=Verdana]The ability to c**trol a camera’s iris opening plays an important role in image quality. An iris is used to maintain the optimum light level to the image sensor so that images can be sharp, clear and correctly exposed with good c**trast and resoluti**. The iris can also be used to c**trol the depth of field. Iris c**trol can be fixed or adjustable. Adjustable iris lenses can be manual or automatic (auto iris and P-Iris). [/F**T][/COLOR]
[COLOR=black][F**T=Verdana]**** details: [/F**T][/COLOR][URL="http://www.axis.com/products/video/camera/about_cameras/iris.htm"][COLOR=#3366cc][F**T=Verdana]Types of iris c**trol - fixed, manual, auto, precise (P-Iris)[/F**T][/COLOR][/URL][COLOR=black][F**T=Verdana] [/F**T][/COLOR]
[B][COLOR=#444444][F**T=Arial]Depth of field[/F**T][/COLOR][/B]
[COLOR=black][F**T=Verdana]A criteri** that may be important to a video surveillance applicati** is depth of field. Depth of field refers to the distance in fr**t of and bey**d the point of focus where objects appear to be sharp simultaneously. Depth of field may be important, for instance, in m**itoring a parking lot, where there may be a need to identify license plates of cars at 20, 30 and 50 meters (60, 90 and 150 feet) away. [/F**T][/COLOR]
[COLOR=black][F**T=Verdana]Depth of field is affected by three factors: focal length, iris diameter and distance of the camera to the subject. A l**g focal length, a large iris opening or a short distance between the camera and the subject will limit the depth of field.[/F**T][/COLOR]
[LEFT][I][COLOR=black][F**T=Verdana]Depth of field: Imagine a line of people standing behind each other. If the focus is in the middle of the line and it is possible to identify the faces of all in fr**t and behind the mid-point **** than 15 meters (45 feet) away, the depth of field is good. [/F**T][/COLOR][/I]
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[LEFT][I][COLOR=black][F**T=Verdana]Iris opening and depth of field. The below illustrati** is an example of the depth of field for different f-numbers with a focal distance of 2 meters (7 feet). A large f-number (smaller iris opening) enables objects to be in focus over a l**ger range. (Depending ** the pixel size, very small iris openings may blur an image due to diffracti**.) [/F**T][/COLOR][/I]
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[COLOR=black][F**T=Verdana]Next topic: [/F**T][/COLOR][URL="http://www.axis.com/products/video/about_networkvideo/image_sensors.htm"][COLOR=#3366cc][F**T=Verdana]Image senso[/F**T][/COLOR][/URL][COLOR=black][F**T=Verdana][/F**T][/COLOR]
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عبده الشاعر 07-04-2011 10:59 AM

اللهم تقبل

عبده الشاعر 07-04-2011 11:01 AM

اللهم تقبل

جابر العلي 09-22-2011 06:35 PM

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الساعة الآن »02:15 PM.

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