Technology in CCTV Surveillance / Security is constantly evolving and moving forward. Consequently, most products are rapidly being changed and improved.
The following offers some fundamental facts and knowledge of basic security surveillance equipment. The information below is offered for those who understand what CCTV surveillance systems are, and would like to educate themselves to the next level.
Please note that not all information pertains to all products & manufacturers in the same way.
THE BASICS OF CCTV
The following is a basic overview of CCTV cameras and their functions and uses:
What good is it?
Advances in CCTV technology are turning the video surveillance into one of the most valuable loss prevention, safety/security and management tools available today. Retailers use CCTV to monitor for shoplifters and dishonest employees, compile recorded evidence against bogus accident claims and monitor merchandising displays in stores that may be hundreds of miles away. Manufacturers, governments, hospitals and universities use CCTV to identify visitors and employees, monitor hazardous work areas, thwart theft and ensure the security of their premises and parking facilities.
Security Applications
Observe and record theft or violence by overtly monitoring retail floor space, office buildings, building perimeters, warehouses, loading docks and parking garages.
- Monitor sensitive areas where infrequent activities occur (i.e. confidential records, safes, etc...)
- Monitor point-of-sale exceptions (cash register voids, over-rings, etc...) reducing cashier theft.
- Observe and record shoplifting activities.
- "Walk a beat" by programming a moving camera to pan, tilt, and zoom within a defined pattern.
- Perform covert surveillance (where legally applicable)
- Integrate with access control systems to provide video of persons entering and leaving the premises
- Complement asset tracking systems to provide video when a tagged asset leaves the premises.
Safety Applications
- Allow operators to see into areas where the environment is hazardous to life or health
- Monitor potential accident areas.
- Monitor halls, common areas, or high-risk areas to ensure safety of an institution's employees and clients.
- Help reduce the severity of some incidents by the timely dispatch of police, fire, and emergency personnel.
Management Tools
- Train employees, check stock on store shelves and monitor retail sales floor coverage, production lines, etc.
- Demonstrate management's due diligence towards protecting employees, clients, and visitors, and perhaps avert or minimize litigation and negative publicity
- Document video images on magnetic tape or optical hard drives to record events. This information may be reviewed and later presented as evidence for prosecution of criminals, or as a training tool.
Getting Started
Many elements must be considered when designing and installing CCTV systems.
The Scene and Light
The scene refers to the objects or area to be observed and the total environment in which the exist. A scene often contains different colors, surfaces and materials that reflect varying levels of light. To select proper equipment, it is necessary to determine the minimum lighting level (day or night) that will arrive from the scene to the camera lens.
A scene or target area can be illuminated by natural or artificial light sources. Natural sources include the sun, the moon, and the stars. Artificial sources include incandescent, sodium, florescent, infrared, and other man-made lights. An axiom in CCTV security applications is: The better the light, the better the picture.
Color vs. Black and White
Color cameras generally require higher levels of lighting than their monochromatic (B/W) counterparts. Color produces a more natural, richer image than black and white and may keep the operator's interest for longer periods of time. It also makes it easier to detect subjects. For example, with a color system a viewer can easily distinguish a red car from a green one, while on a black and white system both cars would appear a similar shade of grey. In retail applications, a color system can help security personnel identify shoplifters and their clothing more easily and convincingly. Color accuracy is extremely important in gambling casinos, where hundreds of dollars can ride on the ability to recognize the difference between maroon chips and red ones.
While the use of color cameras is growing as the cost difference decreases, black and white cameras continue to offer some distinct advantages. Black and white cameras are better suited for extremely low light situations. The ability to capture good quality images in low light situations increase the cost of both black and white and color cameras. Before purchasing cameras, you should consider the crossover points between lighting costs and camera costs. It's possible that low-light cameras will cost less than paying to increase the lighting level of a parking lot. It is also a possibility to employ "Day-Night" cameras which are color in good light and switch to a B&W mode when the light levels drop to a set level.
TERMS AND DEFINITIONS
ALC (Automatic Level Control) On AL lenses, aka peak/average control. Adjusting this control allows the auto iris circuitry to either take bright spots
Activity Detection A feature of the generation 3 multiplexor range that uses video motion detection techniques to improve camera update
Alarming The ability of CCTV equipment to respond to an input signal, normally a simple switch closure. The response varies
Analog A signal in which any level is represented by a directly proportional voltage; not digital.
Angle of View The maximum scene angle that can be seen through a lens.
Aperture The lens opening that controls the amount of light reaching the pickup device (imager)
Aspherical Lens A lens designed with a non spherical shape so that it refracts the light passing through it either lower the lens aperture so that it passes more light or decreases barrel distortion on wide angle lenses.
Attenuation A decrease or loss in a signal. Reduction of signal magnitude (loss) normally measured in decibels
Auto White Balance Feature on color cameras whereby the camera constantly monitors the light and adjusts its color to maintain white areas.
Automatic Frequency Control (AFC) An electronic circuit used whereby the frequencies of an oscillator is automatically maintained within specific units,
Automatic Gain Control (AGC) An electronic circuit used whereby the gain of a signal is automatically adjusted as a function of its input or other specified parameter.
Automatic Iris Lens A lens in which the aperture automatically opens or closes to maintain proper light levels on the cameras imager.
Auto-Terminating Feature whereby equipment automatically selects the correct termination depending on whether the video output is connected to
Back Lighting Compensation (BLC) A feature of modern CCD camera which electronically compensates for high background lighting to give detail which would normally be silhouetted.
Back Focus The mechanical aligning of the imaging device with the focal point of the lens. Most important on a zoom lens to ensure the image stays in focus throughout the zoom range.
Balanced Signal Method of transmitting video, usual over twisted pair cable, that consists of two equal but opposite signals being sent down two semiconductors.
Bandwidth The frequency range of a signal. The span that the information-bearing signal occupies or requires or the difference between the lowest and highest frequency of a band.
Base-Band Video Unmodulated video signal suitable for display on a monitor, but not a domestic TV
Black Level The level of the video signal that corresponds to the maximum limits of the black areas of the picture
Blanking (Field and line fly back blanking) The operation of turning of the monitor display, or pick-up device, during sync pulses to avoid thin white lines that appear in the picture
Blooming The halation and defocusing effect that occurs around the bright areas of the picture (highlight) whenever there is an increase in the brightness intensity.
BNC Video connector, the most commonly used video connector in the CCTV industry
Bridging A term indicating that a high impedance video line is paralleled, usually through a switch, to a source of video. A separate overall video output.
C Mount / CS Mount The two industry standards for mounting a lens on a camera. The C-Mount lens has a 17.5 mm flange back distance. The CS-Mount lens has a 12.5 mm flange back distance.
CCIR The European TV standard 625 lines 50 fields
Camera A device that translates light into a video image and transmits that image to a monitor for viewing. It contains the image sensor and other electronic circuitry to create a video signal.
Cathode Ray Tube (CRT) The picture tube in a monitor that can reproduce the picture image seen by the camera.
CATV (Cable Access Television) The method for distributing RF signals via coaxial cable rather than radiated through the air.
CCD (Charged Coupled Device) This is a solid state semi-conductor imaging device often referred to as an integrated circuit, chip or imager. Solid state cameras are sometimes referred to as CCD cameras.
CCTV The common abbreviation for Closed Circuit Television. A private or closed television system.
Chrominance (C) The part of the video signal corresponding to the color information.
Coaxial Cable A type of shielded cable of carrying a wide range of frequencies (video and radio) with very low signal loss.
Composite Video The complete video signal consisting of the video information, the sync pulse and the threshold reference.
Contrast The range of light and dark values in a picture or the ratio between the maximum and minimum brightness values.
Cross talk An undesired signal that interferes with the desired signal
Decibel (Db) A measure of the power ratio of two signals. It is equal to ten times the logarithm of the ratio of the two signals.
DC Type Lens An auto-iris lens with internal circuit which receives voltage and a video signal from the camera to adjust the lens.
Depth of Field The area in focus in front of and behind the subject.
Digital A signal that levels are represented by binary numbers. These can be kept in a store.
Digital Recording Recording images on digital mediums such as optical or hard drives instead of analog tapes.
Distribution Amplifier A device that accepts a video signal and sends it out over a number of independent outputs.
Duplex Multiplexor A multiplexor with two frame stores allowing it to show multi-screen pictures while performing time multiplex recordings.
Dwell Time The length of time a switcher displays a camera before moving on to the next one in the sequence.
EI (Electronic Iris) Shutter The ability of the camera to compensate for moderate light changes in indoor applications without the use of an auto iris lens.
EIA (Electronic Industry Association) US TV standard 525 lines 60 fields.
Equalization The process of correcting losses of certain components in a signal.
External Sync The ability of CCTV equipment to accept one or more of the standard sync formats so as to align itself to the rest of the system
Fiber Optic The process of transmitting light through a long, flexible fiber such as glass or plastic, for the purpose of transmitting video, audio, or data over long distances.
Field One half of a frame of video (262.5 scan lines)
Field of View The maximum viewed image a lens "sees". The horizontal or vertical scene size at a given distance from the camera.
Flange Back The distance from the flange of the lens to the focal plane.
Focal Length The distance (in millimeters) from the lens to the surface of the imager. The shorted the distance, the wider the view, the longer the distance, the narrower the view.
Focus The focal point. An adjustment to the lens optics to improve the images clarity.
Foot Candle (FC) A measurement of light. 1 lumen per square foot.
Format The size of the cameras imager. Current standards are 1/2, 1/3, 1/4 inches.
Frame A complete picture. 2 fields of video information.
F-Stop The speed of a lens. The smaller the f-number, the greater amount of light passing through the lens.
Gen. Lock (See External Sync) To synchronize one piece of equipment to the sync pulses of another.
Ghost A shadowy or weak image in the received picture, offset to either the right or to the left of the primary image.
Grey Scale The number of variations from white to grey to black.
Ground An electrical connection point that is common to either a metal chassis, a terminal, or a ground bus.
Ground Loop Affects video pictures in the form of a black shadow bar across the screen or as tearing in the top corner of the picture. Caused by different earth potentials in the system.
Hardwired Method of controlling camera points using multi-core cables.
Horizontal Resolution The maximum number of individual picture elements that can be distinguished in a single scanning line.
Hertz (Hz) Cycles per second
IR (Infrared) A range of frequencies lower than visible red light used for covert surveillance or as a low cost wireless video link.
ISDN (Integrated Services Digital Network) Digital phone lines from which allow transmissions of video signals via fast scan at speeds of 128Kb / sec; used with terminal adapters.
Image Burn (Retained Image) A change produced or in or on a target pickup device (monitor) which remains after the removal of a previously stationary light source.
Image Device (Imager) The detector in the camera, either a tube or CCD solid state device.
Impedance The effective resistance of an electronic circuit to an AC signal.
Insertion Loss The signal strength loss that occurs when a piece of equipment is inserted into a line.
Intensifier An electronic device that creates an output image brighter than the input (original) image.
Interlace A scanning process where every other horizontal line is scanned in one field while the alternate lines are scanned into the next field to produce a complete picture frame.
Internal Sync (Crystal Controlled) A camera that generates its sync pulses without reference to any other sources. Normally using a crystal controlled oscillator.
Iris The adjustable opening through which light can pass and is regulated.
Lens A transparent optical component that converges light rays to forma two dimensional image of that object.
Level Control Main iris control. Used to set the auto-iris circuit to a video level desired by the user. After set-up, the circuit will adjust the iris to maintain this video level in changing lighting conditions. Turning this control towards high will open the iris, towards low will close the iris.
Line Amplifier (Video Line Corrector) A device to make good the loss of a signal strength and quality due to long cable runs.
Line Lock Used to synchronize the field sync pulses of an AC powered camera to the frequency of the voltage input (line voltage).
Looping A term indicating that a high impedance device has been permanently connected in a parallel to a video source. Individual balanced video outputs for each video input.
Luminance (Y) The part of the video signal that consists of the monochromatic data.
Lux A unit of measurement for the intensity of light. One foot-candle (FC) = 10 Lux.
Manual Iris Lens A lens with a manual adjustment for the iris opening in a fixed position. Generally used for fixed lighting applications.
MATV Multiple Access Television. The method of distributing RF TV signals by broadcasting them through the air.
Mechanical focus (Back Focus, Racking) lenses that ensure the image stays in focus throughout the zoom range.
Microwave Radio frequencies between 1 and 30 Ghz.
Minimum Object Distance (MOD) The closest distance a given lens will able to focus upon.
Modulate to change or vary some parameter such as varying the amplitude of a signal. A modulator is the circuit that modulates the signal.
Monitor A device that converts electronic signals into the video image that was generate by a camera and lens. The picture end of a CCTV system.
Multiplex (Time Multiplex) Using one carrier to send more than one signal. In video Multiplexors this achieved by sending a different camera output in each successive field or frame of a video signal.
Mux A Multiplexor.
NTSC (National Television Standards Committee, see EIA) Color TV system used in the United States.
N/D (Neutral Density) A filter that attenuates light equally over the whole visible spectrum.
Noise Undesired signal(s) that corrupts the original video signal and may reduce image quality.
PAL (Phase Alternate Line, see CCIR) Color TV system used in the United Kingdom.
PTSN (Public switched Telephone Network) Standard phone lines used to transmit fast scan pictures via modems.
Pan Side-to-side movement of a camera (horizontal axis).
Passive A non-powered element of a system.
Peak to Peak The amplitude difference between the most positive and the most negative incursions of a signal.
Peripheral An optional device that can enhance a CCTV system. (i.e. quad, VCR, multiplexor, etc...)
Phase Adjustment (See Line Lock) The ability to delay the line locking process so as to align cameras fed from AC voltages on a different phase.
Photocell Automatically switches on the infrared (IR) if ambient light levels fall below a preset level.
Pinhole Lens lens used for applications where the camera must be hidden. A 1/32 or 1/16 inch lens mounted on a camera that is capable of viewing an entire room.
PIP Picture in Picture
Pixel The smallest cell or area in a picture. The greater number of pixels, the higher the resolution of the scene.
Presets The pre-positioning of a pan, tilt, zoom camera by the use of its potentiometer.
Quad A device that compresses up to 4 video signals and simultaneously displays them on a monitor.
Rack Mount An industrial standard housing 19" wide, its height is measure in units of U (1.75 inches ea)
RF (Radio Frequency) Signals with a repetition rate above audible hearing, but lowed than frequencies associated with heat and light.
Random Interlace A scanning technique in which there is no external control over the scanning process. There is no fixed relationship between adjacent lines and successive fields.
Range Finder Used to determine the focal length needed and what the picture will look like on a monitor.
Raster The rectangular patter of scanning lines upon which the picture is produced. The illuminated face of the TV monitor without the information present.
Reflected Light The scene brightness or the light being reflected from a scene.
Resolution A measure of the ability of a camera or a television system to reproduce details.
Roll A loss of vertical sync which causes the picture to move up and down on a TV screen.
RS232 A commonly used computer serial interface.
SECAM (see CCIR) Color TV system used in France.
SVHS (Super Video Home System) New format high resolution VHS recorders. VHS compatible.
S/N (Signal to Noise Ratio) Measure of the noise levels of a video signal. The higher the number, the better.
Saturation (Color) The measure of the vividness of color. The amplitude of chrominance.
Scanner A panning only unit.
Scanning The rapid movement of an electronic beam in a pickup device of a camera or in the CRT of a television receiver. When referring to a video surveillance field, it is the horizontal panning camera motion.
Sensitivity A camera's ability to reproduce a given scene with a given light level. Usually expressed in Lux or foot-candles.
Sequential Switchers A video device that alternates the displayed video image from one camera to the next. A video device that sequentially steps through and displays all connected camera signals.
Simplex (Multiplexor) A Mux with 1 frame store that can either multiplex record or show multi-screen pictures in live or play back mode. It cannot record multiplexed images while showing a multiplexed mode.
Spot Filter A small insert used in a lens to increase the F-Stop range of the lens.
Switcher See sequential Switchers above.
Sync Abbreviation for synchronization. Electronic pulses that are inserted into video signal for the purpose of assembling the picture in the correct position.
TVL (Television Lines Resolution) The maximum number of changes between light and dark pictures across 3/4 the width. Dictates the resolution of a CCTV product.
Tearing A picture condition in which the horizontal lines are displaced in a irregular manner.
Telemetry A system utilizing control code transmitters and receivers. These use the video cable or a simple twisted pair cable to send their information.
Telephoto A lens used to produce a larger image of a distant object.
Terminated (75 Ohm Terminated) Video Input of a piece of CCTV equipment, wired to be the last in a particular video line.
Tilt The up and down (vertical) movement of a camera.
Time Zone Time intervals during which activity is permitted. Used digital motion setup.
TBC (Time Base Corrector) Electronic circuit that aligns the unsynced video signal before signal processing. Used in Muxs and quads.
Time-Lapse VCR Video recorder that can record frames with pauses between them thereby extending the time that a standard cassette will hold.
Triplex (Triplex Multiplexor) Mux feature that gives you the ability to simultaneously view both playback and live cameras within the same multiscreen while still recording.
Twisted Pair A cable, often screened, that consist of two conductors twisted together along their length.
UPS (Uninterruptible Power Supply) Equipment that supplies power to a system in the event of a primary power loss. It may consist of batteries or an auxiliary generator.
Unterminated (Hi-Z) Video input of a piece of CCTV equipment wired so as to allow the video signal to be fed to further equipment. Does not necessarily include extra sockets for external coaxial cables.
Varifocal Lens See Zoom lens. Usually not motorized (manual)
VCR a device that records video information to a tape.
Vertical Interval Switching Sequential switchers waiting until the current field has finished before displaying the next camera even though the dwell time has elapsed.
Vext A Mux feature that uses a pulse generated by the VCR so that the MUX automatically adjusts to the VCR time lapse speed.
Video Motion Detection (VMD) A system that uses the video signal from a camera to determine if there is any movement in the picture and set of an alarm.
Video Printer A device that prints a hard copy of images from a video signal.
Visible Light Wavelengths of light visible to the human eye.
White Light The entire spectrum of visible light.
Y/C A method of sending video pictures in 2 separate parts down 2 separate cables. The component parts are Y and C
Zoom The ability to change the magnification of a scene.
Zoom Lens A lens with a variable focal length. This lens may be effectively used as a wide angle, standard, or telephoto lens by varying the focal length of the lens. A varifocal lens.
Zoom Ratio The ratio of starting focal length (wide position) to the ending focal length (telephoto position) of a zoom lens. Usually represented as a multiplier (i.e. 32x)
Switchers & Recorders
(Economy) A video switcher enables different cameras to be switched to call-up monitors. In a smaller, cost-conscious applications, a manual switcher allows users to select the camera they want to see by pressing the appropriate button. The most popular type of switcher, a sequential switcher, contains circuitry that will switch one camera to another automatically. The operator can set the length of time (dwell time) that a scene remains on the monitor before sequencing automatically to the next camera. This allows operators to keep tabs on numerous cameras with only one monitor, but also creates a drawback, some call ... "switcher dilemma".
To illustrate switcher dilemma, imagine a system with eight cameras, each programmed to switch after dwelling on the monitor for 5 seconds. In this scenario, a considerable gap will occur between the time the first image is displayed and the time the eighth image is displayed. The situation worsens when recoding the video for review at a later time. On playback, you may see a door opening on camera 1, then suddenly the video switches to camera 2, followed by camera 3 and so on. By the time camera 1 appears again, the door is closed and you are left wondering who came through the door while camera 2 through 8 were flashing sequentially on the monitor.
Matrix Switcher
A matrix switcher is a more complex design enabling the user to switch any video signal to any call-up monitor in a large-scale system. They normally incorporate PTZ control and other features such as preset and alarm inputs and outputs.
Multiplexers & Digital Recorders
Unlike conventional recording systems, a Multiplexor collects full-screen pictures from up to 16 cameras and displays them simultaneously on a monitor. The video is simultaneously recorder to a hard drive as data, just like your computer. 
The camera data (video) is recorded sequentially at a high rate of speed. As mentioned earlier, a standard video signal is comprised of 30 separate frames each second. In a video system containing 15 cameras, the Multiplexors selects two frames from each camera and records them to a single archive source. The result is an effective frame rate of 2 frames per second, instead of the standard 30. With the advent of the unbelievable PC processing capabilities today, there are now digital recorders available for reasonable prices that provide 480 frames per second and beyond. Most Multiplexor technologies contain motion detection features that enables the system to record more frames of video from cameras showing motion than from those not showing any motion. The Multiplexor does this by reallocating frames from one camera to another as needed. The net result is higher quality recordings of scenes that are more likely to be important to security personnel.
The camera data (video) is recorded sequentially at a high rate of speed. As mentioned earlier, a standard video signal is comprised of 30 separate frames each second. In a video system containing 15 cameras, the Multiplexors selects two frames from each camera and records them to a single archive source. The result is an effective frame rate of 2 frames per second, instead of the standard 30. With the advent of the unbelievable PC processing capabilities today, there are now digital recorders available for reasonable prices that provide 480 frames per second and beyond. Most Multiplexor technologies contain motion detection features that enables the system to record more frames of video from cameras showing motion than from those not showing any motion. The Multiplexor does this by reallocating frames from one camera to another as needed. The net result is higher quality recordings of scenes that are more likely to be important to security personnel.Cameras
Cameras convert the visible scene captured by a lens into an electronic signal and transmit that signal to a monitor for viewing and in many cases to an archiving device such as a digital recorder. Several considerations should be taken into account when choosing the proper camera/lens for any video system:
- The purpose of the video system (detection, assessment, identification, etc.)
- The overall sensitivity & resolution of the camera needed based upon the actual application
- The amount of varying levels of light available at the scene
- The environment in which the camera will operate (indoors/outdoors)
- The field of view (FOV) required by the application
- The lens
- Cost
Camera performance depends largely upon the reflected light at the scene and the quality of the cameras imager
Where the level of available light can change dramatically, a camera equipped with automatic iris control and back light compensation features can help assure consistent image quality. Automatic iris control enables cameras to open or close an auto iris lens to adjust the amount of light passing through the lens. On a bright, sunny day an auto iris lens camera will close the lens' iris to prevent strong light from reaching into the camera's imager. At night, the camera will open the iris to allow greater amounts of light into the camera.
Cameras are available in various "formats" expressed as 1/2, 1/3, or 1/4 inches. These measurements represent the overall usable size of the cameras imager. In general, you should match the cameras format to the lens format. For example, a half inch camera should be paired with a half inch lens. This only applies to camera that need a separate lens (i.e. C-Mount types) as opposed to cameras with a built in lens.
Fixed and PTZ Cameras
CCTV cameras can be fixed or have pan, tilt, and zoom (PTZ) capabilities. Fixed cameras are mounted on a fixed bracket and cannot move in response to operator commands. PTZ cameras are motor driven and can pan left or right, tilt up or down, and zoom in & out. A camera housing protects the camera and lens from vandalism and the environment. It also can enhance the appearance of the camera installation and conceal the equipment from the casual observer. All outdoor cameras require a housing of some type. The National Electrical Manufacturers Association (NEMA) rates housings on their ability to withstand environmental conditions. Protection from cold, heat, dust, dirt, or other elements is needed to ensure optimal performance and extend the life of the camera.
Dome Cameras
Many PTZ cameras today are disguised in dark or mirrored Plexiglas housings called domes. Dome cameras are found in practically every major department store and in many industrial/commercial locations such as hospitals, colleges and government facilities. They are particularly popular wherever aesthetics are valued. Dome cameras offer three primary benefits:
- Deterrence - Domes make it virtually impossible for suspects to determine where the camera is pointed.
- Economy - Domes equipped with a camera, lens, and PTZ capabilities can sometimes cover and area it would take many fixed cameras to view
- Aesthetics - Domes conceals all the internal equipment in a nice clean shell.
Smoked Plexiglas bubbles have the same effect as wearing sunglasses. It reduces the amount of light reaching the lens affecting the color accuracy picked up by the camera.
of light reaching the lens affecting the color accuracy picked up by the camera.The Lens (optics)
Lenses (Optics) play an important role in the design of a CCTV system. Their primary function is to collect reflected light from a scene and focus a clear, sharp image on the camera's imager. Typically the more light that passes through the lens, the better the quality of the picture. Selection of a lens is especially critical because it directly affects the size, shape, and sharpness of the image to be displayed on the imager. Factors such as distance from the scene, focal length, desired field of view, lighting and format affects the size and clarity of the image on cameras imager.
Field of view
The field of view (FOV) is the actually picture size (height and width) produced by the specific lens. If the field of view is not suitable, you may consider using a different lens (wide angle, telephoto, etc...) to increase or decrease the field of view. Camera lenses can be divided into two basic types.: fixed focal and varifocal (sometimes known as zoom). A fixed focal lens has a constant focal length, while the varifocal lens can change its focal length. Focal length is simply the distance from the optical center of the lens to a focal point near the back of the lens. This distance is written on the lens and expressed in millimeters (mm). Fixed focal length lenses are available in various wide, medium, and narrow fields of view. A lens with a "normal" focal length produces a picture that approximates the field of view of the human eye. A wide-angle lens has a short focal length, while a telephoto lens has a long focal length. When you select a fixed lens for a particular view, bear in mind that if you want to change the field of view, you must change the lens.
When both wide scenes and close-up scenes are needed, a varifocal lens is best. A zoom lens is an assembly of lens elements that move the change the focal length from a wide angle to a telephoto while maintaining focus on the camera's imager. This permits you to change the field of view between narrow, medium, and wide angles.
F-Stop
The ability of a lens to gather light depends on the relationship between the lens opening (aperture) and the focal length. This relationship is symbolized by the letter f, which is commonly referred to as the "F-stop," and can be found printed on the side or front of the lens. The lower the F-Stop number, the larger the maximum lens aperture and the greater the lens' ability to pass light to the camera imager and the better it can view a low light scene. For example, a lens with an F-stop of f/1.2 can gather a great deal more light than a lens with an F-stop of f/4.0. A lens with a low F-stop number is sometimes also called a "fast lens".
Depth of field
Another consideration when determining the proper lens is depth of field. Depth of field is the area in focus before and behind a subject. This means that when you focus precisely on a subject a certain distance in front of and behind the subject also will be in focus, although not as sharp. Depth of field increase or decreases based on the Iris.
- Lens length short lens (wide angle lens) ... longer depth of field
- Long lens (telephoto) ... shorter depth of field
- Aperture wide aperture (low F-Stop) ... shorter depth of field
- Narrow aperture (high F-stop) ... longer depth of field
- Distance to object short distance ... shorter depth of field
- Long distance ... longer depth of field
Purchasing and planning decisions should take these factors into account since depth of field can affect image quality (and may jeopardize the ability to identify and prosecute subjects). If depth of field is important, you may want to explore options such as increasing artificial lighting or installing cameras with normal lenses rather than telephoto lenses, etc...
Video Transmission Methods
The purpose of the transmission medium is to carry the video signal from the camera to the monitor. Today, many video transmission methods exist: coaxial cable, twisted data pairs, fiber optic, phone lines, microwave, and radio frequency. Due to varying application technologies in use within the same CCTV system. The choice of transmission mediums depends on factors such as distance, environments, cost and facility layout. In addition nearly all methods of transmission suffer from various forms of interference or loss. The essence of good design is to minimize this impact. Examples of current video transmission include:
- Coaxial Cable
A coaxial cable is one that provides a continuous physical connection - or closed circuit - between the camera and the monitor. The cable is shielded to minimize interference from any nearby electronic devices or electrical wires. Copper, at least 95% braided coaxial cable is recommended to maximize conductivity and minimize potential interference. For traditional CCTV systems, as well as many applications today, this is the most common economical method of signal transmission over relatively short distances (few hundred feet).
- Fiber Optics
Fiber optic technology changes an electronic video signal into pulsed or laser light and injects (transmits) it into one end of a glass rod (the fiber optic cable). At the other end, a receiver translates the pulsed light back into an electronic signal capable of being displayed on a monitor. The transmission is unaffected by any kind of interference, water in conduit or high voltage being run in the same conduit. Fiber optic capable have a large signal capacity (bandwidth) and no possibility of a spark from a broken fiber. Hence, there is no fire hazard to a facility even in the most flammable environment. Fiber optics offers a cost-effective method of sending large transmissions over long distances.
- Telephone line
A telephone line is a standard twisted pair of wires that can transmit the image for distances up to one kilometer without signal boosting. The dedicated line connects the transmitter (camera end) with a receiver (monitor end). Through the use of specialized transmission and receiver equipment, it is possible to use standard telephone lines for video signal transmission (RSM).
- Microwave
If already in place, microwave can be a very efficient and cost-effective method of delivering black and white or color video. Microwave turns the video and data signals into high radio frequency signals and transmits them from one point to another via free air and space. A receiver then converts the transmission back into the video and data signals and displays the scene on a monitor. Good quality transmission can be achieved over a line of sight path. Microwave technology offers a large bandwidth to carry video, however it can be affected by environmental conditions. It is a practical option when a wire path between the camera and the monitor locations cannot be established or is prohibitively expensive. Microwave transmissions are regulated by the FCC and a license is required.
- Radio Frequency
Radio frequency (RF) is a reliable, but short distance, line of sight video transmission technology. It is becoming increasingly popular where hardwiring methods are easier impossible or impractical, and has been used successfully to reduce cabling costs even within large buildings. Environment conditions or other RF in the area can affect it.
CCTV
'Lens Tech Info'
from CBC / Computar
