CCTV Basics

CCTV Basics

The first thing you should ask yourself before purchasing a security surveillance system is: What do I need this system to do? Do you need detection of an incident only, or do you need to identify the object in question. Keep your answer in mind when reading this document.

The second thing you should ask yourself before purchasing a system is: What is my application? The most common applications are: security applications, safety applications, and management applications.
 

Scene and Lighting

The scene refers to the objects or area to be observed as well as the environment in which it will be observed. One important aspect that must be considered is the environment. The environment contains multiple colors, materials, reflective surfaces, and varying degrees of light within the picture. To select the proper equipment you must determine the amount of light present during peak times of operation. The amount of light on the scene determines everything from picture clarity to focus. What is the minimum light that will be available? Will it be more cost efficient to go with a better night-view camera or adding more artificial lighting to the scene?

Area of illumination in the camera field of view . A minimum illumination of 70 percent of the camera field of view is recommended. A camera is an averaging device. If too little of the field of view is illuminated, the camera will average between the illuminated areas and the non-illuminated areas, resulting in blooming and loss of picture detail in the illuminated area.

Lighting position . The position of lighting in relation to the camera field of view is also important. As much as possible, light sources must be kept out of the camera's field of view. Lights that are illuminating a camera scene should be mounted higher than the cameras. When determining a location and field of view for a camera, extraneous light sources, such as building-mounted lighting for pedestrians that will be in the camera view, must be considered. Extraneous light sources can cause blooming and streaking in a camera, rendering portions of the field of view unusable. Distant light sources that are relatively dim are usually not a problem.

Other lighting . Another type of lighting is known as infrared (IR) or near infrared. The spectrum for this lighting is just below red and is not visible to the human eye. Most black-and-white cameras have sensitivity into the infrared. A black-and-white camera can be used with this type of lighting to observe areas at night without having lighting that is visible to humans. To make use of IR lighting, the camera must not have an IR cut filter. Cameras can be ordered without IR cut filters; be sure to specify no IR cut filter when ordering.

The Cameras
A camera's performance largely depends on the amount of light present, as well as the imager used. When the level of light changes dramatically, usually a camera equipped with automatic iris control can help ensure consistent image quality. Auto iris enables a camera to open or close its lens accordingly to the varying levels of light, limiting or increasing the amount of light passing onto the sensor. Cameras are available in several imaging formats expressed as 1/2, 1/3, 1/4 inches. These are the sizes of the imager used. Generally speaking, you should match the camera's format with the lens format (ex. 1/3" sensor with 1/3" lens). It is crucial to understand a camera's format, resolution, and corresponding lens focal length when determining what camera will best suit your needs. Without proper equipment and studying all these variables, you may be deceived on what really is causing poor imaging. For example, what might look like a poor camera, may be a poor monitor. You can't simply take a high resolution camera and expect its high performance to be visible on a poor monitor or display. Each component in the system will affect the overall performance. Your overall quality is only as good as the weakest component in your system.

The Lens
The lens plays a large role in a system's design. The primary function of the lens is to collect light from a scene, focus the image to produce a sharp image on the camera's imager. Selection of a lens is critical. The lens directly affects the size, shape, and sharpness of the image to be displayed on the imager. Factors such as distance to the scene, focal length, desired field of view, lighting, and format affect the size and clarity of the image.

The field of view (FOV) is the actual picture size (height/width) produced by a specific lens. If the view is not suitable, consider a different lens to increase or decrease the field of view.

Camera lenses are divided into two major categories: fixed and varifocal (manual zoom). A fixed lens obviously has a fixed focal length, while a varifocal lens enables the user to change its focal length to produce a zooming effect (narrowing the FOV). Focal length is the distance from the optical center of the lens to the focal point near the back of the lens. This focal length distance is displayed on the lens (in millimeters). A lens with a focal length of 8mm on a 1/3" camera produces a field of view similar to the view produced by the human eye. A wide-angle lens has a short focal length, while a telephoto lens has a long focal length. In order to change the field of view, you must change the lens.

The ability of a lens to gather light depends on the relationship between the lens opening (aperture) and focal length. This relationship is symbolized as the letter F, also know as F-stop. The lower the F-stop number, the larger the aperture, thus the greater ability to pass light through the lens to the camera's imaging device. For example, a lens with an F-stop of F1.2 can gather much more light than a lens with an F-stop of F4.0. A lens with a low F-stop number is sometimes referred as a "fast" lens.

Depth of field is another consideration when determining the proper lens. Depth of field is the area in focus ahead of and behind the main object. When you focus on a particular object there is an amount of area behind the object and in front of the object that will still be in focus, although not as sharp. Depth of field increases or decreases based on the length of the lens, the len's aperture, and distance from the camera to the subject.

Video Transmission Methods
There are many transmission methods that exist today. The purpose of the transmission medium is to carry the video signal from the camera to the monitor or other device. The most common mediums include: coaxial cable, RCA cable, fiber optic, CAT5 cabling, phone lines, microwave, and radio frequency. The choice of determining which medium to use depends on many factors including distance, environment, cost, and facility layout.

Coaxial cable is the most popular in the CCTV industry. The cable, preferably copper, is shielded to minimize interference from any nearby electronic devices or electrical wires. Usually no longer than 500 feet. This type of cable is used for direct connections with no special conversions.

RCA cable is probably the least expensive and can provide audio, video and power connections in one convenient cable jacket. RCA cable should not be run more than 200 feet due to the risk of signal and power attenuation.

Fiber optic transmission technologies convert an electronic analog signal into a digital signal using a series of light pulses or lasers. The medium that carries these light signals come either in plastic or glass rods. Fiber optic transmission is unaffected from almost any type of interference. Fiber optics have a large signal capacity (bandwidth) and have no possibly for spark. Fiber optic cabling offers a cost-effective method for sending large amounts of data over long distances (miles). Special conversions and devices are needed to facilitate this type of media transmission.

Telephone line is a standard twisted pair of wires that can transmit signal up to 1 kilometer. It is possible to use standard telephone lines for video transmissions with the use of specialized transmission and receiver equipment.

Radio Frequency (RF) is a reliable, but short distance, line-of-sight video transmission technology.  It is becoming increasingly popular where hardwiring methods are either impossible or impractical, and has been used successfully to reduce cabling costs even within large buildings.  Environmental conditions or other RF in the area can affect it.

 

Peripherals
There are many devices on the market today to bring multiple video signals and channel them through one device, either enabling multiple channel viewing on a single display, sequencing multiple channels on a single display, recording capabilities, and many other features. The most common include quad processors or splitters, digital video recorders, and time lapse VCRs.

Quad processors allow up to four cameras to be displayed on a single screen. Each camera will occupy a quarter of the screen and a single camera can be selected to display full screen. If a VCR or recording device is attached to the quad, then on playback you will see all four screens. Some quad processors have additional features, such as motion detection that triggers a recording device to record only when motion has occurred within the images.

Digital Video Recorders (DVR's) are quickly replacing quad processors and time lapse VCR's since they have built-in quad processors and have better playback resolution than time lapse VCR's. Since the video images are stored digitally, the image quality will not degrade overtime, as would a VHS tape when recorded over multiple times. The time-saving search capabilities of a DVR will enable the user to locate the desired video clips via user defined parameters (camera, event, time/date, etc.) versus the fast forward and rewind functions of a VCR. Other advantages of a DVR include multiple camera inputs, motion recording, view all cameras at one time or individually and they can be accessed remotely from anywhere in the world using the Internet.