CCTV System Front-end Equipment Selection
In CCTV system, there are numerous cases that due to improper device selection lead to decreasing image quality or losing much capital. Reasonable selecting CCTV device and forming high cost performance system is the permanent goal for CCTV people. Article introduces how to practically choose CCTV front-end equipment, providing useful experience for system setup.
CCTV front-end equipments include camera, lens, pan / tilt, housing, bracket, decoder and spotlight. At present, there are many companies produce such products, but their brand, model and functions are different each other. Thus reasonable selecting these products is important to enhance CCTV image quality. Follows are some practical experiences.
Camera selection
As the core device in CCTV system, according to customer requirements, the selection of camera is very important. At present, CCTV system usually adopts CCD camera which includes features (vidicon camera not having) such as small size, light weight, low inertia, high sensitivity, good uniformity, high anti-hitting and ling expectancy. In practical projects, if object illumination is low but image requires high resolution, we should choose black & white CCD camera. If customer requires color camera, we should add lighting device or select color to black & white CCD camera which can auto switch to black & white mode when object illumination is lower than that of color camera.
After choosing black & white camera or color camera, we should consider camera specifications. The follows are the main specifications:
Resolution (definition)
Show camera ability of analyzing image. It usually shows with TVL. It is determined by CCD pixels, resolving capability of lens and bandwidth of recording system.
Black & white camera commonly adopts 450TVL. Considering construction factor, final definition should meet GB/T16676－1996 or 380TVL.
Color camera definition commonly adopts 350 TVL or more, because human eye is hard to recognize more specific color. Such choice can also meet GB/T16676－1996 or 270TVL.
Sensitivity
In aperture size being fixed condition, capturing min illumination required by signal level. For example, adopting F1.2 lens, when object surface illumination is 0.04Lux, signal-out amplitude is 350mV (50% max amplitude), and camera sensitivity is 0.04Lux / F1.2. If object surface illumination is even lower, monitor screen will be a mass of grey image which is hard to recognize layer. Based on experience, the optimum sensitivity of camera is 1/10 that of object surface.
SNR (Signal to Noise Ratio)
That is the ratio between signal voltage and noise voltage. CCD camera SNR is topically among 45 to 55dB. Commonly, CCTV system adopts 50dB rounded which can not only meet industry standard (no less than 38dB) but also realizes that in lacking illumination condition, the more the ratio, the clearer the image.
Work temperature
-10~+50℃ is the common index for most camera enterprises. Protection or dedicated protection is added according to temperature difference among areas.
Power supply voltage
Abroad DC voltage of camera ranges from 198 to 264V during which anti-voltage change ability is strong; domestic camera DC voltage ranges from 200 to 240V during which anti-voltage change ability is weak, so regulated power supply is needed.
Lens selection
Lens kind is numerous, even classification mode is many also. We just from control mode talk about how to select camera lens.
Manual aperture mono-focal lens
Such lens applies in conditions of fixed objects monitoring and small luminance change. Such lens costs little. Focal length can be calculated with bellow formula:
f=A·L/H
F-----Focal length
A----Aperture height
L-----Length of object to lens
H-----Object height
The larger the focal length, the smaller the view angle, and the more narrow the monitoring area; the smaller the focal length, the view angle the larger, and the wider the monitoring area.
Auto aperture mono-focal lens
When luminous flux changes, electric charge of CCD changes accordingly, making signal level changes and creates a control signal, driving micro motor run forward or backward in order to adjust aperture size. Such lens should be adopted when illumination changes more than 100times. But the caution is: if viewing angle is non-uniform, especially when object and background light contrasts greatly, adopting such lens can not get ideal effect.
Auto aperture motorized zoom lens
According to auto aperture mono-focal lens, this lens adds two micro motors, one motor occluding with lens zoom ring, can change lens focal length; other motor occluding lens focus ring, can focalize for lens. Because of adding two motors, this kind of lens costs much.
Three motorized zoom lens
Comparing with auto aperture zoom lens, three motorized zoom lens adopts remote adjust mode rather than auto adjust. It includes three micro motors which connect to controller by a 6-cores cable. At present, such lens applies widely.
When choosing lens, we should not only choose different controlling-mode lens according to different conditions and different requirements, but also consider below problems:
- Lens size should accords to aperture size, e.g. 1/3″camera should adopt 1/3″ lens. If two sizes are hard to reach accordance, big lens equips small camera; converse fails.
- Lens mount should accord to camera mount or add washer if mounts can not keep accordance, e.g. C-mount lens installs CS camera should add washer. If not adding washer, CCD glass may be destroyed, damages CCD camera.
Selections of pan / tilt, housing, bracket and controller decoder
Pan / tilt
Pan / tilt can be simply considered as tray for installing camera, and this tray can turn with all directions (horizontally or vertically). So pan / tilt application makes camera view more widely. In CCTV system, loop monitoring environments (e.g. hall) should adopt pan / tilt. Pan / tilt application should notice below points:
- Pan / tilt payload capacity must be 1.2 times larger than practical payload. That is to say total weight of all devices on pan / tilt must be lighter than payload capacity of pan / tilt. If operates with full payload or over payload, pan / tilt can work difficultly and high inertia, especially vertical turns more difficultly, though affects monitoring effect greatly.
- Pan / tilt should have good auto-lock function, backhaul angle in stopping should be smaller than 1.
- Indoor pan / tilt with full payload should have noise lower than 50dB.
Housing is applied for protecting camera. There are indoor type and outdoor type. Indoor housing mainly uses for dustproof; outdoor housing mainly uses for dustproof as well as protecting camera in poor conditions (rain, snow, low temperature, high temperature). This not only requires concealed structure but also having wiper and spray kit, even with heating and cooling function. Thus outdoor housing is more expensive than indoor housing.
Bracket
It is used for fixing camera components, including wall mount and ceiling mount. Bracket selection is simple, which has more payload than all devices weight on pan / tilt, otherwise, it is possible to lead to bracket distortion, pan /tilt turning with shock, affect image quality.
Control decoder
In CCTV system with pan / tilt, motorized lens and outdoor housing must equip control decoder. Thus some buttons in control room can control front-end equipments action & function. Control decoder must be same brand with system host; the reason is that different companies have different control decoders and system host protocols & codes. Only if some control decoder point out the device compacting with some brand host, otherwise, we must never adopt different brand decoder with host.
Spot light selection
At present, most CCTV systems adopt spot light (turning follows camera) in order to help lighting. Spot light has features of flexible, convenient and cheap. Black & white camera system should equip high pressure mercury lamp. Color camera system should equip Iodine tungsten lamp. Night, concealed monitoring should adopt IR spot light. IR spot light includes two kinds: one is that adding optical filter on common lamp, the other is IR LED array. The former consumes more power, often producing “IR exposure” (filter not thoroughly, some IR ray is seen by eye); the latter seldom produces IR exposure“, but covered distance is relative short. Besides, notice: IR spot light can not affect cooler camera system at all; the reason is that IR ray is filtered off by color filter of color camera.
