CCTV Security Camera Specifications explained
CCTV Security Camera Specifications like AGC and AES are not straight forward and plain to everybody. Below, we have put some effort into providing our potential clients with a comprehensive CCTV Specification Guide. Ideal CCTV's quotations are among the most clear and transparent in the security market, showing all important features and specifications of every camera on the project. In order to evaluate and compare propositions effectively, one needs understand why for example a camera specified at 560TVL is much more advanced than a random wireless 320TVL import. CAMERA SPECIFICATIONSAny camera data specification sheet has a many specifications shown like resolution, sensitivity, camera voltage, signal to noise ratio, chip type, and operating temperature. Some data sheets are elaborate, while others are relatively basic and cover the bare minimum. To define a security camera, most people will first look at the resolution and sensitivity in the data sheet. These two values are the most important. In this lesson we will look into these specifications in more detail. There is confusion surrounding these terms and I would like to demystify them by explaining them in simple terms. CCTV Specification Quick Link Reference
AES - Automatic Electronic ShutterCameras need to be able to measure the present scene illumination, in order to determine the overall video brightness. In modern day the most advanced and commonly used method is Auto-Iris. Auto-Iris is established through team-work of a professional CCTV CCD Camera in conjunction with a modern lens - both supporting Auto Iris function. The camera measures the light level - decides whether it needs more or less light - and then adjusts the motorised iris in the Auto Iris Lens (open further or close), via a small control cable. The most common cause for not equipping cameras with auto iris lenses is the cost-saving factor. Utilising AES, a standard Manual Iris Lens will be used on an average setting (determined by needed and average scene illumination. The electronic in the camera will modify the shutter speed of the CCD Chip. Increase in shuttering = darker image, decrease of shutter speed = brighter image. Adavantage = Cost Saving. Disadvantage = less crisp image quality and less quality when capturing moving objects. Engineer Tip: Be sure to TURN OFF AES whn an Auto Iris is being used. Further Study: Shutter-Speed
DSS - Digital Slow ShutteringFirst, please read the preceding paragraph above (AES) to understand about shuttering in general. In essence DSS is trickery and at best a trade-off, but not really a technology that enhances any signals. DSS basically electronically spreads the light reflected onto the CCD chip of the camera, onto fewer images per second. This leads to drastic increase of the usuable illumination LUX, often to the point of operation in star light 0.0002 LUX and lower. DSS generally impresses customers more than Infra Red camera video, as it is often producedin colour and seems to give brilliant footage at all times. One has to however consider the trade-off in images reflected per second. Moving objects are therefore displayed a lot less crisp and sometimes completely blurred. If people and moving objects are to be detected by the system, we suggest a demonstration or trial before committing to buy. An easy online video demonstration might do. Further Study: Shutter-Speed and AES - Automotic Electronic Shuttering
CCTV CAMERA RESOLUTION - TVL - Television LinesResolution is the quality of definition and clarity of a picture and is defined in lines meaning more lines = higher resolution , means better picture quality. The Resolution depends on the amount of pixels (picture elements) in the CCD chip. If a camera manufacturer can put in more number of pixels in the same size CCD chip, that camera will have a higher resolution. In other words the resolution is directly proportional to the number of pixels in the CCD (Charge Coupled Device) chip. In many data sheets, two type of resolution, vertical and horizontal are indicated. Vertical Resolution Vertical resolution stands for the number of horizontal lines The Vertical Resolution is limited by the number of horizontal scanning lines. In PAL it is 625 lines and in NTSC it is 525 lines. Using the Kell or aspect ratio factor the maximum vertical resolution is .7 of the number of horizontal scanning lines. Using this the maximum vertical resolution is PAL 625 X .75 = 470 lines Vertical resolution is not critical as most camera manufacturers achieve this figure. Horizontal Resolution Horizontal resolution = no. of vertical lines Theoretically horizontal resolution can be increased infinitely, but the following two factors limit this It may not be technological possible to increase the number of pixles in a chip. There are different methods to measure resolution: 1. Resolution Chart
The camera is focused on a resolution chart and the vertical lines and horizontal lines are measured on the monitor. The resolution measurement is the point were the lines start merging and they can not be separated. Problems
Bandwidth method This is a scientific method to measure the resolution. The bandwidth of the video signal from the camera is measured on a oscilloscope. Multiply this bandwidth by 80 to give the resolution of the camera. Eample. If the bandwidth is 5Mhz, the camera resolution will be 5 * 80 = 400 lines Typical Resolutions of Cameras
As you can clearly see in the table above; only IP Cameras are able to enter into the HD High Definition CCTV Resolution range. Analog CCTV Cameras have reached the end of their thread. The signals transmitted via coaxial cables cannot carry any more information. The 'Great Exodus' from analog to IP will begin between now and the next couple of years, as increase of identification chances, camera intelligence and installation cost reductions are moving into the foreground. Ideal CCTV recommends to seriously consider IP based solutions for complete new installations. This will ensure a future proofed investment and compliance with future industry and government standards. Further Study: Megapixel CCTV Facts! LUX & MINIMUM SCENE ILLUMINATION / SENSITIVITYSensitivity, measured in foot candles or also called lux indicates the minimum light level required to get an usable / acceptable video picture. There is a great deal of confusion in the CCTV industry over this specification. There are two definitions "sensitivity at faceplate" and "minimum scene illumination". Sensitivity at faceplate indicates the minimum illumination required at the CCD chip to get an acceptable video picture. This looks good on paper, but in reality does not give any indication of the light required at the location of use. Minimum scene illumination indicates the minimum light required at the scene to get an acceptable video picture. Though the correct way to show this specification, it depends upon a number of variables. Usually the variables used in the data sheet are never the same as in the field and therefore do not give a correct indication of the actual light required. For example a camera indicating the minimum scene illumination is 0.1 lux. Moon light provides this light level, but when this camera is installed in moon light, the picture quality is either poor or there is no picture. Why does this happen? It is because the field variables are not the same as those used in the data sheet. How does it work? Usually light falls on the subject. A certain percentage is absorbed and the balance is reflected and this moves toward the lens in the camera. Depending upon the iris opening of the camera a certain portion of the light falls on the CCD chip. This light then generates a charge, which is converted into a voltage. The following variables should be shown in the data sheet while indicating the minimum scene illumination.
Reflectance Light from a light source falls on the subject. Depending upon the surface reflectivity, a certain portion of this light is reflected back which moves towards the camera. Below are a few examples of surface reflectivity.
Most camera manufacturers use a 89% or 75% (white surface) reflectance surface to define the minimum scene illumination. If the actual scene you are watching has the same reflectance as in the data sheet, then there is no problem, but in most cases this is not true. If you are watching a black car, only 5% of the light is reflected and therefore at least 15 times more light is required at the scene to give the same amount of reflected light. To compensate for the mismatch, use the modification factor shown below. Modification factor F1 = Rd/Ra Rd = reflectance used in the data sheet Lens Speed The reflected light starts moving towards the camera. The first device it meets is the lens, which has a certain iris opening. While specifying the minimum scene illumination, the data sheet usually specifies a F Stop of F1.4 or F1.2. F Stop gives an indication of the iris opening of the lens. The larger the F Stop value, the smaller the iris opening and vice versa. If the lens being used at the scene does not have the same iris opening, then the light required at the scene requires to be compensated for the mismatch in the iris opening. Modification factor F2=- Fa² / Fd² Fa = F-stop of actual lens
Acceptable Video After passing through the lens the light reaches the CCD chip and generates a charge which is proportional to the light falling on a pixel. This charge is read out and converted into a video signal. Usable video is the minimum video signal specified in the camera data sheet to generate an acceptable picture on the monitor. It is usually measured as a percentage of the full video. For Example : 30% usable video = 30% of 0.7 volts (full video or maximum video amplitude) = 0.2 volts. The question here is: Is this acceptable?. Sadly there is no standard definition for usable video in the industry and most manufacturers do not indicate their definition in the data sheet while measuring the minimum scene illumination. It is advised to be aware of the useable video percentage used by the manufacturer while specifying the minimum scene illumination in the data sheet. The minimum scene illumination should be modified if the useable video used in the data sheet is not acceptable. Modification Factor F3 = Ua/Ud Ua = actual video required at the site as % of full video AGC - Automatic Gain ControlAGC is the short cut for Automatic Gain Control. As the light level reduces the AGC switches on and the video signal gets a boost. Unforunately, the noise present also gets a boost. However when the light levels are high, the AGC switches off automatically, because the boost could overload the pixels causing vertical streaking etc. A data sheet should indicate if the AGC is On or Off while measuring minimum scene illumination. If the data sheet indicates AGC is "on" yet, if in reality the AGC is "off" then the minimum scene illumination in the data sheet should be modified Modification Factor F4 = Ad/Aa Ad = AGC position in the data sheet If AGC off = 1, then AGC on = db figure from the data sheet Shutter SpeedToday most cameras have an electronic shutter speed which allows one to adjust the timing of the charge read of the CCD chip. The standard read out is 50 times (PAL) and 60 times (NTSC) per second. If the shutter speed is increased to say 1000 times per sec, that means the light required at the location should be 20 times more (for PAL). Increasing the shutter speed allows the picture to be crisper, but requires more light. Use the following modification factor Modification Factor F5 = Sa/Sd Sd = Default shutter speed (PAL - 1/50 sec NTSC - 1/60 sec) Adjusted Minimum Scene Illumination The minimum scene illumination of the camera must be adjusted because of the mismatch between the actual conditions in the field and the variables used in the data sheet. Ma = (F1*F2*F3*F4*F5) * Md Ma = adjusted minimum scene illumination
In Comparison Compare the actual light at the scene (L) with the adjusted minimum scene illumination (Ma). If the light available is more than the adjusted minimum scene illumination, then the current camera can be used. If the actual light at the scene is lower than the adjusted minimum scene illumination of the camera, then the camera setting may require adjustment or an alternative solution is necessary. The following steps will help resolve the issue.
Example It maybe worth while to study an example so that all the above concepts can be understood correctly. Let us assume that the security camera is focussed on green grass (20% reflectivity). The actual light level at the scene is 50 lux. The colour camera data sheet indicates the minimum scene illumination is 2.5 lux. The table below compares the variables as indicated in the data sheet and also the actual situation in the field.
Modified Minimum Scene Illumination = ( 4.45* 1.36 * 3.3 * 1 * 1 ) * 2.5 = 45 lux This security camera would work as the light level at the scene (50 lux) is higher than the modified minimum scene illumination of the camera (45 lux).
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