“Understanding CCTV Series”. In this series, we cover some of the important CCTV components and explain some of the issues in simple user-friendly language.
We start the series with cameras. Cameras are the starting point of the video signal and is therefore are a critical component of a CCTV system. The word camera comes from the Latin ‘camara obscura’ and means ‘dark chamber’. Artists in the Middle Ages used a dark box to trace images. Since then the camera has come a long way. Today there are three types of cameras most commonly used:
- film cameras
- photographic cameras
- video cameras.
The construction and type of Charge Coupled Device (CCD) chip used in a camera is important. Some of the better quality cameras have superior chip design incorporating many innovative features such as On Chip Lens (OCL), Back Light Compensation (BLC), excess charge drainage technology etc. In this article we won’t look into these aspects, but will to understand some of the important camera specifications.
CAMERA SPECIFICATIONS
Any camera data sheet has a number of specifications shown, such as resolution, sensitivity, signal to noise ratio, camera voltage, chip type, and operating temperature. Some data sheets are detailed while others are quite sketchy and cover the bare minimum. To classify a camera, most people will first look at the resolution and sensitivity in the data sheet. These two specifications are the most important. In this article we will discuss these specifications in more detail. There is confusion surrounding these terms and I would like to demystify them.
RESOLUTION
Resolution is a picture’s quality of definition and clarity is defined in lines:
more lines = higher resolution = better picture quality.
Resolution depends on the number of pixels (picture elements) in the CCD chip. If a camera manufacturer can put a higher number of pixels in the same size CCD chip, that camera will have more resolution. In other words the resolution is directly proportional to the number of pixels in the CCD chip.
In some data sheets, two type of resolution, vertical and horizontal, are indicated.
Vertical Resolution
Vertical resolution = number of horizontal lines
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
for PAL 625 X .75 = 470 lines
for NTSC 525 X .7 = 393 lines
Vertical resolution is not critical as most camera manufacturers achieve this figure.
Horizontal Resolution = no. of vertical lines
Theoretically, horizontal resolution can be increased infinitely. However, it may not be technological possible to increase the number of pixles in a chip. As the number of pixels in the chip increase, the pixel size decreases which affects the sensitivity. There is a trade off between resolution and sensitivity. If only one resolution is shown in the data sheet, it is usually the horizontal resolution.
SENSITIVITY / MINIMUM SCENE ILLUMINATION
Sensitivity, measured in foot candles or lux, indicates the minimum light level required to get an 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 light 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 scene.
Minimum scene illumination indicates the minimum light required at the scene to get an acceptable video picture. Though there is a correct way to show this specification, it depends on 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. Moonlight provides this light level, but when this camera is installed in moonlight, 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
- F Stop
- Usable Video
- AGC
- Shutter speed
Reflectance
Light from a light source falls on the subject. Depending upon the surface reflectivity, a certain portion of this light is reflected back and moves towards the camera. Below are a few examples of surface reflectivity.
- snow = 90%
- grass = 40%
- brick = 25%
- black = 5%
Most camera manufacturers use an 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 so. 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
Ra = reflectance of the actual scene
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 an F Stop of F1.4 or F1.2. The 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 needs to be compensated for the mismatch in the iris opening.
Modification factor F2=- Fa² / Fd²
Fa = F stop of actual lens
Fd = F stop of lens used in data sheet
Usable Video
After passing through the lens, the light reaches the CCD chip and generates a charge that 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.
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?
Unfortunately, there is no standard definition for usable video in the industry. Furthermore, most manufacturers do not indicate their definition in the data sheet when measuring the minimum scene illumination. It is recommended to be aware of the usable video percentage used by the manufacturer when specifying the minimum scene illumination in the data sheet. The minimum scene illumination should be modified if the usable 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
Ud = usable video % used by the manufacturer
AGC
AGC stands for Automatic Gain Control. As the light level decreases the AGC switches on and the video signal gets a boost. Unfortunately, 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 and cause vertical streaking etc. The data sheet should indicate whether the AGC is On or Off while measuring minimum scene illumination. If the data sheet indicates AGC is “on” already, if in reality the AGC is “off”, then the minimum scene illumination on the data sheet should be modified.
Modification Factor F4 = Ad/Aa
Ad = AGC position in the data sheet
Aa = Actual AGC position
If AGC off = 1, then AGC on = db figure from the data sheet.
Shutter Speed
These days 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 second, that means the light required at the scene should be 20 times higher (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)
Sa = Actual shutter speed being used
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
Md = minimum scene illumination as per the camera data sheet
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.
Step 1
Check if camera variables can be changed
- If AGC is switched off, then switch AGC on
- Accept a lower usable video %
- Reduce shutter speed, if possible
- Use a lens with a lower F stop
- If no success go to step 2
Step 2
- Find a more sensitive camera
- Own grade from colour to B/W camera
- Add Infrared light if B/W camera is being used
- Add more lighting at the scene
Example
It maybe worth while to study an example so that all the above concepts can be understood correctly. Let us assume that the 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.
| Parameter | Data Sheet | Actual | Factor |
| Reflectivity | 89% | 20% | 4.45 |
| F Stop | 1.2 | 1.4 | 1.36 |
| Usable Video | 30% | 100% | 3.3 |
| AGC | On | On | 1 |
| Shutter Speed | 1 / 50 Sec | 1 / 50 Sec | 1 |
| Minimum Scene Illumination | 2.5 Lux | ? | |
| Actual Light level | 50 Lux |
Modified Minimum Scene Illumination = ( 4.45* 1.36 * 3.3 * 1 * 1 ) * 2.5 = 50 lux
This 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).
About the Author
Jayant Kapatker is an international authority on CCTV and is the brain behind STAM InSight, The Award Winning CCTV Program.
