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Camera data sheets have a number of specifications shown like resolution, sensitivity, signal to noise ratio, camera voltage, chip type, and operating temperature. The resolution and sensitivity are the most important.
Resolution is the quality of definition and clarity of a picture and is defined in lines
more lines = higher resolution = better picture quality.
Resolution depends upon the number 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 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 = no. 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.
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.
As the number of pixels increase in the chip, the pixel size reduces which affects the sensitivity. There is a trade off between resolution and sensitivity.
There are different methods to measure resolution; the Resolution Chart method and the Bandwidth method. At i-Lax the latter is used to establish the resolution of our cameras. 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.
For example. If the bandwidth is 5Mhz, the camera resolution will be 5 * 80 = 400 lines
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Typical Resolutions of Cameras
||380 - 420 Line
Sensitivity / Minimum Scene Illumination
Sensitivity, measured in foot candles or lux indicates the minimum light level required to get an acceptable video picture.
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 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.
* F Stop
* Usable Video
* Shutter speed
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.
* snow = 90%
* grass = 40%
* brick = 25%
* black = 5%
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
Ra = reflectance of the actual scene
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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
Fd = F-stop of lens used in data sheet.
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.
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 and most manufacturers do not indicate their definition in the data sheet while measuring the minimum scene illumination.
It is recommended 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
Ud = usable video % used by the manufacturer
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AGC stands 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.
The 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
Aa = Actual AGC position
If AGC off = 1, then AGC on = db figure from the data sheet
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 sec, that means the light required at the scene 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)
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
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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.
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 step 2
* Find a more sensitive camera
* own grade from color to B/W camera
* Add Infrared light if B/W camera is being used
* Add more lighting at the scene
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.
Modified Minimum Scene Illumination = ( 4.45* 1.36 * 3.3 * 1 * 1 ) * 2.5 = 45 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).