Technical notes
Introduction to CCD cameras

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In this page you can find the main terms used when specifying cameras,
 useful to select the more suitable product for your application.
Our technical service will be glad to help you, in case further clarifications will be needed.

Image format size
There are presently four main image format sizes commonly used in CCTV cameras:  1", 2/3", 1/2", 1/3" (these measures refers to the sensor diagonal).


Moreover, 1/4" and 1/5" format sizes have been introduced recently in the market.


Picture elements (pixels) 

These are the individual photo-diodes which forms the sensor matrix. Their number varies according to sensor type and is strictly related to camera resolution. The number of pixels is usually given in two different forms:

a) Total pixels
It is the total number of pixels on the CCD faceplate; however some of them are not usable.

b) Active pixels
It is the actual number of active pixels; this value is always a lower figure than total pixel, but is the only reliable value.

Sensitivity

Sensitivity can be quoted in two different ways.

a) Usable picture
Sometimes this is called minimum illumination. This gives merely an indication of minimum light level at which the camera start providing an output video signal.

b) Full video
This is the real value to be taken into consideration as it gives the lowest light value at which camera supplies a full output video signal "1V peak to peak".   This value is important for the whole CCTV system because many devices need a full signal to operate properly.

Sensitivity can be measured:

a) At the faceplate 

In this way, it measures the camera performance when all light falls directly on the sensor faceplate. Although this is a technically correct figure, it never actually happens, because to get a real picture you need a lens, which reduces in someways the light amount falling on the faceplate.

b) At the lens

This is a more effective value of the actual sensitivity of the camera + lens system. Obviously the lower is the F-No the better result you get (for details about the F-No go to the introduction to CCTV lenses)

For instance:
A camera having a value of 0.1 lux (full video) with a f1.4 lens has a higher sensitivity than a camera showing the same value of 0.1 lux with a f1.2 lens. By using a f1.2 lens with the 1st camera its sensitivity would be possibly around 0.08 lux.

Another value to be taken into consideration is the scene reflectance (which is the light amount, in percentage, reflected by the scene).
The higher is the reflectance value, the higher is the light amount available to the camera faceplate.
Usually the reflectance is something between 65% and 90%.

For instance:
A camera having a sensitivity of 0.1 lux (full video) with a f1.2 lens with a reflectance value of 70% is more sensitive than a camera having the same sensitivity (full video) with the same lens but at 90%.

The sensitivity value is normally specified in lux. However some American manufacurers give this value in footcandles. 1 lux = 10 fc.

Resolution

It is the camera ability to show details. The higher is the resolution, the sharper will be the picture details. However the high resolution cameras are more expensive. The resolution value generally shown is the horizontal resolution measured in TV lines. The horizontal resolution is related to the number of pixels on the faceplate.

S/N ratio

It is a measure showing the amount of interference caused by noise. Its value is important especially at low light values. The higher is its value, the better will be the picture quality at low light levels. It is usually measured with AGC switched on ON position.

Automatic gain control (AGC)

The video signal is amplified at low light levels by an automatic gain control circuit. Unfortunately noise is also amplified. In some cameras the gain level can be controlled manually.  (MGC = manual gain control).

Gamma

It is a corrective value to take in account the fact that picture display reproduced by monitor is not linear. Generally it is set at 0.45; however with some industrial applications it is preferably set to 1. By changing gamma value the contrast of image is also changed.

Spectral response

It is a curve showing the camera performance vs.the wavelenght of the falling light.

Syncronization

Most cameras have an internal sync generator which generates both the sync pulse for CCD senssor scanning  and the syncronization pulses used by monitor to scan the displayed image. However the internal sync generator is not enough, when the system includes more than one camera. It is thus necessary to have an external syncronization.
There are two ways, nowadays, to have an external syncronization:

a) gen-lock
A number of cameras is syncronized by an external sync signal. Often the video ouput signal of one camera is used to syncronize the other cameras, because, being a composite signal, it has the sync pulses.

b) line-lock
In this case, the power supply frequency is used to lock the camera sync and phase lock must be set. This method cannot be used if camera is DC powered.

Knee

Expands horizontal video frequency response in highlight areas of picture to compensate for aperture correction fall-off due to gamma compression of correction signal.

Reference products

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Security cameras

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Industrial cameras

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Last modified 19-02-2013
 
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