An excerpt from a CCTV Design website to help guide you on the proper installation, buying, and angles of CCTV Cameras.
Generally observe/monitor behavior within a broad area
At this resolution, an operator can observe activity in a scene but not be able to determine what a particular person is doing. Typical applications are crowd control for congestion or disturbances in public areas. This type of coverage is of minimal use outside of live monitored systems where an operator can respond to changes in the scene and would often have a PTZ to zoom in to the scene for a more detailed image.
In the image above, we can see a the back center of the scene a black man at 35 meters from the camera using a 4CIf camera with a 67 degree lens.
The minimum person size is 29 pixels tall, which at 4CIF is 5% of the scene. This would be a pixel density of 18 pixels / meter.
Detect the presence of a person or object in the scene
A person can be detected in the scene but not recognized or identified. Typical applications include tracking a persons movement around a scene as an overview, with some detail to distinguish what they are doing. An example would be an isle in a supermarket.
When combined with other cameras for, it helps to convict shop lifters. This scene would like like this where the black person is able to be detected and tracked from 18 meters away from the camera using a 4CIF camera with a 67 degree lens.
The minimum person size is 58 pixels tall, which at 4CIF is 10% of the scene. This would be a pixel density of 35 pixels / meter.
The ability to recognize a person known to you
A person in the scene should be able to be recognized if they are already known to you. For example, a staff member, regular customer, or registered police suspect.
It is common that a surveillance camera will only provide recognition for a portion of the visible scene where the person is close to the camera and the further regions will only provide detect coverage.
The pixel density required for recognition is greater than 176 pixels / meter (vert) which is around 44 pixels for the persons head. On a PAL 4CIF signal the person would occupy at least 50% of the scene height.
The ability to identify a person you do not know
This is the requirement of most CCTV surveillance systems but also the one most often not achieved. In any surveillance system intended to cover a general public area, at least one camera should provide a identification image, ideally this is located at a check point where the public must pass such as an entry door, corridor or counter.
At a counter, it is possible to achieve a good quality identification image and also provide areas of recognition, detect, and secure.
The black man is at the maximum this camera could provide ID based on the above settings, using a 4CIF camera with a 67 degree lens. The distance is only 1.1 meters due to the camera angle exceeding 30 degrees.
If the camera is lowered by 30cm to 2 meters, the camera obtains a significantly better result as per the second image above.
The pixel density required to recognize is greater than 352 pixels / meter which is around 88 pixels for the persons head. On a PAL 4CIF signal, the person would occupy at least 100% of the scene height.
The ability to read the characters of a vehicle number plate
Also referred to NPR or LPR (Number or License Plate Recognition), this is where the human eye can read each character. This is different to ANPR where the recognition is automated via character recognition software.
Each character needs to occupy 5% of the scene height based on a PAL 4CIF signal. Because each country has different number plate sizes, this is going to vary depending on where you live but based on an average height of 7cm, the total scene height must be less than 1.4 metres
To read the number plate, we need to tighten the lens angle. This image shows a car at 8 metres with a 17 degree angle of view.
This is a pixel density of 410 pixels / metre or for an Australian number plate, or 29 pixels tall for each character. This is also suitable for person identification but often the camera will be pointed too low to cover faces.
Other factors that should ne considered for NPR applications are the amount of reflected light from the number plate at different times of day and nigh as well as shutter speed to accommodate the speed of the vehicle. Typically a camera with a wide dynamic range is required such as the Panasonic Super Dynamic range.