Identification and Recognition Tutorial
Required resolution
The traditional way of defining requirements for resolution of an analog CCTV system has been by specifying what percentage of the full screen the observed object occupies. Different surveillance objectives require different percentages.
For example, detecting the presence of a person in a scene could require that the person occupies 10% of the view. Recognizing a known person, however, could require that the person occupies 50%, and further identifying that person could require 120% or more.
Examples from Stockholm Subway using AXIS 225FD Network Camera
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| 20% | 40% | 140% | ||
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Too small for recognition |
Face turned away from camera |
Face partly turned away from camera |
With today’s wide range of available resolutions, it is practical to translate the percentage requirements to pixel resolution in order to compare and specify cameras as well as for viewing and recording solutions. In Europe, the percentage requirements are based on a 400 TVL (Television Lines) analog image. Table 1 shows the equivalent of required resolutions in pixels/m and the number of pixels a 16 cm wide face will cover for the different surveillance objectives.
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You should note that these numbers are minimum resolutions, and that if conditions are less than ideal, you will need to compensate with better resolution. Today, stronger identification is often requested. For example, recommendations from SKL, the Swedish National Laboratory of Forensic Science, suggest that resolution for identification purposes should start at 500 pixels/m. This means that a 16 cm wide face would be represented by 80 pixels or more.
Other criteria are valid for objects such as license plates, where typical recommendations are that the height of letters should be represented by 15 pixels (corresponding to about 200 pixels/m) to ensure legibility.
It is also important to take legal and regulatory requirements into account when determining the resolution needed in order to be able to use camera footage as evidence in court.
Finding a camera to match resolution requirements
The resolution of a captured scene is determined by the camera resolution and the size of the scene. For example, if you are using a camera that delivers 4CIF (704 x 576 pixels) resolution, you can cover a scene that is, at most, 1.4 m wide, if the linear resolution is 500 pixels/m or more. You will need to select a camera and lens that will allow the field of view to match the scene size at the desired distance between the camera and the scene.
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Axis lens calculators and the AXIS Object Recognition Tool are useful tools that help finding a suitable camera and focal length. For advanced users, a pixel and distance calculator spreadsheet is also available.
Higher camera resolution means fewer cameras and better overview
Since the maximum size of a scene covered at a given resolution only depends on the camera resolution, cameras with higher resolution can cover larger areas. For example, if your 7 m wide scene requires five cameras delivering 4CIF resolution, these can be replaced by two cameras at 1080p HDTV resolution (1920 x 1080 pixels). Also, a camera with higher resolution can be used to give a better overview, by covering a larger scene while maintaining the required linear resolution.
Cameras with higher resolution can cover larger areas
Depth of field
The larger the depth of field is, the larger the area where persons or objects are in focus. With a large depth of field, your chances of identification increase. Depth of field is determined by the iris opening, the focal length and the distance to the camera.
The depth of field increases with smaller iris openings. This means that good lighting conditions can help increase depth of field. The P-Iris feature of some Axis cameras will adjust the iris to optimize depth of field for different lighting conditions. You can learn more about the P-iris from the P-Iris white paper:
