Pixel can be defined as the minimal visible part of a digital image. A common parameter, when you are designing video surveillance systems, is the pixel density at a certain distance from the camera.
Imagine that your image consists of tiny squares (pixels). Larger the area of an image occupied by an object, the greater are the number of pixels in it and the fine details of the object can be distinguished.
Pixel density depends on camera resolution (usually vertically, because this value is smaller), and distance of the camera from the object.
(Image quality reduces with increasing distance from camera)
Pixel density is measured in number of pixels per meter or number pixels per foot. Using the pixel density value, the designer of video surveillance system can estimate the distance of an object from the camera, where it is possible to detect the presence of a person in the frame, recognize a person known to the observer or get an image for identification of the person.
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Image quality vs pixel density: 250, 125, 60 and 25 pixels per meter.
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