Intensity images measure the amount of light impinging on a photosensitive device. The input to the photosensitive device, typically a camera, is the incoming light, which enters the camera's lens and hits the image plane. In a digital camera, the physical image plane is an array which contains a rectangular grid of photosensors, each sensitive to light intensity. The output of the array is a continuous electric signal, the video signal. The video signal is sent to an electronic device called frame grabber, where it is digitised into a 2D rectangular array of integer values and stored in a memory buffer.

The interpretation of an intensity image depends strongly on the characteristics of the camera called the camera parameters. The parameters can be separated into extrinsic and intrinsic parameters. The extrinsic parameters transform the camera reference frame to the world reference frame. The intrinsic parameters describe the optical, geometric and digital characteristics of the camera. One parameter, for example, can describe the geometric distortion introduced by the optics.