Grey level: A calibrated sequence of grey tones, ranging from black to white. In remote sensing, electromagnetic radiation falling on a photosensitive receiver generates an electrical current proportional to the intensity of the radiation. The receiver is usually tuned to specific wavelength bands and the signal from each receiver is amplified and its intensity classified into different levels, usually 0 (black) to 256. These are the digital numbers of each of the pixel units that together make up a remotely sensed frame. In multi-spectral scanning systems, the digital numbers are classified into grey-scale bands, of which there are usually 10 to 12, but up to 256 shades of grey (brightness) may be used.