DN
Digital Number. In remotely sensed imagery like satellite images, each pixel in an image or colour band is assigned a numerical value between 0 and 255 (for 8 bit imagery) describing the relative quantity of electromagnetic energy captured by the sensor. Black represents a DN of 0, meaning all the energy was absorbed by the surface material, white represents a DN of 255, meaning all the energy was reflected by the surface material. Graytone images represent of the variation in spectral reflectance values of the surface materials across a region.