(1) A shade of grey representing a given radiometric level, or intensity, on the image. (2) A number or value assigned to a position (x, y) on the image proportional to the integrated output, reflectance, or Transmittance of a small area, usually called a Resolution cell or pixel, centered on the x, y position. Grey level can be measured or expressed as: transmittance, reflectance, brightness, radiance, luminance, density, voltage, current, or brightness value.