Tone is defined as the relative brightness in a B&W. Hue refers to the colour as defined in IHS colour space. Tonal variations are an important interpretation element. The tonal expression of objects in an image is directly related to the amount of light (or other forms of EM radiation) reflected (or emitted) from the surface. Different types of rock, soil or vegetation are most likely have different tones. Variations in moisture conditions are also reflected as tonal differences in an image: increasing moisture content gives darker grey tones. Variations in hue are primarily related to the spectral characteristics of the imaged terrain and also to the bands selected for visualization. The advantage of hue over tone is that the human eye has a much larger sensitivity for variations in colour (approximately 10,000 colours) than tone (approximately 200 grey levels).