This defines the number of possible intensity/colour values that a pixel may have and relates to the quantization of the image information. For instance a binary image has just two colours (black or white), a grey-scale image commonly has 256 different grey levels ranging from black to white whilst for a colour image it depends on the colour range in use. The bit resolution is commonly quoted as the number of binary bits required for storage at a given quantization level, e.g. binary is 2 bit, grey-scale is 8 bit and colour (most commonly) is 24 bit. The range of values a pixel may take is often referred to as the dynamic range of an image.
Consider the following example for Binary, Grayscale and a RGB Image.
|Binary Images only just two colours (black or white)|
|Grayscale Images (0-255 different grey levels)|
|RGB Color Image (8 bits for Red, 8 bits for Green,8 bits for Blue)|
It is important to recognize that the bit resolution of an image does not necessarily correspond to the resolution of the originating imaging system. A common feature of many cameras is automatic gain, in which the minimum and maximum responses over the image field are sensed and this range is automatically divided into a convenient number of bits (i.e. digitized into N levels). In such a case, the bit resolution of the image is typically less than that which is, in principle, achievable by the device.