Bit Depth is a commonly advertised specification of scanners. It is the scanner's ability to accurately analyze and reproduce the full color spectrum of the scanned information.
Let's start off with a black and white scanner:
A 2 bit scanner would only produce four shades of gray (=4 shades)
A 4 Bit scanner would produce 16 shades of gray. (=16 shades)
An 8 Bit scanner would produce 256 shades of gray. (=256 shades)
When applying the same principal to color scanning, you must remember that there are three channels of color (RGB = Red, Green, Blue). Therefore, a 24 Bit scanner is actually three separate color channels of 8 Bits each (256 shades of red, 256 shades of green, 256 shades of blue). The total color range available with a 24 Bit scanner is 16.8 million colors. (=16,777,216 colors)
A 30 Bit scanner is made up of three 10 Bit color channels (1,024 shades of red, 1,024 shades of green, 1,024 shades of blue). The total color range available with a 30 Bit scanner is 1 billion colors. (=1,073,741,824 colors)
A 36 Bit scanner is made up of three 12 Bit color channels (4,096 shades of red, 4,096 shades of green, 4,096 shades of blue). The total color range available with a 36 Bit scanner is 68.7 billion colors. (=68,719,476,736 colors)
The higher the Bit Depth of the scanner, the more accurate and precise it becomes when analyzing available tonal information and reproducing color. Since a scanner with a higher Optical Density obtains more tonal information to analyze and reproduce, it is more precise. Data that was never obtained can not be analyzed and reproduced.