Table of Contents
48-Bit Color
Definition
24-bit color is 8 bits per red, green, and blue channel. Since there are 3 color channels, we multiply 8 bits per channel by 3 color channels to get 24-bit color. “24-bit color,” “8 bits per channel”, and “8/24-bit color” all mean the same thing.
Similarly, 48 bit color is 16 bits per channel or “16/48-bit color”.
Why Choose 48-bit Color?
The human eye can only see about 7-10 million colors which is less than 24 bits, so 24 bits is enough to produce a digital image that looks identical to the original. But a faded photo has information compressed across the histogram, and decompressing that information when restoring colors can create banding artifacts unless there are more bits of information to start with. 36 bits of information should be enough to prevent banding, but since we can only choose 24 or 48 from today's scanners, let's make it 48 bits.
Here's an example of how information can be compressed across the histogram. In the image below, the vertical blue lines represent the 8 bits per channel that your monitor can display. A and B are identical histograms in 8 and 16 bits per channel. Notice how B aligns perfectly with the blue lines, and A has more information than the monitor can display.
C and D had their gamma corrected in Photoshop or similar software, but D has fewer lines on the left side of the histogram than blue lines:
This means that D will have fewer colors than your monitor is capable of.
The result of correcting gamma looks something like below. The top row is an underexposed photo scanned at 4, 8, and 14 bits per channel (42-bit color). The bottom row is the result when the same 4-, 8-, and 14-bits per channel photo is brightened in Photoshop:
The middle photo on the bottom row shows what can happen when scanning in 8/24 bits and correcting the photo in Photoshop. Do you see the “banding” effect in the sky compared to the 14/42-bit scan?
Scanners don't support 14/42-bit scans so we choose 16/48-bits which is better.
Always scan in 48-bit color if your scanner supports it.
Please keep the following in mind when scanning at 16/48-bits:
A 48-bit image will look dark and have low contrast. This is normal.
What About Black & White Photos?
Even black and white photos should be scanned in color. Scanning B&W photos in color will help you detect and remove stains and writing. Afterwards they can be converted to black and white image files to save storage space.