2004 October 25

48-Bit Color

Topics

Scanning Software

The 4870 comes with two scanning programs: SilverFast SE (a light version) and Epson’s proprietary software. Athough SilverFast SE has a few more professional controls, I quickly ruled it out because (unlike the full version) it can’t scan in 48-bit color.

About 48-Bit Color

You may know that computers and TVs break pictures up into little dots called pixels (short for picture elements). You may even know that each pixel actually comprises 3 dots: 1 red, 1 green, and 1 blue. It would be nice if a single dot could glow any color we asked it, but the technology right now restricts each dot to a single color. Luckily, these 3 colors (RGB)—from roughly the top, middle, and bottom of the visible light spectrum—can produce a wide range of colors when combined. The dots in each pixel are so close together and so small that we perceive each pixel as a single unit.

To produce pure red, for example, you display the R dot at full strength and turn off the G and B dots. By turning R and G to full strength with no B, you get yellow. And all 3 dots at full strength makes white light. R + B = magenta, and G + B = cyan. So how do we get, say, brown? Well, although we can’t change a dot’s color we can change its intensity. A typical brown might be something like 50% R, 25% G, and no B.

From the above, you might be able to guess that the number of colors a computer can display is determined by the levels of intensity each pixel can display. Based solely on hardware (e.g., video memory), this could be arbitrarily high… but in practice, human eyes can’t distinguish more than about 15 million colors, so that’s where monitors draw the line. It takes 24 bits of information to handle a number as big as 15 million. (A single bit can have one of two values; 2 bits can hold 4 values; 8 bits hold 256 values; 24 bits hold 16,777,216 values.) So most of today’s monitors and imaging software use 24-bit color information.

In contrast, 48-bit color means 281,474,976,710,656 colors! Monitors can’t display that many, and even if they could we couldn’t tell the difference. So why bother? Because just about every scan you do will involve color correction. Your brain does color correction all the time without your knowing it…making interior walls look white even in yellowish light, for example. But the camera doesn’t. You’ll probably need to correct the color on every scan, and doing so involves throwing out improper color information. What happens if you start with 16 million colors but throw lots of them away? You don’t have 24-bit color anymore. But if you start with 280 trillion colors, you can throw out as many as you like and still end up with better than 24-bit color. After the corrections, you can go ahead and save the picture in 24-bit color because it’ll produce a smaller file size.