Why can one scanner "see" better than another?

Q: What do scanner vendors mean by 30-bit or 36-bit quality and by  specs like Density = Dmax 3.6? Is Dmax 3.6 much better than Dmax 3.2?

A: The whole 30-bit versus 36-bit and Dmax stuff relates to how  much detail a scanner can capture in the highlights and shadows of an  image you're scanning. It has nothing to do with resolution, which is  the number of dots per inch a model may scan. Dmax is a rather complicated way of measuring opacity; see Density Revealed for the technical details.

What we're basically talking about is dynamic range--how much the scanner "sees" in  the light and dark parts of a picture. Stand in a room and start dimming the lights. At a certain point, you can't distinguish colors. Eventually, you can't see anything. There's still some light in the room, but your eyes aren't sensitive enough to see by it. Your cat,  however, could probably see just fine. Our eyes simply have different degrees of sensitivity, in other words, a different dynamic range.

Scanners also differ in light sensitivity. Cheap scanners can cover the range of light and dark (the dynamic range) necessary for typical photographs. But transparencies and negatives have a wider range, dipping much farther into the dark and light  extremes. Beyond a certain point, scanners with limited dynamic range see very dark details as black (100 percent dark) or very bright areas as totally white. The detail is there, but the scanner's light sensors can't pick it up.

The Bits

All this 36-bit stuff ties in to the sensitivity issue. Your cat not only has more sensitive  eyes, but he also has more nerves connecting his eyes to his brain and he probably has  a higher proportion of brain dedicated to sight. Similarly, more sensitive scanners convert their light detection readings to a larger range of digital data values--not just 24 bits of data, but often 30 or 36 bits.

Here's how it works. All scanners convert each pixel of an image into an electrical voltage, using a chip called a CCD array; the more light energy at a pixel, the more voltage. An analog-to-digital (A/D) chip converts the voltages into bits of digital data.  A higher voltage results in a higher digital value.

Years ago, almost all desktop flatbed scanners converted CCD voltages to eight bits of  light-to-dark (grayscale) information for each red, green, and blue color, for 256 light  levels each, which works out to a combined 24 bits of RGB color (a range of about 16
million colors or shades). But when vendors started using more sensitive CCD arrays,  they upped the A/D conversion to 10 bits per color, for a total of 30 bits (about 1 billion colors). Many high-end flatbed models have 12-bit A/D converters for a total of 36 bits
(68 billion colors) and up to 48 or 54 bits.

But most of us work with 24-bit color files. What happens to the extra six or 12 bits  captured by the scanner? With all that extra color information, the scanner has more options in selecting the best range of 24-bit colors to represent the image. It can boost  detail in highlights and shadows, trim off any visual noise, and still have lots of good  values left to fill in the middle range.

The Catch

Unfortunately, a scanner's sensor– converter–image–data processing system is only as  good as its weakest part. To save bucks, a vendor might pair a 30-bit A/D circuit with  a low-sensitivity CCD, so it can market the unit as a 30-bit scanner. But those 30 bits
may not look as good as another model's 24 bits. You can't tell from the specs.

Other vendors have been known to play fast and loose with their specs. Visioneer and  Umax got into a scrape over Umax's claim that its Astra 1220 and 2420 scanners have 36-bit "quality," thanks to Umax's sophisticated Bit Enhancement Technology.  Visioneer rightly claimed that the Umax models sport only 10-bit A/D converters and hence are only 30-bit scanners. The judge ruled that Umax could use the term "36-bit  quality" because the scanners may well achieve quality equal to 36-bit scanners, but Umax's literature must clearly indicate that these units feature 10-bit (30-bit total) A/D conversion. This is yet another reason to compare scans from different units when shopping, since the specs may not tell the whole story.