General
A talk about Scanners
By
Apr 29, 2003, 00:26

Resolving The Scanner Ratings Confusion

Scanner manufacturers like to use numbers such as 600-by-1200dpi but sometimes these numbers can be misleading.

The optical resolution is determined by the scanner's sensor usually a CCD array and the number of individual elements in the sensor. For example, a 600-by-1200 dots per inch scanner has 600 elements per inch. But the sensor reads only one scan line at a time. The second number is the mechanical resolution, which indicates the number of steps the stepper motor takes when moving the sensor down the flatbed. So a 600-by-1200 dpi scanner offers a 600 dpi optical resolution and a 1,200 dpi mechanical resolution.

The problem with using a number such as 600 by 1,200 for a scanner's resolution is that computers need the image handed to them with the same resolution vertically and horizontally. In turn, this means that for a 600 by 1,200 dpi scanner, 600 by 600 is the maximum resolution without interpolation, which is another way of saying that the maximum raw image resolution for a 600 by 1,200 dpi scanner is 600 by 600.

Although the second number is misleading, it isn't meaningless. To understand why, you have to understand that the claimed resolution isn't always the resolution you get. By definition, resolution means the ability to resolve detail. In reality, however whether a 600-dpi sensing element can resolve 600 lines per inch depends on the quality of the optics. And whether a 600-dpi mechanical resolution can resolve 600-lines per inch depends on the precision of the stepper motor.

As you might expect, a 1,200-dpi stepper motor will be more precise at 600-dpi than a 600-dpi stepper motor. And the stepper motor in a 600 by 2,400-dpi scanner will be even more precise. To determine a scanner's maximum optical resolution, use the lower of the dpi settings. In the absence of empirical results, use the higher number to determine the likelihood that the scanner can deliver the optical resolution it promises.

On a related subject, you’ll get the best output if the pixels per inch (ppi), which means that sometimes you’ll need a higher resolution than the maximum optical resolution. But this doesn’t mean that the best place to do the interpolation is during the scan process.

Figure 1

With continuous tone images, you'll get the best results from a technique that creates new, interpolated pixels by averaging the color values of all the pixels around each new one. A scanner can't do that during the scan process because it has to insert the new pixels for each line before it gets to the next line. As a result, using a scanner's ability to interpolate to a higher resolution can degrade your pictures. Once you've finished a scan and imported it into a program like Photoshop, you can improve the image by interpolation.

Line art is a different story.  You don't want to create the interpolated pixels with an averaged gray level. You want each pixel to be either black or white (or for color line art, full color or white). This approach does not need to take the next scan line into account. So using the scanner's interpolation will improve your image, but using the more sophisticated interpolation in image editing programs will degrade your image. The rule of thumb is to interpolate when scanning line art and to interpolate in an image-editing program for continuous tone images.