A drum scanner consists of a rapidly spinning glass cylinder to which an image is taped. A full-spectrum light source is beamed through the image on the drum and is read by a photo-multiplier tube (PMT). A PMT is a vacuum tube enclosing a phosphor sensor. Reflective images are mounted on an opaque cylinder, generally black glass, and the sensing transport has the light source mounted at 45 degree angles to the sensors. In the PMT the phosphor screen is scanned and cleared thousands of times every second as each pixel is read off the image. As the entire phosphor screen is dedicated to reading a single pixel at a time, PMTs have a high signal-to-noise ratio (S/N), the key determinant of D-Max. Although simple in concept, to attain the high image quality required by commercial graphics applications, drum scanners employ extremely tight mechanical tolerances along with precise control of their motion.
It's easy to be blown away by drum scanned pictures, but before you click over to buy.com, consider a few operational realities of owning a drum scanner:
To minimize Newton's rings the air between the film and the glass drum surface is eliminated by mounting it on a thin film of mineral oil or naphthalene. Of course, after the scan is completed, the coating must be removed.
Charge-Coupled Device Scanners
The great majority of consumer scanners employ charge-coupled device (CCD) technology. Mechanically simpler and thus amenable to mass-production, CCD scanners are much cheaper than drum scanners. The sensing component is a chip with an array of several thousand CCDs arranged in a line. CCD scanners use fluorescent, cold-cathode, or light emitting diode (LED) light sources. A typical flatbed scanner with a 4000-array CCD scanning an 8"-wide image has a resolution of about 500 dpi (or more accurately samples per inch, spi).
The small size of each CCD limits the strength of the signal it can capture. Unfortunately they are small enough so as to be affected by stray electrical signals, including noise among adjacent pixels, that may interfere with and obscure the relatively weak signal generated by the incident light source. A stronger signal may be generated by increasing the size of each CCD, but this comes at the cost of resolution because fewer CCDs can be placed on the chip. At present, it appears that CCD technology may be near the limits of its potential as a scanner technology.
It is received wisdom that flatbed CCD scanners produce inherently inferior results to drum scanners. This is not true. For example, their quality is such that the publishing company that produced the drum test images included here has both types. Manufacturers of high-end CCD scanners address the S/N problem by employing stratagems such as highly refined tri-linear CCD arrays and elaborate circuit cooling mechanisms. Ironically, the result is a scanner about as expensive as a drum scanner.