How does a drum scanner work?

The drum scanner operated by affixing a film or print image to the outside of a cylinder of clear glass. The drum was powered up causing it to spin at high speed. A light source located inside the cylinder outputs a focused spot of light that passes through the glass cylinder and traverses the film.

The color and intensity of this beam of light is thus modified by film. This emerging light then plays on a photo multiplier tube. A photo multiplier tube is old-school, a cousin of the old-fashion radio / TV glass enclosed tubes. Light hitting this tube converted light energy into an electric charge. The photo multiplier was outstanding due to its ability to react to minuscule changes in light intensity by outputting an amplified antilog signal.

The location of the spot of light playing on the film is mechanically moved. The photomultiplier is also moved in step with the focused light source. These movements and the spin of drum generate an analog representation of the film’s image. To record a color image, three scans are made by passing the light through red, green and blue separation filter. If the image is a print on paper, the light reflects back into the cylinder after traversing the print emulsion. It is then directed to a photomultiplier inside the cylinder.

The analog signal could be used by devices that exposed photo film or photo paper. Some accepted the analog signal, otherwise the signal was converted to a digital signal. These exposing devices were constructed using red, green, and blue laser light sources that were caused to trace a path across film or paper. Various analog and digital storage methods were used. These included giant 10 inch floppy disks and various hard disks not far removed from the ones used today.