X-bit labs

Flatbed Scanner Guide: Glance from Inside (page 6)

Category: Other

by Sergey Samarin

[ 01/11/2004 | 11:56 PM ]

About Light Sources

Every scanner has an internal lighter. It is a small and powerful unit responsible for turning the scanner lamp (or anything that serves as a lamp) on and off. A CIS scanner uses a LED line as a source of light, and this is why it consumes so little power.

CCD scanners generally illumine the original with a fluorescence lamp with a cold cathode. This lamp is incomparably brighter than light-emitting diodes, but high voltage is required to make the gas inside the lamp shine. A unit called inverter provides this voltage.

A high-voltage unit is required for supplying power to the lamp

The inverter pulls the voltage up from 5 volts to several thousand volts, and also transforms direct current into alternating current.

Generally, there are three types of lamps used in scanners:

• Xenon Gas Discharge lamp;
• Hot Cathode Fluorescent lamp;
• Cold Cathode Fluorescent lamp.

However, for several reasons, SOHO scanners only use lamps of the last type (with a cold cathode).

Cold Cathode Fluorescent lamp

The scanner lamp rests on a plastic chassis of the scan head right above the lampshade. The lampshade is shaped like a reflector (a “collector” and “thrower-off” of light) or magnifying glass. The reflected light is more intense and shines at the object placed on the bed. Reflecting from the original, light goes through the chassis slit (its contour is outlined with blue in the snapshot) and falls onto the first, longest mirror of the optical system.

An evident advantage of a cold-cathode lamp is its long service time, about 5,000-10,000 hours. That also explains why some scanners don’t turn the lamp off after a scanning operation is over. Then, lamps of this type do not require any additional cooling and have low manufacturing cost. The disadvantage of these lamps is slow turning-on. It takes from 30 seconds to several minutes for a lamp like that to warm up.

The lamp matters a lot for the quality of scanning. Even a slight deviation in the characteristics of the light source may affect the light stream that is falling onto the reception matrix. This fact explains why it is necessary to warm up the lamp properly before scanning. Some drivers allow reducing the warm-up time if you don’t want to have excellent scanning quality (when scanning text documents, for example). The lamp’s characteristics degenerate with the time, which is a natural inevitable process, but scanners can automatically calibrate themselves according to a black-and-white pattern that is integrated into the case.

As you can see in the snapshot, the plastic of the case
and the calibration target get dimmer after being exposed to light

The scanner I took as an example complies with the rule. The photo above shows the color pattern according to which the scanner adjusts colors before scanning to compensate for “ageing” of the lamp. The plastic inside the case, which is permanently illumined by the lamp, as well as the calibration pattern become dimmer with time. This brings in more color distortion.

A cold-cathode lamp resembles a daylight lamp… only a small one

If you are inventive enough, you can design a desk lamp
from the inverter and a cold-cathode lamp

That’s another possible use for the scanner lamp. The inverter unit was connected to a standard computer PSU with wires and an adapter. Add a holder and you will have a bright desk lamp.