Some active slide-adapters are equipped with their own light source shining through a transparent original. Some models feature a moving carriage with a light source, which is moved with a motor and a belt mechanism. The light source is moving along with the scanner head. The scanner’s own lamp is turned off in this case.
Today, SOHO scanners without moving parts in the slide-adapter unit are the most popular. We have recently tested one device like that, EPSON Perfection 3200 Photo. Its light source is built into the scanner lid and occupies its entire useful surface. For the adapter and the scanner to work together, a cable comes from the lid and gets attached to a special connector in the scanner back panel (marked as “XPA”). The light of the adapter is automatically activated when you change the type of the original in the control program; an indicator in the scanner lid informs you about that. Transparent originals are installed into stencils included with the device: 35mm film of 12 stills, four 35mm slides in frames, 120/220 (6x9cm)/4x5” film. Then you lay the stencil onto the glass.
During the scan process, light stream goes through the transparent original, hits the input of the scanner’s optical system and is handled just like a non-transparent original. Of course, such features of the scanner like optical resolution or color depth do not change when you use a slide-adapter, but the range of optical density does. This last parameter directly depends on the brightness of the light source and the time of exposure. It can be described as follows:the darker is the original, the less light can go through it, and the more time it takes the CCD matrix capacitors to collect a necessary charge. The darkest of all transparent materials is X-ray film (up to 3.6D). In order to get a quality scan of such a film, you need a bright light source. However, the range of reproduced optical densities doesn’t depend on the lamp brightness alone. It is also determined by the precision (bit capacity) of the analog-to-digital converter, by the abilities of the optical system and of the light-sensitive matrix.
A passive adapter is a much simpler device. It uses the scanner’s own lamp and you get poorer results than with an active adapter (intensity of the light stream is lower just like the quality). However, you can use passive slide-adapters for scanning images for Web. They are cheaper.
The scanner is a sophisticated electronic device and I could discuss it for ages, getting deeper into specifics and nuances. So far, we have learned the following:
- CCD scanners produce better digitized images than CIS scanners;
- The bit capacity of a converter is very important;
- Optical and mechanical resolutions are quite distinct things;
- There are different light sources and they affect the quality of scanning in different ways;
- There are no “universal slide-adapters”.
- Overall, I tried to be clear and precise trying to explain the characteristics of modern SOHO scanners. Your feedback would be most welcome.