Tutorial: Reflective Scanning vs. Transmissive Scanning

If you have shopped for a scanner, you may have come across the terms "reflective" and "transmissive". This article will explain the meaning of these terms in the context of scanner hardware.

Reflective scanning is the process used to scan a subject that does not allow light to pass through it. A piece of paper is a good example of an object that would have to be scanned using a reflective scanner. The reflective scanner is composed of a cover, a subject, a glass screen, a light source, and a lens. The light source produces an intense light that goes through the glass, and is reflected off the subject and back to the lens. A charge-coupled device detects the light. A charged coupled device is a chip that is light sensitive.

A transmissive scanner, on the other hand, scans a subject that allows light to pass though. A good example of such a transparent or translucent object is a negative or slide. Scanning a transparent or translucent object does not work correctly on the reflective scanner because most of the light will pass through the subject to the back of the scanner and be reflected back to the lens and CCD.

Transmissive scanners have nearly the same components as reflective scanners, but they are arranged differently. Notice that in this type of a scanner has no glass surface (though some film holders do use glass to hold the film in place). Instead, the film being scanned is held rigidly between the light source and lens (for example, by holding the film edges). The light source is on the opposite side of the scanner as the lens. The light shines through the subject and onto the lens instead of having to bounce off the subject in order to make an image.

The film scanners that we use to scan your slides and negatives are a special kind of transmissive scanner that are catered to scanning only film. Though some flatbed (i.e., reflective) scanners can function as both reflective and transmissive scanners (by enabling a "transparency adapter"), the results are better using these dedicated transmissive film scanners because the light sources and optics of the scanner are created specifically for film scanning.