Dropout color means color that does not appear on the scanned image.
The mechanism of this feature (described later on this page) is similar to that of copy machines that drop the color green when in monochrome mode.
This happens because copy machines generally use a green lamp for monochrome mode. With the dropout color feature, you can make images that exclude particular parts of a document.
For example, if scanning a stack of questionnaires printed in red, you can extract only the answers by specifying red as the dropout color.
[Dropout color mechanism]
When a red lamp is put on the document surface, only red reflects the light. When the lamp is green, only green reflects.
However, as the white area on the document surface reflects all light, both the red and white areas on the document reflects light without distinction when illuminated by the red lamp. The scanner then regards the red area as the white area, dropping out the red color in the final file.
Changing lamp colors is one way to achieve the dropout feature.
Another is to attach a filter to the CCD used for scanning, or to use a color-switching CCD