Tune the following four settings to suit your specific needs. These settings interact with, and sometimes counteract each other, so start with something simple and refine the settings as needed.
RESOLUTION: 300 X 300 DPI is the normal recommended resolution setting. Typically, you should not have to go higher than this DPI – even for OCR needs. Increasing the resolution increases the size of the image.
BRIGHTNESS: Controls the balance of light and dark shades in your image. By increasing brightness, you can actually discard objects on the scanned image.
CONTRAST: Refers to the range between the darkest pixel and the lightest pixel in an image. Increasing the contrast can make objects on the scanned page stand out more, thus contrast is desirable for barcode scanning / OCR applications. Too much contrast can increase artifacts and noise found on the page, so increase it carefully.
THRESHOLD: Defines the darkness level an object must be to register on the scanned image. The higher the number, the darker the object must be to register. This is the key parameter of tuning out coloured paper. The threshold can be a little confusing – just remember that a lower number = more dark, so to make the scanner not see coloured paper, lower the number. Changing it from 128 to 90 is a good place to start.
The following web page article provides further excellent advice on how Brightness and Contrast affect the scanned images http://www.scantips.com/bce.html