Why do some scanners have red light sources and others infrared?
One of the major decisions to be made when choosing a scanner is the wavelength of its light source. In general scanners are available in two wavelengths; the visible red region around 650 nanometers and the infrared region around 950 nanometers. The wavelength, in nanometers, is listed strictly for reference. Don't worry if you don't have the slightest idea what nanometers are!
Scanners with red light sources are most common. They have the big advantage that the light source can be seen, making it easy to tell where the scanner is "looking". This makes alignment a lot simpler. You can also get some visual feedback that the scanner is actually working. Visible scanners can also read codes where the bars may be printed in a color other than black. Generally bars in the blue-green color area, printed on a white background, will look "black" to a red light scanner. Also, black bars can be printed in non-carbon based inks.
Infrared scanners are generally used in specialized applications. One example is in the photo processing industry where visible light would expose photographic products. Another example is where infrared transparent films are placed over barcodes so they cannot be photo copied. This material is transparent at infrared wavelengths but looks completely opaque to the human eye. It is commonly found on barcoded identification cards. Barcodes printed with inks other than carbon based black will not be visible to infrared scanners. Since the IR scanner does not operate in the visible spectrum it is virtually color blind.
In general, a red light scanner is your best choice for the majority of applications unless there is some compelling reason to use infrared.