Carrier is a general term used in xerography to describe the toner transporting species in a two-component dry developer.

In such a mixture, the smaller toner particles are carried on the surface of the larger carrier granules.

Here, a coated, spherical carrier is shown in cross section. In practice, the carrier may be coated or uncoated, spherical or irregular in shape.

Toner is the “ink” that actually makes the mark on the paper. It is a particle size ranging from about 12 microns in older products to about 6 microns in the more recent “microfine” toner powders.

The carrier is also a granular material which ranges from 3 to 50 times the average diameter of toner particle.

Xerographic developer mixtures typically contain from 1 to 10% by weight of toner. The toner is transported by the carrier and brought into close proximity with an invisible, electrostatically charged image on a photosensitive surface  in a copier, laser printer, fax machine, or multifunctional imaging device. At this point, toner particles abandon the carrier surface and deposit either in charged areas on the latent image, or are repelled into neutral image areas, depending upon the relative charge polarity if image and toner.

One of the most important functions of the carrier is to impart a static charge to the toner particles.This is accomplished by frictional surface contact with the toner during mixing, a phenomenon known as triboelectrification. Another example of tribocharging is the unpleasant experience of walking on a carpet on a dry day and being shocked by touching a doorknob or other grounded element.

In the case of xerographic developers the combination of toner and carrier properties must be chosen to produce the correct level and polarity of electrostatic charge on toner. This will ensure that the desired amount is attracted to the oppositely charge Image area. 

It is important that the charge on the toner be neither so high that it cannot be stripped from the carrier nor so low that it is not held tightly to the carrier. Loose toner can float around the imaging device, depositing on optical and other machine components, producing dust in the environment, and settling in nonimage areas of the print as unwanted level of background Density.