In a photocopier, the light-induced conductivity of the drum is exploited to create a latent image in the form of electrical charges on the surface of the drum. This image is made visible and transferred to paper using a special, charged toner. Here's how it all comes together to make a copy:
- For the photocopier to work its magic, the surface of the photoconductive material must first be coated with a layer of positively charged ions by the corona wire.
- When you hit the Start button, a strong lamp moves across the inside of the copier and casts light onto the paper you're copying, and the drum starts to rotate. As light reflects off of blank areas of the paper, mirrors direct it through onto the drum surface. Like dark clothing on a hot sunny day, the dark areas of the original absorb the light, and the corresponding areas on the drum's surface are not illuminated.
- In the places that light strikes the rotating drum, the energy of the photons kicks electrons away from the photoconductive atoms.
- Opposites attract -- the positively charged ions coating the photoconductive layer attract the freed electrons. The marriage of one ion and one electron produces a neutral particle. Charged particles remain only in places where light didn't hit the drum because it wasn't reflected from the original -- the dark spaces taken up by text and pictures on the page! This part of the process loosely resembles how a camera takes a picture. If you've read How Photographic Film Works, you know that when film is exposed to light, the energy of the photons causes chemical changes in the silver halide grains coating the film. This creates a negative image of what you see through the viewfinder. With a photocopier, however, you end up with a real image created from a pattern of positive charges left after exposure to light. And while you have to develop film using special chemical processes and print it on light-sensitive photographic paper, the photocopier produces a visible image with only dry ink, heat and regular paper.
- Voltage is applied to the aluminum core of the drum. Since light renders selenium conductive, current can flow through the photoconductive layer while the drum is being illuminated, and the electrons released by the atoms are quickly replaced by the electrons that form the current flowing through the drum.
- The exposed areas of the drum rotate past rollers encrusted with beads of toner. Tiny particles of toner are pressed against the drum's surface. The plastic-based toner particles have a negative charge and are attracted to areas of positive charges that remain on the drum's surface.
- The corona wire passes over a sheet of paper so that the paper's surface becomes electrically charged.
- The area of the drum freshly coated with toner spins into contact with a positively charged sheet of paper. The electric field surrounding the paper exerts a stronger pull than the ions coating the drum's surface, and the toner particles stick to the paper as the drum passes by.
- Once the entire original has been recreated on toner in the page, the paper proceeds on through the copier to the fuser. The weak attraction between the toner particles and the surface of the sheet of paper can easily be disrupted. To fix the toner image in place on the paper's surface, the entire sheet is shunted through the fuser's heated rollers. The heat melts the plastic material in the toner and fuses the pigment to the page.
By the time you reach for your copy in the collection tray, the photocopier has already prepared for the next go-round by again cleaning off the drum's surface and applying a fresh coat of positively charged ions to it.
For more information on photocopiers and related topics, check out the links on the next page.