I want to understand how a laser printers laser writes on a photosensitive drum. I am trying to understand this process.

the photosensitive cylinder is illuminated by a laser beam that projects above the negative of the image.

  1. After you illuminate with laser beam what happens exactly on the photosensitive drum? What is this projection?
  2. negative image? Where does this come from?
  3. When the drum receives this laser light, the photosensitive drum material generates a current, voltage? How and what is handled this photosensitive drum variation due to the absorption of light?

I understand the process drum-toner but I don't understand this step well:

when you want to perform printing, it is loaded with positive electric energy from the corona wire, an electric current thread that runs through it in all its length.

  • $\begingroup$ That second phrase in the description looks very poorly written, are you sure it's the best you can find? $\endgroup$ Commented Dec 22, 2016 at 16:56

1 Answer 1


You are apparently asking about the Xerographic process, such as used in original copy machines and now in laser printers.

The process works by charging up the surface of a cylinder (the drum) to a high voltage. The drum then rotates so that any parts of it that are charged pick up black particles, called the toner. Then a piece of paper is pressed against the drum as it rotates further. That causes the black stuff to stick to the paper. The paper is then heated and squished to make the black stuff stick to the paper permanently.

Note that you end up with black on the paper wherever there was electric charge on the drum. As I described things above, the drum was charged everywhere, so the paper ends up all black.

If only we could selectively discharge parts of the drum, then we could end up with white areas on the paper. Fortunately there is a way. Instead of making the drum out of purely insulating material, we make it out of material that insulates unless it gets hit with light. Early drums were made from selenium, for example. If we connect the inside of the drum to low voltage (discharge potential), then all we have to do is shine light on the areas where we ultimately want the paper to be white. The light discharges the high voltage on the surface of the drum, which causes that area to not pick up toner, which won't make the paper black.

So now we have a way of exposing a image onto the drum surface, and ending up with that image on a piece of paper. This is exactly what early copiers did. A strip of bright light was shone on the original to be copied. The image of that was optically focused onto the drum as it rotated and the bright light strip moved along.

Laser printers use the same principle, except that a modulated laser "writes" onto the drum to discharge it, instead of the optical projection of the paper to be copied. Since the modulation of the laser doesn't have to come from scanning a original, you now have a "printer" that can discharge any pattern on the drum, which means it can produce any pattern of black/white on the resulting piece of paper.

Modern copiers are really digital scanners with a laser printer back end. There is no longer a direct optical connection between the original and the drum. This allows for image processing, enhancements, storing the result, or even generating the pattern in a computer without ever scanning any original (printer versus copier).

  • $\begingroup$ @Olin Thanks for your answer. Could you be more precise about the laser modulation? What modulation value/measure is needed to print with laser and how you control this modulation ? How do I know that the modulation is correct? If modulation is wrong what happens ? $\endgroup$
    – user332153
    Commented Dec 23, 2016 at 15:35
  • $\begingroup$ @user: The light output of the laser can be varied, or modulated. As the laser point is swept across the drum, its intensity is changed. The laser essentially paints the image onto the drum. The laser light is scanned in one dimension, and the drum rotates in the other dimension. Together they allow the laser to create a 2D image on the drum, or more accurately, a 2D pattern of charges. $\endgroup$ Commented Dec 23, 2016 at 15:43
  • $\begingroup$ mm.. but this modulation change with what settings, what electronic component drive this laser ? The laser paint on drum like a negative image - this is clear for me, but this drum painting (or laser projection) how do you get precisely the image that we expect ? There must be a precise value of the modulation from computer to printer, otherwise, in theory, the output image may be any on our sheet of paper. Also, I imagine that there is a range through which the modulation can operate, it can not exceed a certain range - this step I am not clear on how it is managed. $\endgroup$
    – user332153
    Commented Dec 23, 2016 at 17:02
  • $\begingroup$ @user: The lasers used in these devices can have their output intensity controlled electrically. Due to the drum either picking up toner or not, half tones are often created with a dither pattern than with in-between light intensity. $\endgroup$ Commented Dec 23, 2016 at 17:36

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