The lasers used in some photosensitive components like laser printers 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.

I don't understand:

  1. Laser modulation change with what settings/parameter (and what math formula is used to make this) to 'print' the right negative image on photosensitive component (called also laser projection) ? what electronic component drive this laser modulation usually ?

  2. What values, frequency is need to have the correct modulation ?

  3. What is a dither pattern and how is made it ?

I imagine that there is a range through which the modulation transfer function can operate, it can not exceed a certain range - this step I am not clear on how it is managed physically.

EDIT: some references connected seems to be Refractive index, Pockels effect & Kerr effect but how you control electric-electronically laser modulation to print correct patterns on photoreceptor belt or drum ?


Most lasers used in printers and such are laser diodes, in which the energy is pumped into the semiconductor by passing a current through the depletion region.

More current = more light output. It is very simple to modulate current at high frequencies, although you have to be a bit careful about the inductance of the circuit (and the nonlinear characteristics of the diode) to do this properly.

This is why dithering with a simple "on-off" mechanisms is a nice way to achieve well controlled gray scales without the need to understand and characterize the nonlinearity.


You are asking clarifying questions about the operation of the laser printer which are a bit outside the usual scope of this physics site, but let me try:

First, the printer's "brain" converts the requested image (which may include characters) into a finely rastered "bit image" - in the same way that your video card does, before displaying images on your screen, but at very high resolution. Now you need to know "where is the laser beam pointing" and "what gray scale should I display there".

The first question is answered with a position encoder on the mirror - this produces a precise location of the mirror (as a digital signal), and from the position of the mirror you can compute where the beam is pointing. This takes some calibration that is normally done at the time of assembly.

Once you know where the laser is pointing, you then look up the corresponding location in memory, and convert that value into a current sent into the laser diode. This is usually done with a DAC (digital to analog converter) with a current output.

All this happens very, very quickly. And it results in little bits of charge being removed from some of the plate, while remaining on other parts. The plate then picks up toner where it is charged, the toner is transferred to the paper, and there it is melted into place.

There is a nice description with some diagrams at [this link}(http://www.explainthatstuff.com/laserprinters.html).

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  • $\begingroup$ To re-word this a bit, the key point is, for any kind of (quasi-)CW laser the output power can't to be more than the pump power. In the case of direct-injection (electrically pumped) diode lasers, it's pretty easy to modulate the pump power to adjust the output power. $\endgroup$ – The Photon Dec 24 '16 at 17:36
  • $\begingroup$ @ThePhoton of course when I turn off the pumping power there is still some energy stored but it depletes quickly... $\endgroup$ – Floris Dec 24 '16 at 18:04
  • $\begingroup$ yes, but that's a transient effect. That's why I specified CW. $\endgroup$ – The Photon Dec 24 '16 at 18:47
  • $\begingroup$ @Floris thanks but mm.. ok, but how the electronic circuit inside printer control laser diode ? Laser project light on moving mirrors that reflects on photosensitive drum but when laser is modulated how you control that modulation is always correct ? $\endgroup$ – user332153 Dec 24 '16 at 19:36
  • $\begingroup$ Are you asking about intensity or synchronization? $\endgroup$ – Floris Dec 24 '16 at 20:53

A dither pattern is just a pattern with some pixels filled and some empty to create from a distance the impression of a half tone.

Variable lasers tend to be difficult, when cascade is met the intensity ramps up due to a feedback loop to its maximum. That feedback loop makes them very intense. So they are on or off type devices.

Getting the toner to hold prior to fixing requires a good strong force, if you start varying it you will probably have toner slipping or not being help prior to fixing, so I suspect, but do not know, that using this technique to vary intensity would be quite difficult.

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  • $\begingroup$ mm.. but what techniques are used ? I find this, this and this but I don't know if this articles are about laser modulation control on photosensitive component (like printer drum) $\endgroup$ – user332153 Dec 24 '16 at 15:13
  • $\begingroup$ I find this about a simply electro-optical modulator but patent is old 1974 so I don't understand if this principles is used integrated like nanochip in modern laser printers $\endgroup$ – user332153 Dec 24 '16 at 15:59
  • $\begingroup$ try this en.wikipedia.org/wiki/Xerography $\endgroup$ – JMLCarter Jan 13 '17 at 20:45

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