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By laser projection I mean the laser beam that is projected onto the photosensitive drum in the laser printer. The printer receives digital image data from a computer processes it and projects it onto the photosensitive drum using laser beams.

What I want to understand is:

  1. How does the conversion from digital image data to a laser beam projection actually happen? What is the process? What are the steps involved? What are the components (eg. electronic circuits) involved?
  2. Does the laser beam project the entire image at once onto the drum like a slide projector projects a entire slide onto the screen or does the laser beam like a pencil "draw" the image onto the drum like how a child would draw a picture on a paper using a pencil?
  3. If the laser beam draws like a pencil, how is the laser guiding mirror that directs the laser beam onto the drum able to move so fast that a page is printed every few seconds?
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  • $\begingroup$ Entering "how does a laser printer work?" into an Internet search engine results in thousands of articles on the subject. There is no need for anyone to write one here for you. What aspect of the published material requires further explanation? $\endgroup$
    – Transistor
    Feb 14 '20 at 16:40
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    $\begingroup$ @Transistor I've read a lot of articles on internet explaining how laser printer works. These articles explain the process starting from the step involving the laser being projected on the drum and don't explain how the actual conversion from digital-image data to a laser beam takes place. My question is not about how laser printers work, Its about how digital image from the computer is converted into laser beam projection. That something none of the articles I read on the internet has explained. Don't know if I had accidentally missed reading articles that might have an explanation I want. $\endgroup$
    – Somanna
    Feb 14 '20 at 17:13
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    $\begingroup$ @Transistor Also none of the three sub questions I have asked in the question have been answered in the articles I read. So I have asked it here in case some people here knows the answer. $\endgroup$
    – Somanna
    Feb 14 '20 at 17:16
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I'll answer the questions in reverse order.

  1. If the laser beam draws like a pencil, how is the laser guiding mirror that directs the laser beam onto the drum able to move so fast that a page is printed every few seconds?

enter image description here

Figure 1. A simplified drawing of the laser and mirror arrangement. Source: How Stuff Works.

The mirror in many cases just rotates continuously causing the beam to sweep from one side to the other in very rapid succession and always in the same direction. You can see something similar in many barcode scanners. This very simple arrangement requires no acceleration or deceleration of the mirror.

  1. Does the laser beam project the entire image at once onto the drum like a slide projector projects a entire slide onto the screen or does the laser beam like a pencil "draw" the image onto the drum like how a child would draw a picture on a paper using a pencil?

One 600 dpi (0.06 mm) dot at a time building up to one line at a time.

  1. How does the conversion from digital image data to a laser beam projection actually happen? What is the process? What are the steps involved? What are the components (eg. electronic circuits) involved?

This will vary but the general idea will be:

  • If the printer has a byte per pixel then 28 = 256 grey-scale levels are possible.
  • Synchronise the memory reads with the mirror at the start of each line.
  • Read a byte of memory and adjust the laser power to suit.
  • Read the next byte and at the right time adjust the laser power to suit.
  • Repeat until the page is complete.

The speeds involved are quite impressive. If we take a modest 600 dpi printer that can print six pages per hour (10 s per page) we get about 11 × 8 × 6002 = 31 million pixels for a letter / A4 page. For a 10 s print that works out at 31 ns per pixel or a data transfer rate of 3.2 MB/s.

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