The Common Printing Tech You Haven't Heard Of (b done)

 Today’s video is about printers. I know, you’re probably filled with excitement at the mere mention of printers, but please remain calm. If you were to go shopping for a printer, you’ll almost certainly find yourself choosing between an inkjet printer and a laser printer. Or perhaps a 3D printer, but we’re not talking about those today.

Now, I won’t get into the many reasons that I don’t really like inkjet printers for anything but photo printing because that’s not really the point of this video. Instead, we’re going to talk about a subset of the laser printer that seems to fly under the radar. The LED printer. Now, if you’ve never heard of this before I wouldn’t blame you. Fundamentally LED printers are the same as a laser printer. They still use powdered toner rather than liquid ink. They use photosensitive drums to transfer toner to paper (along with corona wires and/or other charge inducing or destroying doodads).

And a fuser melts this toner onto the paper, so a tall stack of freshly printed printouts is nice and warm, just asking you to give it a hug. But where they differ is in how the image is drawn on the drum. And to understand why that’s important, let’s have a quick lesson into how laser printing works and how it came about. Laser printers are essentially an offshoot of the photocopier.

Using a process called xerography, which comes from the Greek for dry writing (and now you know where Xerox got its name), analog photocopiers use a cylindrical drum coated in a photoconductive material. This material becomes conductive when exposed to light. To duplicate the image, the drum is first charged by a corona wire, which produces a high voltage and gives the drum a static charge. The now negatively charged drum is rolled underneath a piece of paper to be copied, where a lens focuses the image of the paper onto the drum, and a bright light source provides illumination.

Because the photoconductive material will conduct electricity when exposed to light, any bright areas become discharged, as a path to ground can now be completed. Dark areas, where printing or handwriting exist on the original,  will remain negatively charged. This drum is then rolled against a supply of powdered toner, which is positively charged.

This toner will thus want to stick to any areas of the drum that remained negatively charged from the original exposure. Now the drum has a coating of powdered toner in the same pattern as the writing or image or whatever in the original document. Next, another corona wire creates a stronger negative charge in the paper that is to receive the toner, and this stronger charge will attract the toner off of the of drum and onto the paper. And finally the paper, now covered in powder, goes through the fuser unit, which melts this powder to the paper, and thus a stable photocopy is made.

Fast forward to the late 1960’s, and Gary Stark weather, an engineer from Xerox’s product division, had the idea of using a laser beam to draw directly onto the imaging drum of a photocopier.

With computer control, you could draw text and images directly on the drum with the laser, thus turning it into a printer. And that’s exactly what happened. Using a laser diode, lenses, and a spinning mirror, laser printers draw onto the drum in lines, and the laser is pulsed on and off to create the image. The resolution of the printer is determined by the number of lines it can draw in a given unit, in addition to the maximum number of times the laser can be pulsed on or off within that line.

Often the resolution is measured in dots per inch, so a printer with 600 dpi will scan the drum 600 times along the length of one inch, and the laser can pulse on and off 600 times within one inch of each line, meaning each square inch has 600 by 600 discrete points that can be either on or off--that is, black or white. Incidentally, this method of creating an image using lines of light is strikingly similar

to how analog television works. I made a series on television if you’d like to check it out, but the pattern is called a raster.

Raster scanning in laser printers requires that the entire image, in its full resolution, be loaded into its memory before printing can commence, as it has to be done in one shot. It can’t start and stop like an inkjet printer, particularly because the fuser unit is liable to burn some paper (or at least singe it a little) if it were to suddenly stop mid-print. So, laser printing works. And it works really well! But the actual laser mechanism is kind a big, and relying on a spinning thing to make the image introduces more moving parts and complexity. And that’s where the LED printer comes in.

When did it come in?

And who invented it?

I’m not sure. It’s a weird footnote into the development of the laser printer, but Oki claims to have made the world’s first LED printer in 1981. It’s surprising how little info there seems to be about LED printers, though as we’ll see, perhaps that’s to be expected. What makes LED printers different?

Well, rather than use a scanning laser, LED printers use LEDs. Shocking. But in a unique way--this Brother HL-3040CN is a color LED printer. If I lift on the lid, you’ll see these four bars here that kind a flip out of the way when the lid’s all the way up. There are four because this is a color printer--one each for cyan, magenta, yellow, and black (more correctly referred to as key). These bars are the key. Each of these things is an array of tiny individual LEDs.

Each LED handles one column of pixels--or dots--on the page. This printer has a resolution of 600 DPI, so along this entire bar there are roughly 5,000 LEDs. If you look closely you’ll see a pattern of dots--these are lenses that focus the light from multiple LEDs behind them onto the correct spot of the drum.

The LEDs themselves are so small as to be nearly invisible to the naked eye. Below these assemblies lie individual drum and toner cartridges. You can see that when the lid is closed, the LED bar sits right on top of the drums. You’ll also find evidence of toner mishaps, but, ehhh.

So when this printer prints, rather than spinning a mirror and pulsing a laser on and off, each

LED simply pulses on its own.

    

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