Early models of photocopy machines had the annoying attribute of copies becoming progressively more faint the more copies you made. This technical issue seems to have been solved. How did they do it?
For one thing, no copy machine any more makes direct copies. They all scan an image into a computer, and then print that image from the computer. While it’s in the computer, they can do whatever processing they want on the image, for good or ill.
How a photocopier works:
The image is blurred and degrades with successive copies because of the limitations of the resolution the photoreceptors and the degradation of charge distribution on the drum. Improvements in the technology allowed the photoreceptors to be smaller and the charge distribution on the drum to be more precise and more fully reset between scan cycles but copies always had artifacts and degradation with rapid copying.
Modern “copiers” are actually digital scanners which make a high resolution image of the original document which is stored in the controller, and can produce an endless supply of virtually identical copies up to the precision of the resolution of the original scan, which can also be saved and transmitted digitally as an image file.
Stranger
I believe the ink used is basically carbon black or something like it that cannot fade. I have photocopies that are over 50 years old and show no fading. There were other duplicating processes that did produce copies that faded, but they were based on masters. The one I recall is the one we called stinky because of the solvent used. You prepared the master by writing on a sheet that had an ink backing sheet. Where you pressed, it picked up ink. You then inserted it in a machine and turned a crank (usually electrically operated). Every turn used the solvent to pick up some ink and deposit on a sheet of paper. The ink was purple and faded over the years. I guess you could also type the masters.
Did you mean “copy of a copy of a copy”? Or “making 200 copies at once of the same item”? Even back in the early Xerox days I never saw the latter phenomenon.
First is improved resolution. Early copiers and laser printers used to be “good enough”, usually somewhere between 200dpi and 300dpi. Line up the copy so the edge of the dark area is half the sensor pixel, and 50-50 if it detects dark or light. So each iteration of copying, resolution can suffer. Plus, the process of attracting toner onto the drum would result in “splash” where some of the toner stuck to the drum near the black area in what was supposed to be white. With the lack of resolution, fine lines and details on the original would blur or disappear.
(I recall some guy taking an arts class when I was in university where he made successive copies of the same picture, and created a sequence where the image became progressively less detailed and less recognizable.)
Like everything else computer-wise, the resolution and detail of scanning has improved. As Stranger points out, modern copiers are actually digital scanners. Plus the original laser printers (using a laser to project the scanned/composed image on a copier drum) like the original HP LaserJet were 300dpi, now many are 600 to 1200dpi. They also use this high reolution for dithering to allow for grey-scale photo reproduction. Since these are the same print engines, the same tech applies to copiers. This means it would take a lot more re-copying to lose detail the way it used to.
Ah, that sounds like a mimeograph machine and ‘dittos’ you’re describing. Many a time as grade schoolers, we used to pick up the dittos and smell the still damp vaguely astringent banana-scented solvent on the pages our teachers had freshly run.
There were also fax machines thirty years ago or so that printed on special thermal paper, so those documents would fade if exposed to light or heated or possibly over time.
Modern digital scanners/copiers use sensors with 1-D or 2-D arrays of pixels. But were there in fact discrete “photoreceptors” on the photosensitive drums of old analog copiers?
I remember the early copiers had special toner packs. There was a toner sheet, blank paper, something else? You put the original in the middle and ran the entire pack through the machine. It was slow. Took about 30 secs. The copies were on thin, tissue like paper.
I worked in the school office. I got in trouble for ruining a original for a teacher. I put it in the toner pack backwards. The toner covered the original and blacked it. I wasn’t allowed to use that copy machine again that semester.
Most of our work was on the mimeograph. All the teacher tests were printed on it.
You are thinking of a spirit duplicator, of which Ditto was a well known brand name. That’s how all of our tests and handouts were made in grade school in the 1970s. I remember the handouts in class smelling like methanol.
The mimeograph process was entirely different: it was a stencil process, using a stencil created by a typewriter punching holes in a special sheet of paper.
I only once saw a real proper mimeographed document: it was an announcement sent to residents in Princeton, NJ during WWII regarding ration coupons. I found it in a box of random stuff in the attic of our church. Mimeographed documents look very different from Ditto documents.
I think the copier our school used was 3m 051 copier. Or a similar model. It was only used for limited copies.
For early 1970’s it was surprisingly compact.
Thermal paper is still pretty widely used for labels (like UPS barcodes) and, especially, cash register receipts. And they still fade over time (or blacken with heat) but are used since the ink is built into the media. They’re also pretty fast and quiet.
I know all about thermal label printers; we have dozens where I work. I was specifically talking about the fax machines that used a roll of special thermal paper.
Yes, chips with thousands of sensors or transistors or whatever are very recent. As late as 2000, a digital camera was good if it had 2 megapixels. I have a Radio-electronics reprint for their new home computer, that uses the 8008 processor, plus an add on board with an additional 256 bytes (!) of RAM from 1975.
The original Xerox going back to the 60’s (50’s?) used a lens system and a very bright light to project the image of what was on the platen onto a photosensitive drum that was statically charged; no image sensors involved. Where the light hit the drum, the coating conducted, where it didn’t (black on the original) the charge stayed. Then the parts that were statically charged attracted the toner dust, which was then transferred to and melted onto paper.
The same process applies today, except instead of a lens system, the image is scanned and a laser or LED array paints the picture onto the drum. But considering how much memory is required, it’s something from the late 1990’s. (The originals few models of HP Laserjet, for example, you would need a memory upgrade for enough RAM to hold a full page as image; the Laserjet would receive text and vector graphics otherwise and compose the image to the drum on the fly.
An 8.5x11" paper full image at 300dpi would require over 8Mp; that’s 8 Megabytes uncompressed for 256 shades of B&W, back when a computer with 1 megabyte was a big computer.
Supermarket receipt printers even do two-sided receipts now. So the paper must be coated on both sides, and the printer itself must be two printers in one.
Fun fact: You might think that you can protect such a printed paper from fading by putting clear scotch tape over it. Wrong! If you put scotch tape over some printed part of a modern cash register receipt, the print under the tape will fade completely in just a few days or sooner.
Back in the early 1960s, I had a job with 3M in Wales. They made Sellotape there and I worked in the office.
They had a photocopier that actually made photographic copies involving liquids on photo-receptive paper. It was a big machine and required training to use it. I remember that the copies were often quite poor - black on white came out okay, but any other colour came out grey. or no at all. The copies came out too hot to handle as the machine had to dry them.
The factory was strictly no smoking (loads of volatile solvents around) apart from the canteen. Matches and lighters were banned, so there was a naked gas flame for people to light up.
I’ve only seen preprinted things (local ads, return terms and conditions, etc) on the back of receipts. I’m not sure how the heat from the print head that activates the ink could be isolated to a single side of a thin scrap of paper.
I can remember the scent just reading this. Although I never would have associated that smell with bananas.
God, I’m old. This was how it was in the office world when I started working.
In the mid-1980s I worked at a medium-smallish Silicon Valley company that made laser printers, in the days when laser printers were still quite a novelty. We didn’t actually build the print engines, which we bought from companies like Canon mostly. What we made were the image processing machines – the combined special-purpose computer and software to convert the data from the print file to pixels for the printer, and drive the printer. Our company was founded by some graduates from Stanford who had developed the algorithm for rasterizing the vector-to-pixels images on the fly, in real-time as the paper moved through the printer.
This enabled our printers to print at the rated speed of the engine – the full 8 pages a minute that the desk-top Canon engines could do, and we had a full-size floor model that could do 20 pages a minute, full duplex (two-sided), multiple copies, various collating options, up to 11x14 pages. Nobody else could come close to that in those days.
When Adobe came out with their Postscript-based printers, they could only print 4 pages a minute on the same Canon engines because they just couldn’t process the data any faster than that.
We even had a prototype printer that printed about 60 pages a minute. It used a technology called “ion deposition”, which I’ve never heard of before or since. Unfortunately, the physical machine (which I’ve also never seen before or since) was some kind of prototype that was way too clumsy and jammed incessantly, so that model never got out the door.
Then some biggie company (Kodak, IIRC) came out with a copier that could physically move 90 pages a minute, that controlled the movement of every moving part with little tubes with air pressure going through them. We built a version of our processor for that, and we had the printer really printing 90 pages a minute, with occasional hiccups that we were still trying to work out. And this was for arbitrarily complex vector drawings, not just plain text stuff.
But, Postscript took the whole world by storm, and our printers did not use Postscript. The upper brass at Kodak decided (apparently correctly) that the whole world really wanted Postscript printers that could print 4 pages a minute rather than our printers that could print 8 or 20 or 90 pages a minute.
So that’s what the world wanted, and that’s what the world got. Printers that could print 4 pages a minute (considered quite a feat with the 386-era processors in those days), and not our printers that could print 8, or 20, or 90 pages a minute.
Postscript conquered the world, and our company slowly fizzled out over the next few years.
Two-sided receipt printers are a real thing. I’ve got some right in front of me now. I’m not sure if they still use the same technology (heat) to print, although I don’t know of any other tech. I think it just takes a minuscule amount of heat or whatever. But mainly, I think the paper for these printers is a little thicker.
PM me with your snail-mail address (if you dare) and I’ll send you some samples.