Laser Printer: Resolution

Laser printers produce their output by raster scanning. The word "raster" apparently derives from the word for the close spaced lines on a musical stave and has been in use from the 1920s onwards to describe the scanning pattern used by TVs and more recently by many printers.

An image broken up into a raster looks better the more lines there are. The raster scanning used by televisions usually makes under 100 lines per inch on the screen - although it does depend on the size of the screen, a 40 inch diagonal TV has about 55 lines per inch

Analogue TV actually does just break the image down into lines. Digital equipment breaks it further into pixels. Pixels are usually square or circular so there are as many pixels per inch along a line as there are lines per inch.

With printers the customary measure is dots per inch (dpi); dots per millimetre would at one time have given inconveniently small figures. A basic fax is at about 100 dots per inch which is why the image looks a bit crude, printed pages are held at arms length where TV screens are seen across a room. Dot matrix printers could produce about 250 dots per inch. Early laser printers had resolutions of 300 dots per inch and progressive improvements in technology have given machine rated for 600 and now 1200 dpi. A few machines claim hardware resolution of 2400dpi.

The human eye does not very clearly discern dots less than 0.1mm which is 250dpi. (Actually the eye can resolve a bit less than 1 arcminute, so defects in print become more evident the closer the page is held). The need for print resolutions going beyond 300dpi is partly to handle very fine lines but mostly a way to deliver greyscale. Many digital print processes either print a dot or they do not, there is non of the shading and gradation expected in photographs. To get an area of grey a printer can use many small dots, some black and some white. As it happens some recent laser printers can handle some levels of grey so there may not be a great need for really high resolution print.

The print resolution of a laser printer is limited by several things:

• abilities of the motors and particularly the laser scanner.
• size of the toner
• abilities of the electrostatic fields
• the communication, processing and memory abilities of the printer

Where the limitations come from emerges from an outline of the process.

Printing is actually done by several mechanisms clustered around a drum (big roller) of metal coated in organic photoconductor. As it's name suggests the photoconductor is an electrical conductor in the light but an insulator in the dark. Giving a photoconductive surface a charge in the dark then painting a raster across it gives an electrical "latent image". Some areas still hold charge, others do not.

An electrical field will attract material with an opposite charge and repel it if it has the same charge. Materials can be given a charge by contact with another charged surface or by the triboelectric effect where materials are rubbed and take on a charge. Laser printers use both techniques. Toner powder is charged by stirring and then further charged on a developer roller. The drum, developer roller and transfer roller get their charges from a High Voltage Power Supply (HVPS).

Electrostatic Print Process

The page creation process goes as follows:

The printer assembles an image of the page about to be printed in memory. The page has to be ready in memory because once the printer starts the various rollers and drums rotating it cannot stop, it must complete the page, so the data has to be ready. Depending on the content and how they are held page images can take a significant amount of memory and until recently this limited print resolution.

The page is positioned by pickup and feed rollers at a registration station - often a pair of pagewidth hard rollers that nip the paper tightly.

With the data ready the printer can start the laser scanner and drum turning and as they come to speed it releases the clutch on the registration station so that the pattern being developed on the drum meets the top of the page at just the right point with a couple of millimetres spare for margin.

The print mechanism is arranged with a photoconductive drum at its heart. The drum turns and the motor driving it also turns other rollers in the printer so that all the parts move in synchrony with one another. Any jittering in the motion would be noticable and the higher the resolution the smoother the action has to be.

The drum is given a charge of several hundred volts by a conductive pre-charge roller connected to the HVPS. As the drum turns the laser scanner paints it with a raster creating the latent image. There is more on the scanner below, it is a key limitation on resolution.

Further round the drum passes the developer roller. On the developer's surface is a thin coat of toner. In a write-white system the toner has been charged by stirring and by the developer itself to have much the same charge as the photoconductor drum so it is repelled by the charge - except where the laser has discharged the drum where it is attracted. In the places the laser has written toner particles move from the developer to the drum, the image is now visible. Toner particle size is important to the resolution of the process. It is difficult to make very fine grained powders with an even, sperical grain and one of the factors allowing high resolution laser printers has been technical progress on this front.

The drum now passes over the transfer roller. By now paper should be progressing across the transfer roller and it is being charged to a voltage that will attract most of the toner off the drum and onto the page. the motion of drum and page have to stay perfectly in step or the image will be distorted.

The page now passes through a pair of rollers called the fuser. One roller is heated, the other is spring loaded against it. The fuser softens the toner and crushes it into the page so it adheres. The fuser doesn't really have anyy impact on page resolution except that mechanical and electrostatic effects of the rollers shouldn't detract from the look of the page.

The page is now finished. The photoconductor passes through a cleaning process to ready it for another cycle. Actually most photoconductors have a circumference of 3 to 4 inches so they turn 3 or 4 times in printing one page. Smaller rollers have shorter lives, hence small printers tend to cost more to run.

Resolution is limited in the following ways:

• The laser must scan 3,300 times across the page to produce one piece of A4 at 300 dots per inch and four times as many at 1200dpi - more than 13,000 scans.
• The Laser signal must change up to 2,400 times in each line at 300 dpi and nearly 10,000 times at 1200dpi. In a 60 page per minute 1200dpi printer that means writing something like 126,720,000 dots in a second. That isn't high by comparrison with communication lasers running at 10GHz but they aren't delivering a quanta of light sufficient to change the adhesion of a drop of toner.
• Laser focus needs to be somewhat better than 1/1000th of an inch to produce a page at 1200 dpi and the photoconductor itself must operate with this degree of precision.
• Toner needs to be quite a fine powder, not the nanoscale powders used in some processes but at 1200 dpi (50 dots per mm) each dot is about 20 microns across. Older toners with irregular grains and a mean size of 8 microns are too big. Toner grain size has reduced to about 5 microns.
• Electrostatic fields need to be much more evenly distributed and in the case of colour printers quite precise. Rather than corona wires printers tend to use conductive rubber pre-charge and transfer rollers.

Technological Progress

The first laser printer was built in 1970, based on a photocopier. The idea was impractically expensive until the mid 1980s, partly because of the cost of the formatter which was based on a minicomputer.

Early laser printers were not fast, and by today's standards not particularly capable at just 8 pages per minute and 300 dpi graphics. For instance:

• Apple LaserWriter. 300dpi, 8 ppm, 1.5mB RAM, 12MHz MC68000, PostScript, full page graphics - $6,995.
• HP Laserwriter. 300dpi, 8ppm 128kB RAM, 8MHz MC68000, PCL, text with small graphics - $2,995.

Today, printers costing less than £100 are rather better than these machines; in fact price is where the real progress has been made.

The cost of memory and processing power has meant that high resolution has been a bit impractical, or has meant that the printer has had to slow down to deal with complicated images.

The problem is in the figures above which mention that an A4 page printed at 1200 dpi has around 127 million points of data in it. If that was binary it would be 16 megabytes of data and if the printer could manage a 256 level greyscale it would need 128 megabytes - for a mono printer. A colour printer could need 4 times as much - 512MB.

Until very recently when gigabyte memory chips became cheap enough to be commonplace formatter boards with that sort of memory capacity and processors powerful enough to manipulate it were expensive items.

Inkjet printers can spit out very small dots and at the end of a line of print they can stop whilst the user's computer prepares the next line of data. Inkjets can therefore have very high resolution - as much as 9600dpi. Thermal inkjets can't manage much of a greyscale (piezo can) but there is not a great expense in adding printheads so inkjet printers designed for photographic purposes can have ten colours. Making a laser printer with more than four colours is probably impractical. For photographic purposes inkjets outshine laser printers and probably always will.

Practical Resolutions.

Inkjet printers can give almost any level of resolution - at 9600 dpi individual dots are 3-4 microns across and lost in the flock of the paper; in aggregate they give a beautiful picture although photographic materials could still in principle be ten-fold better.

There doesn't seem to be a reason why laser printers can't compete with inkjets giving hardware resolutions down to about 1 micron - the wavelength of infra-red light. The problems are:

• Nanometer sized toner powders. Toners are quite complicated with a carrier, colourant and fuser lubricant and ideally we want these delivered by rounded particles with a very tight size distribution.
• Very fast spin of the scanner polygon mirror. There is no limit to how fast things can spin but at around 1500rpm we move from ball bearings to hydrodynamic, then to air bearings and then magnetic. At each stage the electronics gets more complicated and expensive.
• Fast modulation of the laser. The laser in a printer has to deliver sufficient energy to chage the behaviour of a photoconductive material through a finely focussed optic at hundreds of millions of bits per second.
• The limiting factor has always been the laser printers need for a page image in memory. If we are going to have 1 micrometer resolution on A4 then we need something like 1 nanometer resolution on the surface of a memory chip - or several memory chips and a higher cost. Memory chips will shrink to those dimensions but that is several generation of chips in advance of technology today.

Laser printers are generally used for office documents, newsletters and catalogues. Photo reproduction is acceptable but not necessarily excellent. Pages have the robustness of having a plastic material like toner crushed onto them rather than an inkjet liquid squirted on them.

What laser printers generally aim for is a combination of speed, cost per page and page quality.