(selected transparancies on printers from Lecture 10)

Highlights in the history of printing

• invention and refinement of paper in China over several centuries. The Chinese had developed "rag" paper, a cheap cloth-scrap and plant-fiber substitute for cumbersome bark and bamboo strips and for precious silk paper, by A.D. 105. The secrets of the craft that were revealed to Europeans in the twelfth and thirteenth centuries.

• Chinese innovations in ink, block printing and movable clay type all advanced printing technology, but because their alphabet employs thousands of visually specific ideograms, the use of movable type was much more labor-intensive for the Chinese than it would be for Europeans.

• In the early 1450's rapid cultural change in Europe fuelled a growing need for the rapid and cheap production of written documents. Johannes Gutenberg developed a printing press by combining features of existing technologies: textile, papermaking and wine presses.

• In 1476, William Caxton set up England's first printing press.

• Until the 19th century Gutenberg's print technology had not changed dramatically. In the early 1800's the development of continuous rolls of paper, a steam-powered press and a way to use iron instead of wood for building presses all added to the efficiency of printing.

Useful links : The Examiner: THE HISTORY OF PRINTING and Printing: History and Development

XEROGRAPHY (PHOTOCOPYING)

Up until the late 1950’s copying a document was laborious and messy. Now photocopiers are highly advanced and use photoconductive materials with no ink involved - hence the term "xerography" from the Greek for "dry writing".

The US market for copiers was set to top $15 billion in 1996 ...

Whilst photoconductivity was discovered in 1873 it wasn’t until 1938 that xerography was demonstrated by Chester Carlson in New York. A sulphur film was used as the photoconductive surface and the latent electrostatic image rendered visible by frictionally charged powder. The process did not take off, perhaps because :

• the sulphur film had little sensitivity to visible light.

• the initial charging involved rubbing the sulphur film with a handkerchief.

• the developing agent was lycopodium powder (club moss spores) which is not very dark and gave poor contrast.

AMORPHOUS SELENIUM AND IMPROVED COPIERS

Up until the 1930’s work on semiconductors was almost exclusively interested in crystalline materials, which were not suitable for page-size films or drums for photocopiers.

The situation changed in the late 1940’s with the discovery of photoconductivity in amorphous selenium - the photoreceptor used in the first commercially successful photocopier, the Xerox 914, in 1959.

The impact was impressive with market research predicting 3000 sales, whilst over 200,000 were eventually sold.

The 1960’s and 70’s saw continuous improvements in quality and speed. The first xerographic colour copier appeared in 1973 and the mid-1970’s saw the birth of digital xerography with the first use of lasers to write the image onto the photoreceptor.

PHOTOCONDUCTORS FOR XEROGRAPHY

Amorphous selenium was critical in the development of xerography but had shortcomings:

• it is mechanically inflexible and tends to crystallise

• it possesses poor photoconductivity for red light

A flexible, red sensitive photoreceptor was desirable to allow replacement of the drum with a belt (allowing full-page exposure) and also higher speeds.

In the 1960’s organic solids, including polymers, were developed. Polymers typically do not respond to visible light so that dual-layer photoreceptors now dominate in the xerographic industry. A second, thinner, photosensitive layer is attached to the polymer transport layer and acts as the source of charge.

More recently, amorphous hydrogenated silicon has been developed as a commercial photoreceptor.

THE BASIC STEPS IN XEROGRAPHY

(1) An insulating but photoconductive layer (the photoreceptor) is uniformly charged.

(2) An optical image of the document is projected onto the photoreceptor and selective discharges leave a replica of the document in the form of latent surface charge.

(3) The image is then developed and electrostatic attraction used to fix charged powder (toner) to the latent image and also to transfer the toner to paper.

(4) The image is fixed to the paper by heating and excess toner and charge removed from the photoreceptor.

These steps are integrated in an analogue xerographic copier, using a photoreceptor drum.

(The ion charging device is called a corotron).

In a digital xerography a scanned laser beam is used to write a digitised image directly onto the charged photoreceptor. Cumbersome gas lasers were used initially but compact semiconductor lasers brought significant advantages - in size, ease of modulation, cost, etc. The first semiconductor lasers emitted infra-red radiation, however, requiring the development of infra-red sensitive photoreceptors. Solid organic photoreceptors (e.g. phthalocyanines) provided the solution. More recently arrays of light emitting diodes are being used and can eliminate the rotating mirror used to scan the laser beam.

PRINTERS

(link to : 09-00. Photo Printers)

1. Ink-jet Printers

Liquid ink-jet printers propel fine droplets of liquid ink toward the surface of paper. In today's marketplace, this technology is the low-cost entry point for personal printing and low-volume color printing.

These printers use ink-jet cartridges – with cyan, magenta, yellow and black (CMYK) inks. The ink dries partly by absorption and partly by evaporation. Although you can print on plain-paper, liquid inks tend to soak into the paper taking the colour along with them. Richer colours are obtained using coated papers that are less absorbent.

The cartridge of ink is attached to a print head with up to hundreds of nozzles, each thinner than a human hair. The number of nozzles and the size of each determines the printer’s resolution. As the print head moves across the paper, a digital signal from the computer tells each nozzle when to propel a drop of ink onto the paper.

2. Laser Printers

Laser printers revolutionized black-and-white printing, making graphics and desktop publishing possible.

Laser printers use a technology similar to that used in copiers. A laser beam is focused on a photoelectric belt or drum, creating an electrical charge in areas where toner is to adhere. Charged toner is then attracted to those places on the belt or drum. Electrostatic charges cause the toners to adhere to the belt. With black and white printers, this process happens once but with colour printers it is repeated for the cyan, magenta, yellow and black components of the image. The image, composed of the four toner colours, is then transferred to a drum which rolls the toners onto the sheet of paper or transparency. The toners on the paper are then fused using either heat or a combination of heat and pressure.

However, colour laser printers are on the margins of photographic printing. Not only are their costs high, but their quality has not yet matched the very inexpensive ink-jet printers.

The invention of the laser printer

The first laser printer, was developed at the Xerox Palo Alto Research Center in the early 1970’s. An engineer worked out how to add the power of laser light to Xerox copier technology.

"At the Palo Alto Research Center, Xerox was producing amazing prototypes of machines that could produce graphics and proportional-space type on the screen, but computer printers were only capable of printing fixed width characters"

The solution was to use the tried-and-true xerographic process, but move the image directly from computer to the printer drum. Meeting this challenge involved a special mirror that spun at very high speed to spread the laser image over the printer drum.

First laser printer : the Xerox "Dover"

3. Dye Sublimation Printers

Where quality is very important dye-sublimation printers (called dye-subs, but more accurately dye-diffusion) come into play. The "dye" in the name comes from the fact that the process uses solid dyes instead of inks or toner.

Dye sub printers have their coloured dyes in a transfer roll or ribbon. This roll contains consecutive page-sized panels of cyan, magenta, yellow and black dye. These printers require special paper that's designed to absorb the vaporous dye on contact and the cost per page is high (£ per page!).

FAX MACHINES and SCANNERS

Fax transmissions involve sending images by telephone.

Newspapers have been using fax machines to send photos (wire photos) since 1907 when a photo from Paris was wired to the Daily Mirror in London. Photos of the Hindenberg balloon disaster (1937) where the first to be faxed across the Atlantic.

The document to be faxed is moved across a light source (usually a fluorescent strip) and reflected light is focussed onto a light detector. In modern fax machines this will be a linear CCD array which codes the image into a stream of varying electrical pulses, line by line and pixel by pixel. This signal is then sent by telephone to a receiver where a printer is used to recreate the image.

Early fax machines took roughly 6 minutes for an A4 page. Later machines (1980’s) coded the information digitally but used analogue sound-waves for transmission and 30 seconds a page was reasonable. Machines of the 1990’s can both code and transmit digitally cutting the time to about 5 seconds a page.

Flat-bed scanners are now common for digitising images and operate in much the same way as modern fax machines.

Reflected light from the scanned object is detected using CCDs and degrees of light/dark transformed into a digital signal. Colour is handled using multiple CCD chips and colour filters.

Uninspiring picture of a scanner scanned into a computer.