A high-profile use of lasers in modern life is that of printing and graphics, as Peter Rees discovers
In printing, lasers are everywhere from the home to the largest industrial presses. It is a technically diverse sector with lasers of several types finding favour with equipment manufacturers and users. Perhaps this isn't surprising, given the different types of printing in use - lithography, flexography, gravure - and the fragmented nature of the industry.
There are tens of thousands of commercial printing companies worldwide - the US alone has around 35,000 firms - and most are small or medium-sized. The result is that change - at least across the whole industry - comes slowly. Paradoxically perhaps, printing technology is changing rapidly as it becomes increasingly computerised and automated. Fully digital presses - see panel - cannot yet match the overall product quality of traditional offset printing, in which ink is spread on a metal plate holding an image, transferred first to a rubber blanket and then to paper.
There are tens of thousands of commercial printing companies worldwide - the US alone has around 35,000 firms - and most are small or medium-sized. The result is that change - at least across the whole industry - comes slowly. Paradoxically perhaps, printing technology is changing rapidly as it becomes increasingly computerised and automated. Fully digital presses - see panel - cannot yet match the overall product quality of traditional offset printing, in which ink is spread on a metal plate holding an image, transferred first to a rubber blanket and then to paper. So equipment manufacturers have concentrated on digital platemaking. This consists of automating or removing intermediate steps and makes up around half of all traditional printing. This is the main battleground at the moment between two competing laser types - violet and infrared - which are used to create the plates that carry the printing ink.
Despite the relatively short 10- to 15-year history of lasers in printing there have been succeeding waves of technology change. Red and violet diode lasers currently dominate the market after an initial period when argon ion lasers and diode pumped solid state green lasers (DPSS) held the stage.
Because of the capital outlay required to replace machines, many of the older lasers are still in use. These lasers periodically wear out and need replacing well before the machines themselves. Consequently, they remain in production and sometimes under active development. DPSS green lasers are still used for imaging photopolymer plates and in certain specialist areas. And newspaper presses remain a haven for Argon-ion lasers, says Barbara Eschbach of LG-Laser Technologies because of 'unmatched optical criteria and the easy servicing'.
But laser manufacturers' current hopes for growth are focused on computer-to-plate (or CTP) technology, which uses electronic files to create plates for mounting on a conventional press. It has taken off because it is faster and produces higher quality results than traditional methods, which require an intermediate film stage. Early CTP machines were expensive and only big printing firms could afford them. But they have increasingly penetrated mid-sized and small outfits. It is therefore more a case of when, rather than if, printing companies will switch to computer-to-plate technologies that rely on lasers.
In CTP the computerised image being printed is used to drive a visible-light laser, which exposes a photosensitive emulsion coating on a metal (or polymer) printing plate. This cuts out a stage in conventional lithography where a photographic negative of the image is first made and then placed in contact with the emulsion while it is exposed with UV light.
Argon-ion lasers were first used for this type of CTP printing but they have been overtaken by violet diode lasers (with a wavelength of 405nm), which are cheaper and offer a more stable source of light. As well as exposing plates with a silver halide emulsion, they can also be used for plates covered with a light sensitive plastic.
The power requirements are slightly different, with halide plates requiring 5mW and polymer plates 30mW, but systems are available that can expose both. 'The trend is definitely towards machines that can expose both kinds of plates,' says Eschbach.
Its main rival is thermal CTP, where high power IR semiconductor laser diodes (wavelength 830nm) are being used. The disadvantage of thermo plates is that much more energy (around 1,000 times as much as for violet) is required for exposure and the development process is usually slower than with visible laser light. In order to increase the speed, more high-power diodes are needed which makes the whole system more expensive.
Another challenge is that using a large number of high-power laser diodes creates high temperatures resulting in shorter lifetimes than violet lasers. Printers must also take account of the risk of increased downtime when replacing the large number of diodes. 'Despite these disadvantages there are many users out there who swear by this technology and prefer it to violet,' says Eschbach.
Visible light CTP has its own drawbacks. Plates require careful calibration and processing maintenance, because they are susceptible to fogging and partial exposure. Thermal CTP imaging delivers sharper images because thermal plates are sensitive to only one threshold of exposure energy. If there is less exposure energy than required, they are not exposed. If there is more energy than required, it has no further effect.
US print machinery manfacturer Creo, which claims to be the largest developer of thermal CTP systems, is an interesting case because it began by making visible-light systems. Its advice to printers is a useful corrective to any purely technology-based analysis of the systems. 'The decision about which technology to adopt should not begin with an assessment of the imaging technology, but rather with a thorough analysis of what the investment is intended to accomplish,' says Creo.
To underline that, much of the innovation and competition has focused on printing plates, and their emulsions, and other parts of the CTP process - such as workflow - rather than on the lasers used to burn the plates. And now processless plates are beginning to gain ground. At the moment most plates require a certain amount of post-exposure chemical treatment before ink can be applied for printing, but plates that skip this stage are becoming more reliable.
While the focus of commercial printers is on the thermal versus violet debate and on the plates they are going to use, in the photonics world companies are trying to move beyond the current generation of lasers to improve CTP performance. One such firm is Glasgow-based Intense, a private venture-capital backed operation co-founded in 2001 by John Marsh, Professor of Optoelectronic Systems in the Department of Electronics and Electrical Engineering at the University of Glasgow. Earlier this year, Intense won a place in technology magazine Red Herring's top 100 European firms.
Current thermal CTP printing heads use several lasers in parallel to speed the image process, but the heads with their lasers and fibre guides are necessarily expensive. Intense has used quantum-well intermixing to create an array of individually addressable, semiconductor lasers on a single chip. Historically, it has been difficult to achieve good reliability when making chips of this type, says Prof Marsh. But Intense has conquered the problems and can assemble the array, micro-optics and an ASIC driver in a single module with as many as one hundred lasers on it. These will address the problems of 'affordability and reliability' of current thermal CTP heads, says Marsh. But they will also allow for heads with increased power and hence higher plate-processing speeds.
Another advantage of the arrays is that they can be used in digital printing applications other than thermal CTP, including marking, coding and photographic printing. Intense's arrays could also be used in thermal paper-printing applications, where a 'no-contact' laser would be more reliable than a conventional contact print head that wears out because of friction.
For the moment no single laser or technology dominates the printing industry, but laser arrays could shift the balance towards thermal CTP by reducing cost and improving reliability. Intense is preparing for volume production of its arrays and laser print heads could be available early next year.
Whether a small firm like Intense can revolutionise printing with its laser arrays remains to be seen. But the history of print technology tells us to expect the unexpected - and that whatever else happens, today's systems will hang around well into the future.
The digital printing option
Fully digital presses are like large desktop printers and use similar technologies including lasers. A roll (or web) of paper is preferred to sheets and presses run at much higher speeds. Laser systems use laser light (and a mirror) or an array of LEDs to transfer an image stored in a digital file to an electrostatic drum, which attracts toner and deposits it on the paper. As with desktop printers, each page can be different - a process known as variable imaging - but print quality is not as high as in offset printing.
Direct imaging presses transfer the image to a plate - as in offset printing - but one that is held directly on a press. This contrasts with CTP where the plate is created separately before mounting on a press, which is more labour intensive and potentially more time consuming. These presses have been around for more than 10 years and have at various times used different imaging systems including infrared lasers like their CTP counterparts. However, unlike CTP platemakers, which can produce plates for a number of presses, they can run only one job each, which holds up work completely if the press breaks down because a laser needs replacing.
Both direct imaging and digital printing tend to be used for shorter runs than CTP offset printing, when the process of making new plates would make a job too costly.