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Introduction to colour laser printers.

Colour laser printers are more complicated and hence more expensive to buy than black laser printers. They are also less robust and have lighter duty cycles. They are more expensive to operate than a black laser printer but more reliable and cheaper to operate than inkjet printers.

Computer monitors and television screens combine the three primary colours (red, green and blue) to produce additive colour. The wavelength of the light emitted from a source such as the phosphor coating on a CRT screen determines the colour of the image we perceive. By contrast, the appearance of colour prints is determined by light reflecting off an image. Colour printers use a system called subtractive colour. This is a four colour printing system using cyan, magenta, yellow, black and is often referred to as CMYK. In principle, mixing equal amounts of cyan, magenta and yellow should produce black. However, limitations in the pigments and in the media result in a dark green or muddy brown instead of black. Hence, four colours are used for colour printing as opposed to only three colours used on a monitor.

Mixing two primary additive colours in equal amounts at full intensity creates cyan, magenta and yellow light: red plus blue makes magenta; green plus blue makes cyan; and red plus green makes yellow. Conversely, mixing equal amounts of cyan and yellow pigment produces green; magenta and yellow produce red; and cyan and magenta produce blue. Because our perception of subtractive colours depends in part on the colour of the ambient light, subtractive colour literally changes depending on the colour of the light available. For example, the same colour page printed on a colour laser printer will look different when seen under late afternoon light than it will under fluorescent light (HP web: Color Theory Overview).

Most colour laser printers using a PCL driver are optimised for printing red-green-blue colours to better match the colours as displayed on a computer monitor. Most commercial printing systems use a colour interpretation system known as Standard Web Offset Press (SWOP). Another commercial colour interpretation system is known as Pantone which is used for matching printer output to a specific colour. Colour laser printers can produce output conforming to SWOP or to Pantone by using settings available in their PostScript 3 drivers.

Colour printers use four separate cartridges: cyan, magenta, yellow and black. The capacities of the cartridges are rated at significantly less than the number of pages that is typical for monochrome printers. Colour cartridges are also more expensive per each than black cartridges. Hence, when choosing a colour printer, you must examine the cost per page of toner very carefully. The toners are more sophisticated chemically than are the toners used in black cartridges. Manufacturers of after-market toner have found it difficult to match the colour accuracy that can be achieved with the genuine product.

Most new colour laser printers come with only low-yield toner cartridges and you will soon need to replace them with high-yield cartridges. You will find this an expensive exercise. For example, the Lexmark C510N comes with a token set of cartridges that usually run out after printing only 1,000 to 2,000 pages: a complete set of high yield toner cartridges for a Lexmark C510N will cost you $1,200 plus GST and that is more than the purchase price of the printer. Most colour laser toner cartridges come equipped with computer chips that reset the cartridge page count when first inserted into the printer and the printer can report the amount of toner remaining in the cartridge throughout the life of the cartridge.

Laser printers begin the printing process by using a laser beam to form the image on a light sensitive photoconductor drum, the OPC drum. The photoconductor drum is usually incorporated within the toner cartridge of a monochrome laser printer. In colour laser printers, the OPC drum is often separated from the toner cartridge and must be replaced as a separate item when it is worn out. Some printers have a separate OPC drum for each colour while others have one OPC drum for colour and one for black. OPC drums are regarded as consumables and must be replaced after a recommended number of pages have been printed. Some colour printers have computer chips incorporated in their drum kits and, when first inserted, they reset a page counter in the printer. When the drum has been used to print a set number of pages, the printer will cease operation until a new drum is inserted.

In monochrome printers the image is transferred directly from the OPC drum to the paper. In colour printers, the image is transferred to an intermediate component, the transfer belt. Most A4 colour laser printers have their cartridges mounted on a carousel: the carousel rotates and prints a single colour image onto the transfer belt; the carousel rotates again and superimposes the next colour image on top of the previous one. After four movements (corresponding to magenta, cyan, yellow and black), the complete image is built-up on the transfer belt. The complete image is then transferred from the belt to the paper in a single pass. Since each image is built up from four separate colours, you might say that each page is printed four times and hence colour laser printers are usually much slower than monochrome printers. They also tend to make more noise as the carousel rotates. Some colour laser printers have separate page counts for black and colour printing and the total page count is equal to the black page count plus 4 times the colour page count.

Another consequence of the rotating carousel is that any toner that leaks from one of the cartridges tend to be thrown around inside the printer. Colour toner is much lighter than black toner and is more prone to being widely dispersed inside the printer. The HP LaserJet 4500 and 4550 models are known to be somewhat messy whereas the Lexmark models are less so. However, we consider the print quality produced by the HP models is superior to that of the Lexmark models. Because colour toner is so light in weight and so easily dispersed, you should never clean a colour laser printer by blowing it out with compressed air ? you should use a special toner vacuum cleaner instead. Otherwise, you are likely to contaminate one colour toner with another. You are also likely to coat all the mirrors in the print head with colour toner!

Some A3 colour laser printers (eg Lexmark Optra Color 1200, C910N, HP LaserJet 9500) have their cartridges mounted in-line in stationary positions on top of a long, moving transfer belt. This configuration requires that the printer has a separate print-head (laser) for each colour cartridge and this makes them more expensive. However, they are also quieter, faster, cleaner and more reliable as there is no rotating carousel.

The transfer belt and transfer roller are usually sold as a transfer kit. The kit is regarded as a consumable and must be factored in to the cost of operating a colour printer. In the Optra C710, for example, the transfer kit is rated at 100,000 pages. However, the page count here refers to black copies only: this transfer kit is only rated at 25,000 pages for colour printing. It is user replaceable but comes equipped with a chip that also blows a fuse to reset the page count for the transfer kit. When it has produced 25,000 copies, it stops.

Colour laser printers are best installed on a network through a print server. Colour images can result in very large files and colour printing requires much more information to be transmitted from the computer to the printer. You need the speed and reliability of a 10/100 Ethernet network connection via an internal print server to ensure you are not left waiting for long periods while the print file is downloaded. A parallel port connection is just too slow. HP quotes the following figures:

Parallel data transmission speeds from a computer to a printer (under ideal conditions) can be up to 360 Kbps (Kilobits per second)

USB data transmission speeds from a computer to a printer (under ideal conditions) can be up to 12 Mbps (Megabits per second)

Ethernet data transmission speeds (under ideal conditions) can be up to 100 Mbps

If you connect your colour printer to your network by connecting its parallel port to an external print server, your network connection will still be limited to 360Kbps. We recommend that you use a 10/100 internal Ethernet print server because it is approximately 300 times faster than a parallel port.

If you wish to print multiple original copies of the same colour pages, the printer will operate much more efficiently if you have enough memory to store the formatted version of the whole document inside the printer. In this way, the printer can download and format the pages only once and it can hold the resultant data in the printer ready to produce the next copy. The ability to download, format and store the data in the printer for processing multiple original copies (mopies) at full engine speed is referred to as RIP ONCE. As HP explains:

"In order to print a job, the printer must first create a rasterized or dot by dot image of the page. The RIP Once (Raster Image Process Once) process creates an image of a page and it then saves a compressed image of that page into memory. RIP Once will speed multiple copies by using the image created for the first copy to print all successive copies. Pages are printed by decompressing the page image without pausing to process the print job again. The print job will print exactly the same every time because the compressed image is not dependent on any other printer data. This is particularly important in colour printing as it guarantees that the colour tones of the image are reproduced exactly each time. All pages after the first will print at or near engine speed."

In HP printers, you need a RAM disk or a hard disk to enable the RIP Once facility. The process of formatting the pages ready for printing greatly expands the amount of data to be stored. For example, this Catalogue is an 18mb Microsoft Word file. When it is downloaded to a monochrome printer and formatted ready to print, the data expands to about 80mb. Colour files are much larger than monochrome files and need correspondingly more memory. If you wish to take advantage of the RIP ONCE facility, you will probably need to purchase a hard disk option for your colour printer. These hard disks are sometimes mounted into cradles built into the formatter board or can be mounted on add-in cards that fit into the expansion slots of the printer.

Obviously, the printer has to do a lot more computations to format a colour page than a monochrome page. Hence the power and speed of the processor (CPU) is more important to the effective operation of colour printers than it is for monochrome printers. For example a HP Color LaserJet 8550GN with a 333mhz processor will format pages much more quickly than the 8550DN model with a 233mhz processor and that model will be much quicker than an 8500DN with a 133mhz processor. One of the principal advances in newer model colour laser printers is that they have more powerful and faster processors and more memory as standard. They also have improved colour matching algorithms to produce more realistic prints.

The fuser in a colour printer is also regarded as a consumable. The upper and lower rollers in a colour printer fuser are usually both made of silicon rubber and are more prone to wear and damage than, for example, a Teflon coated aluminium roller used in a Lexmark Optra S or T series printer. The two rollers are held together under considerable pressure and both the upper and lower rollers are usually heated. The rollers are prone to develop flat spots if the printer is left idle but kept in a fully-ready state for long periods of time. To minimise the chances of this happening, we recommend that you set the power saver switch on your printer to a relatively short interval, eg 10 minutes.

When buying a colour laser printer, you should firstly look at the print quality it is capable of producing. If you want a printer to equip a graphic arts laboratory, your requirements will be more stringent than if you just wish to print coloured text. Ask to see some sample prints produced by the same model that you are considering. Obviously, a colour laser printer will only produce its best quality output if all of its consumables are in good order and condition ie toner cartridges, OPC drums, transfer belt and fuser. For this reason, we recommend purchasing new toner cartridges and new OPC drum(s) with your refurbished printer.

If you want the highest possible quality of colour prints, we recommend that you consider a solid ink (hot wax) printer such as the Tektronix (now Xerox) Phaser 850N. These printers use melted wax or resin (instead of toner particles) to coat the paper and they produce brilliant glossy prints on plain paper. On the down side, they are expensive to run and are best left operating continuously. We have some of these models available.

Always look for a colour printer with a fast processor, plenty of memory and an internal print server for network connection. Look at the maximum memory capacity of the printer and the cost of purchasing the maximum amount of RAM that the printer can address. If you can afford it, select a model with an internal hard disk or at least select a model that can be upgraded later by the addition of a hard disk.

The running costs of your colour laser printer will be much higher than the cost of a monochrome model. Hence your purchasing decision should be swayed heavily by these projected costs. You must assess the cost and capacity of its toner cartridges (all four of them); the cost and the page rating of replacement OPC drums; transfer kits; and fusers. You should also look at the availability and cost of replacement parts and service. Lastly, look for well-proven drivers and network software that suits your environment.

Until recently, most colour laser printers were heavy duty models designed for commercial use. All colour models featured in this Catalogue come into this category. Many manufacturers have now released relatively cheap, home/office models. Like their monochrome counterparts, these light duty colour laser printers are designed for cheapness of production rather than for economical operation, durability and ease of maintenance. The cost of a replacement formatter board for one current ?home-office? model exceeds the purchase price of the new printer. If they fail, it?s best to buy another one.

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