Ink-jet Printing

Introduction

One of the most useful aspects of having a computer is the ability to easily print whatever you chose. Letters, photos, drawings, the list is endless. Although there are several different types of printer that are used with computers, Laser, Dye-sublimation, the most commonly used in the home and by individual photographers, the most versatile, and currently the cheapest to buy, is the ink-jet.

Choosing which ink-jet printer to buy for your needs is not an easy task given the huge range now available, and many have problems using the model they buy and getting the quality they expect from it. So we will try and give you a guide as to how they work, and what to watch out for when buying one and printing your photos using it. 

Although you may print photos for yourself, equally they may well be printed in magazines. Some of the following contains information on the printing methods used for magazines and why image resolution needs to be higher than for ink-jet printing.

Printing in General

Dictionaries variously describe printing as; the reproduction of text and images by mechanical or electronic means; by pressing ink onto paper; or produced from negatives.  Most people associate it with the printed word, books, newspapers, magazines. It's such a common part of our lives, virtually everything you buy comes in packaging which has invariably been printed in some way, that it's taken for granted like so much else. 

Producing a print of a photograph has not to date been associated as having anything to do with ink printing because, for most, it hasn't. It's been a chemical process, using light sensitive paper. Only in the printing industry has ink been used in the production of images. Now that is changing in the digital revolution the computer has brought.

Early printing methods used individual type to assemble words - typesetting - and in the recent past the type-writer became a well known mechanical printer, using ink impregnated tape, both at work and in the home. As we have moved into the computer age, so has  printing. Instead of the type-writer we now have the ink-jet printer, able to reproduce not only text but images. But the basic principle of printing is still very much the same, the placing of ink onto paper or other media, using some form of mechanical means, even if it is carried out in a different way.

Putting ink on paper.

To understand what effect printing ink onto paper has, we have to look at the colour of light. Pure white light is made up of all the colours of the spectrum. The primary colours of light - Red, Green and Blue -  which between them make up the whole spectrum, are known as additive. If you add all three primary light colours you get white light. Take three torches, one each with a red, blue and yellow light and shine them at the same spot and you would get white light. When you look at a sheet of white paper all light colours are being reflected, and you see white. When you lay ink on paper you block - subtract - the light being reflected. Thus the three primary ink colours, Cyan, Magenta and Yellow, are known as subtractive colours.  Cyan subtracts Red, Magenta subtracts Green and Yellow subtracts Blue. Print all three together - on top of each other - and in theory you should have black as all the light spectrum has been blocked. In practice you usually end up with a muddy dark colour rather than black, as some stray light gets through if the colours are not fully saturated. So a proper Black ink is used to ensure all light elements are blocked and black looks well, black. It's known as Keystone. You may have seen reference to CMYK  - Cyan, Magenta, Yellow and Keystone - in relation to the conversion of RGB images - digital images viewed on a computer screen - for printing. 

Printing Tonal Images

If you are only printing words, symbols, or areas of solid colour on a page no tonal range is involved, and the ink is laid down as a continuous block for each individual letter or area. Block printing is quite common. Ink stamps using ink pads are often used at home and work. An individuals, or company name and address, may be stamped on papers belonging to them to denote ownership, or as a form of advertising. 

Pen and Ink drawings use a single colour on paper are known as 'line' drawings, as are house construction plans, exploded diagrams and similar. Sometimes such drawings give a suggestion of tone by shading. Shading is achieved by placing lines of ink together at varying distances. Close together leaving little white paper showing suggests a dark tone. Spaced wide apart with lots of white paper suggests a light tone. It relies on being viewed from a certain distance by the human eye, which merges - averages - the tones and colours it can't define clearly. From a great distance a black & white checkerboard looks grey. It is only as you get closer and the eye can clearly see the different colours, that it becomes individual blocks of black and white.

Printing uses dots instead of lines or squares to achieve the same result. By placing different sized dots of ink near each other, tones are suggested, whilst use of coloured ink in various combinations enables different shades and colours also to be seen. Under magnification they are just a series of dots.  At the distance normally viewed at the dots blend to form the words, images, and colours we see.

Printing Methods - Commercial

Commercial printing presses use plates - printing plates -  that are the same size as the paper being printed, the whole sheet being printed with one pressing. Some plates are flat, others are curved. Colour printing where the full tonal range is produced, photographic images etc, uses four separate plates, each  of the ink colours - CMYK - being applied individually. This know by the general term 'four colour work'. As it is essential that all 4 plates print in the same place on the paper - otherwise all the colours would look wrong and overlap and the text and images blurry - a method is used known as 'registration'. This uses the outside edges of the paper, four crosses being placed one in each corner on each plate. All four crosses on all four plates must line up, or rather superimpose themselves on each other - register -  to ensure all plates are aligned correctly. Most printing is done on paper larger than the final finished size and is cut to size after printing. Sometimes you can see these crosses on paper that hasn't been trimmed properly, it happens now and then.

Printing plates are made by chemically etching them. The process involved in making the plates is known as Photolithography.  For colour printing, the 4 plates needed are generated from the image by the method known as four colour separation, where coloured filters separate the light into the ink colours CMYK. A Halftone line screen is also applied. In the past all these operations were carried out photographically, using special Lithographic film and paper, and very large cameras known as copy cameras. Today virtually all the operations are undertaken using computers. Many imaging software applications - Photoshop etc - offer colour separation and halftone screen options for those preparing images for commercial 4 colour printing. 

A Halftone line screen is a method used to break up a toned image into a uniform series of dots of varying size depending on the tones. Large dots for deep tones, small dots for light ones. Screen resolutions vary as does the pattern of the dots. Dots can be larger or smaller, but not closer together, or further apart. The same colour can't overlap or cover itself. This means the finest screen size possible needs to be used to get good quality reproduction, and screens requiring images with a resolution of at least 300dpi are used in magazine production.

The level of resolution, and thus the quality of the printing, depends on a number of factors. Newspapers are printed on fast presses, as speed is essential for the quantities needed, so paper is coarse and absorbent and screen resolution is low, to ensure the ink lays on the paper at the speed being printed at. If you look closely at a paper, or under a magnifying glass, you can sometimes see the dots of ink that are used to represent an image.  By contrast, magazine's are printed much more slowly, using higher grade papers and higher screen resolution, resulting in higher quality reproduction. The dots are still there, but they are much smaller and it's pretty impossible to see them under normal conditions. 

Printed Magazines use a fixed resolution as we have said of roughly 300dpi. Whatever size image they are supplied with, they can only print it at this resolution and retain a reasonable quality of reproduction. If you provide a magazine with an A4 image at 300dpi they can print it at any size they wish up to that size. Since that is roughly the size of finished paper stock most use, it gives them plenty of scope, and the option to crop to A5 for a half page illustration if needed. But if you were to provide that same image at say A5 with a resolution of 300dpi, then again that's the maximum they can print it at, and their options are then limited as to what they can do with it

. 

So if you were to supply them with say, an image from a 6mp camera, the biggest size they could print it at would be 10"x 6½". They can print it smaller, or crop it smaller, so long as the crop remains at the minimum 300dpi resolution. It is for this reason that there are continued calls for high resolution digital cameras. Drum scanning, which is the type of scanning used in printing to scan images and film stock for reproduction, can provide this resolution and greater, at a quality not available to the home user. Digital camera pixel counts need to be in the 10mp - 16mp range to give A4 size at 300dpi - 380dpi. The higher figure giving room for manoeuvre, say cropping an image and still providing an A4 size result at 300dpi.

Printing Methods - Ink-Jet.

The name ink-jet describes quite accurately how  these printers work, by firing a jet of ink onto the media - usually paper - being printed. There are several stories of how the basic concept, which had been under research and development for some time, finally came into being. One is that a HP engineer got the idea from watching a coffee pot drip water. Another that someone working in Canon's R&D touched an ink-filled syringe with a soldering iron by accident, causing the the heat to spit the ink out of the syringe. In both cases it led eventually, some eight to ten years later, to the thermal Ink-jet printer, the most widely used printer technology.

HP were first to market a printer using this technology in 1984, the Thinkjet. Canon also introduced a printer around this time, the BJ-80, using similar methods, which they patented in February 1986 as Bubble-Jet. Lexmark [the former IBM printer division], joined the fray in 1992. All their ink-jets have one thing in common. They use heated droplets of ink - the thermal ink-jet process.

There is another ink-jet technology which is also used. Epson printers, instead of heat, use the more expensive piezoelectric technology first used by Siemens in 1977, using ceramics to fire the ink onto the media. This uses pressure not heat. Epson's first printer using this method was released in 1985.

Although today's printers have come a long way since the first models, the basic technology remains. But some of the statistics are mind boggling. HP's head in it's original Thinkjet contained 12 nozzles and used 180 picolitre droplets to produce a fairly crude picture judged by today's standards. Print heads now contain many nozzles, up to 600, and the holes in them are smaller than a human hair, whilst droplets of ink are incredibly small. Most current printers use droplets as small as 3 picolitre's in size. Canon's latest model can manage 1 picolitre drops. A 1 picolitre drop is one billionth of a litre, or one millionth of a cubic centimetre. HP's heads can contain as many as 800,000 heating elements in the space of a 20mm square, whilst Epson's can deliver up to 30,000 drops per second. All printers use variable size droplets, the thermal types achieving this using different sized nozzles and multiple heating elements, whilst Epson's piezoelectric ceramic technology gives it the ability to control droplet size precisely using one size of nozzle. Most print heads are made with similar methods to chip manufacture, using silicon wafers and photolithography, in comparable 'clean' manufacturing plant.

As the ink heads are only small in size they cover the area of paper being printed by continually moving sideways across it in both directions along a 'rack', whilst the paper is moved lengthways under them at the end of each traverse of the rack. Both movements being controlled by what are known as 'stepper' motors. In this way the heads cover the area of the paper being printed, in a regular fashion. How fast they accomplish this depends on the resolution used, which determines how many droplets they must put on the paper, and the size of the paper. The more ink that must be laid down, the slower the printing will be. Variable sized droplets of ink are used, small where detail is fine or shades are light, large in areas of solid colour or where shades are deep and saturated. They can also place the droplets closer together or further apart, overlap them, and put them on top of each other. In order to improve the tonal range, extra ink colours, 'light Magenta' and 'light Cyan', are often to be found in photo quality ink-jet printers, in addition to the standard CMYK colours. All of this helps to provide images that have the best possible detail and colour, for the resolution used to print them. 

This can easily be seen by taking an image and printing it at different resolutions on different paper. Print at say 360dpi on ordinary ink-jet paper and the image can look faded and un-sharp.  Print again at 720dpi on basic photo paper and the difference is clear. Colours are deeper and the image sharper. Print at the highest resolution possible on the best quality paper and you can have an image full of detail and tonal range, with deep saturated colours, a world away from the same image on that ink-jet paper at 360dpi.

Unlike commercial printing, ink-jets can produce comparable quality images at lower image resolutions as a result of the way they are able to print. Whereas 300dpi is the figure needed for magazine images, 150-200 dpi is often more than enough for ink-jets. 

Regular Maintenance

A problem with all ink-jet printers is that they use 'wet' inks, as opposed to some other forms of printing which use ink in solid form, wax tablet, or impregnated in tape as in typewriters and Dye-sub printers. As the nozzle holes in heads are so small, they can easily get blocked as the ink dries out. For this reason it is not wise to leave an ink-jet printer standing un-used for long periods of time. Regular use is needed to prevent the ink drying out. Most cartridges state that they must be used within 6 months of removing from their sealed packaging. If left un-used after opening the ink starts to dry out and can't run freely, blocking up the nozzle holes.

This is not so much of a problem for users of HP and Lexmark printers as each ink cartridge comes with new print heads, which are an integral part of each cartridge. This is because the heads wear out quite quickly due to the particular method of manufacture. If it becomes the case that the heads on the cartridge are unable to be cleared, then replacing the cartridge means new clean nozzles. The advantage of new heads with each change of ink cartridge is offset however by the higher cost of the replacement inks, and the money lost discarding cartridges not fully used up.

Canon and Epson printers use heads which, although more expensive to make, generally last the lifetime of the machine. Because the heads are not replaced at regular intervals, these printers have automatic head cleaning cycles which aim to ensure the heads stay clean. They can also be started manually if the need arises. The problem is again that the head cleaning process uses quite a lot of ink. A further problem is that replacement of the heads, should the need arise for any reason, is so expensive a new printer is often as cheap.

There doesn't seem an easy way out of the basic fact that any ink-jet printer is not as cheap to run as you would like. It is the price you have to pay for the convenience. The most economical method, it seems, is when it's used on a regular basis.

Choosing a Printer

How do you choose a printer from all the ones currently available?  With some difficulty is the usual answer. The best way seems to be to try and work out all the different types of printing you want to undertake, and find one that does it all, at a price and quality you find acceptable. 

Here are some thoughts to bear in mind.

Inks

Ink is expensive. Using a dedicated Photo printer just for lots of ordinary text documents can be wasteful. If you only ever print out the odd photo then an ordinary general purpose printer will still turn out very good results. Perhaps not quite as quickly as a dedicated photo type, and if you are extremely fussy then you might well be able to see a difference between the output of the two types, but these days, and despite what the makers say, the differences in our opinion are small. In the past the standard of ink-jet printing varied greatly. But it has come on in leaps and bounds. Virtually any printer can deliver good results now, whatever the type and the inks used. Those that use just 4 inks, Black and Cyan/ Magenta/Yellow, may not deliver quite the colour quality that the 6 and 8 ink types can, but it is pertinent to remember that these four colours are all that are used in commercial printing, magazines etc.

Once the ink packaging is opened the ink cartridge inside must be used within a certain timescale or it will dry out.  6 months is the norm. This can also happen if the printer is not used regularly. Placing a printer near a heat source such as a radiator is therefore not a good idea. Being able to easily obtain replacement inks is also important. A cheap printer from other than one of the major makers, Epson, HP, Canon, Lexmark, whilst they may seem an attractive buy, won't be if once the initial inks supplied with the printer are used up you find it difficult if not impossible to obtain more. And this does happen.

Printers can't operate if any of the inks are empty. So those that use the least number of cartridges are best in this respect. Keeping a stock of spares is easier, and cheaper. Printers that use individual inks on the basis that you only need replace the one that has run out, are not the advantage they seem. When you print a document other than just text, all the colours are used to generate the colours depicted in it. And on an average general basis most inks run down at around the same rate. Yes, one may run out slightly before another, but it is rare to find the situation that some will last much longer than others. The use of individual ink cartridges is just a marketing way of disguising the extremely high cost of inks. Paying say £7.99 for each ink in a set of 6 or 8, doesn't at first seem as horrendous as paying £47.94 or £63.92. 

Because of the change most ink-jet printer makers have made to individual ink tanks, and the huge price increases that occurred as a result, which we just cannot justify paying, we have stuck to using an old photo printer that uses just 4 inks. At the time it was purchased it produced the best quality available which we were more than happy with, and in that respect nothing has changed just because newer but more expensive to buy and run types have appeared offering even higher quality.

Printers that use dedicated swappable photo inks seem a good idea. Until you realize you must still use them in a certain timeframe. Using replacement inks from 3rd party sources is cheaper, but quality may be sacrificed. Most tests reveal that cheaper inks don't last as long, produce less vibrant colours, and can block the print head nozzles much more quickly, requiring more frequent head cleaning cycles which in themselves use huge amounts of ink. Using such inks also invalidates any warranty.

You can get 'special' inks to use to make certain types of photos, monochrome inks for instance. These are fitted in place of your normal ink cartridges. But you must 'flush' through with cleaning fluids any ink residues before you start, wasting large volumes of your normal inks, and again before you change back again. These inks also seem to clog easily, from the reports we had seen.

Papers

Most ink-jets work best with the paper made to match the inks they use. No two makes of printers use inks chemically formulated in the same way. And some makers have printers that use differently formulated inks depending on what the printer type is. General purpose, photo, etc. New ink formulas are being introduced all the time in an effort to increase what is know as print longevity, the amount of time a print will last before the colours start to fade. 

Picking a printer whose maker also supplies a good range of papers is sensible. Some printers will work well with virtually all makes of 3rd party papers, others can't. Using papers not made by the printer maker means it can be a case of trial and error, and thus very expensive, to find the right settings to use. And in some cases an acceptable result just isn't obtained no matter how much time and effort is expended. 

Ordinary basic ink-jet paper and photo-copier paper is not generally a problem. Most printers can and are designed to work with them. But other papers can be, heavy matt or coated papers, canvas etc. By that we mean, matt, gloss, semi-gloss, pearl, etc. An image printed on the makers own paper may look fine. On 3rd party paper the colours may be different, the ink may run and pool, or not dry out at all. 

Computer Requirements

A sometimes frustrating experience is the time it can take to print a high resolution image. Ink-jet printer reviews always contain information on the time taken to print pages of documents and photos, and makers are often at pains to stress that their printers are capable of so many pages per minute. However, although printers are much faster than they used to be, and the time taken is relative to the resolution a document is being printed at, there is another related aspect that is generally unknown or ignored.

The time it takes an image to be printed is often reliant on the amount of actual RAM the computer has that the printer is connected to. When you send a document to print, the computer has to process the information and convert it from the RGB format that computers use to the CMYK format the printer uses. The larger the image file, the slower this will be. The amount of virtual memory available has no bearing on this. Virtual memory is also referred to by some as 'scratch disk' or 'paging file' . Virtual memory is just a reserved area of hard drive that is used to store temporary information that is greater than the actual physical RAM available can handle. Virtual memory is not of use when sending documents to print.

Tests have shown that there can be considerable differences in processing and printing times depending on the amount of RAM fitted. A file that may take 45mins to process and print using 128mb RAM, may only take 7-10mins if  256mb is used, and just 2-3mins if  512mb is available. These are not actual figures, but they are representative of the general differences that occur between the differing amounts of RAM that might be fitted.

In our experience, if you are using W95/98/98se or ME you should aim for a minimum of 256mb RAM, and preferably 512mb. XP users should consider 512mb as minimum, and 1Gb RAM as preferable. The new Windows Vista O/s has a basic requirement of 1Gb of Ram to work with any degree of usefulness. So with Windows Vista computers 2Gb of Ram should be considered.

details about printing photos can be found at  Photo Printing