Printing has traditionally been primarily about large scale production of pages of text and images. Starting with 14th century carved wooden plates for printing bibles printing has been a matter of more quickly and easily reproducing works of literature or drawing than could be done by hand copying. In the case of drawings though printing also provided a means for the exact same artistic work to be reproduced and widely distributed. With the advent of photography printing took on new meanings because photographic processes could be used to reproduce highly detailed text and images, while cheaper and easier means of reproducing photographs was also necessary.
The Printing Press
When each page of a book had to be carved by hand printing was limited to certain popular works that were sure to sell in large quantities. The huge investment of carving each page meant that once the tooling had been produced it was necessary to sell quite a large number of copies to make ends meet. The story that everyone knows is that this led to the bible being the only book published, it was apparently a sure thing to be able to sell a production run of "the good book". A few other books were however printed over the years, and smaller production runs of single page illustrations may also have been produced. After just a few hundred years of this limited use of the printing press Johannes Guttenberg so famously changed the world with his successful system of moveable type. As the story goes moveable type was known in China for some time, but it took the application of moveable type to the much smaller Latin alphabet for the technology to flourish.
The point that is usually made about moveable type is that books could be printed in smaller quantities, or less popular books could be printed just to test the market to see if they would sell. What turned out to be really significant about moveable type was the way that it allowed information to be disseminated over large distances, and this lead to adoption of a standardized written German language which had never existed before. Previously the only standardized written languages were the Greek and Latin from antiquity. As large numbers of people in Germany began to engage in literacy they naturally adapted their mother tongue printed works, and thus was born the first modern written language.
Moveable type was the "Twitter" of the European renaissance. When a piece of writing came to a town the local printer would read it, and if he liked it (or thought it would sell well) he would set type for his own version of the work. Because this was such a fast process it was in some cases only a matter of weeks before a popular or important piece of writing made it's way across the whole German speaking world. Because the entire printing industry was profit driven most printed work was in the form of sensational storytelling, and the wording of each story was often considerably changed by each printer that produced his own version of the work.
Eventually this sensational story telling form of mass media became the newspaper industry, and by the advent of photography in the mid 1800's daily or weekly productions were routine in cities and towns the world over. Particularly here in the United States though the newspaper industry took off as an absolutely huge driving force in society. One of the reasons for this was that the constant expansion westward into virgin land assured a cheap supply of wood for paper making. The large and expanding country was also the ideal market for fast mass media. Stories sent via telegraph were able to reach large portions of the country in as little as one day.
The incorporation of photographs into newspapers was slow to take off, but eventually became a universal practice. The main reason that photographs were only slowly adopted by newspapers is that the process of converting a black and white photograph into a plate suitable for running on a printing press is a tricky process which normally yields only mediocre results. The problem is that an etched plate on a printing press is not well able to reproduce gray shades, and the photographs tended to become high contrast silhouette representations of the original image.
With considerable skill and experience news paper photographic processors were able to get reasonably life like representations of certain types of black and white photographs to roll off the presses. It was not that the printing press was any good for photographic prints, but it was rather a case of the demand for cheap and quickly reproduced photographs was so huge. Including a photograph of some event made a news story much more compelling to many readers.
Artistic prints of photographs were also produced, and with great care and repetitive trial and error in the etching process, really quite good representations of highly detailed photographs could be reproduced cheaply in large numbers. The best etchings were still severely lacking in their ability to reproduce the rich variety of shades of gray present in photographic prints.
If getting shades of gray to come off of a printing press looking good reproducing color photographs with the printing press was a seemingly insurmountable challenge. The earliest color printing simply used three separate etchings that were exposed using color filters. In this way the information for a color print was stored on a set of three or four plates, but transferring this information to a piece of paper generally did not work well. The problem is that color inks generally don't look good when they are mixed together. This mixing of inks tended to work best for drawings and illustrations that were designed specifically for this type of multiple plate color printing. Color photographs reproduced with high contrast individual plates and the mixing of inks generally looked rather horrible.
The key to getting color prints to look good was avoiding the mixing of the inks, and this became known as offset printing. By the 1950's offset printing of full color photographs had advanced to the point that marketable products were commonly produced. The amount of labor involved with preparing the plates so that the inks would not mix was however substantial. Pretty much what it came down to was a process of hand carving the plates so that the final product looked somewhat like the original image. The thing about offset printing was that there was no limit to how good the results could be made given enough input of labor and the ability to precisely align the paper to each plate. It was the masterpieces of color photographic reproduction found in some books of the 1950's and 1960's that led to a situation where people expected that full color photographs could in fact be reproduced without the use of photographic paper.
It is not entirely clear anymore just when pigment deposition was first used, and it is also not entirely clear which technique of pigment deposition first took over the printing industry. The idea of pigment deposition is that some form of electronically controlled device "draws" the image on each sheet as opposed to a plate applying ink across an entire sheet. In any case removing the long and labor intensive process of hand carving plates allowed smaller production runs and also consistently high quality reproduction of full color photographs.
Somewhat ironically electronic controls can also be used to more quickly and precisely produce metal plates for offset printing, and the changing printing industry in the 1960's and 1970's probably made extensive use of a variety of different computerized printing techniques.
The basic idea behind laser printing is that pigment can be selectively glued to a piece of paper using a thermoset resin as the binder. One way that this is done is to use a laser to "draw" an image onto a roller so that each dot is an area of static charge. When brought into contact with the powdered toner each charged dot picks up a bit of toner that is later fixed to a piece of paper with an intense light source. With three or four separate rollers full color images can be quickly and cheaply reproduced. Smaller more tightly spaced dots allow higher resolution reproduction of images, but as with offset printing the alignment of each color must be precise enough that the dots do not fall on top of each other. The resolution of the final color print is the same as the resolution of each individual color of toner, but there are in fact two different ways that the dots can be put down that yield considerably different results.
For the purposes of the alignment of the machine there is some minimum spacing between the individual dots that will work without the dots overlapping more than a very small amount. If this minimum spacing is 300 dots of each color per inch then the maximum printing resolution is 300dpi. This 300dpi spacing has for quite some time been the industry standard, and all commercially produced toner is well able to operate at this minimum dot size.
The minimum dot size is extremely important as the size of the dots is varied to change the amount of each color of toner applied to each part of the image. The dots can be made large so that they nearly totally overlap for solid areas of one color, or the dots can be made much smaller so that only a small amount of one color is applied to a certain area. Mixing various size dots of the three or four colors of toner is what is able to reproduce lifelike shades of color. Because the dot size can be varied quite good photographic results are possible at the 300dpi print resolution.
Even though the minimum dot size for laser printing has been widely standardized not all laser printers have been able to operate at 300dpi output. Some laser printers lay the same size dots down at a spacing of 150 per inch for an output of 150dpi. This is very confusing because both types of printers use the same type of toner and attain roughly the same minimum dot size. The lower print resolution of the 150dpi printers however means that fine detail is not well represented, and sheets have to be viewed from back a ways for the photograph to look good. Because of problems with lack of fine detail some of the 150dpi printers have recently been beefed up to operate at 200dpi with the same inadequate every other position deposition of each individual toner color. This 200dpi output certainly does do a better job of representing fine detail, but the range of possible dot sizes then diminishes and problems with less than lifelike color gradients can crop up.
There is a reason that printing was always done at 300dpi with dots up to 1/300th of an inch in diameter. The bigger the range of possible dot sizes the easier it is to get colors to come out looking natural and lifelike. Just how good the output from these traditional 300dpi laser printers looked depended on how good the original photograph was as well as how precisely the dots could be aligned. For high speed production where the sheets zip through the machine at a high rate alignment becomes something of a problem, and much of the cheap magazine printing had rather sever problems with sloppy alignment. If the dots are slightly misaligned the printing process still works, but it is harder to get the colors to come out looking perfect and larger quantities of pigment tend to be required. If the dots are more precisely aligned then it is easier to get the desired colors, and excellent results can be attained with lower overall pigment coverage.
The promise of ink jet printing was a selective deposition process that could use roughly the same traditional ink systems as offset printing. The reason that this was appealing was that toner for laser printers has remained extremely expensive while ink is available at much lower prices for the same amount of pigment. The main problem with ink jet printing is that the dot sizes are not variable, meaning that it is a whole lot more difficult to get good color reproduction. The lack of variability in dot size results in a situation where 500 or 600dpi is required to reproduce photographs, and even at that it is extremely tricky to get the colors to come out looking correct. The lack of variability in dot sizes also tends to limit the maximum effective color resolution of inkjet printers, and many 500dpi inkjet printers can attain only about 250dpi representation of fine detail. The real problem though is the difficulty with reproduction of lifelike colors, and printers with extremely high resolution capability are required to produce acceptable photographic prints. The normal way that photographic printing is done with ink jet printers is to use printers capable of 600dpi or more to produce prints at about 100 to 200dpi effective output resolution.
The other main problem with ink jet printing is the severe limitations of the binder systems. The simplest form of ink jet printing is the evaporative solvent method. The ink is applied as a liquid, something evaporates out of the liquid causing the glue to "dry". This can be done with just water as the evaporating solvent, and there are several types of water based glues that can be used. Some water based glues are then not very water proof, and the image will melt if it is exposed to moisture. Polyurethane water based glues can dry to a hard and durable finish, but there is considerable confusion over what has to evaporate from polyurethane glues. In practice evaporative solvent ink jet printing stinks, and requires isolation of the fumes from working and living spaces.
Ink jet printing can also be done with thermoset resins, but this requires radically different handling techniques. The idea behind thermoset resin binders for ink jet printing is that a batch of ink has to be mixed up, loaded into the machine, used for printing and then cleaned out of the machine all within some reasonably narrow window of time. Just how much time is available for the use of the thermoset resin based ink depends on how fast the particular thermoset resin is and what type heat or light source is used to speed up the curing of the printed page once the ink is put to paper. A slow thermoset resin that requires baking at high temperature to cure might be able to be left in the machine at room temperature for up to weeks at a time. Since long periods of baking at high temperature is inconvenient, faster and lower temperature thermoset resins are generally preferable. A reasonable working time at room temperature would be anything from several hours to about one day.
The pigment would already be fully mixed with the resin, then a hardener of about one fifth to one third of the total volume of the finished ink would be mixed in to prepare the ink for use. This mixed ink would be loaded into the ink tanks of the inkjet printer at the beginning of a shift. Once the printer was primed and ready to go any number of prints could be made throughout the day. If new batches of thermoset resin are added the machine can continue to run without cleaning for long periods of time. Thermoset resins that will last for a whole day can be left in the machine over night, and printing can begin again in the morning. A more typical procedure would be for the machine to be cleaned with solvent at the end of the day or whenever the printing was completed.
While the resin is still liquid the cleaning is a simple matter of running just about any solvent through the print head and the nozzles. If the thermoset resin is allowed to harden in the print head and nozzles then cleaning becomes difficult or impossible. There are some powerful solvents that can dissolve cured thermoset resins, but these solvents are not the type of materials that one would want to work with. Generally speaking dissolving cured thermoset resins requires a solvent that is also well able to dissolve the flesh right off of fingers.
Aside from these inconveniences of working with evaporative solvent binders or liquid thermoset resins the main problem with high resolution ink jet printing is that it is a rather slow process. With the print head traveling back and forth across the sheet to lay down just a few lines of dots printing an 8x10 sheet takes many minutes versus the approximately one second printing time for the same sheet on a laser printer. Ink jet printers can be speeded up in a number of ways, but attaining high image quality at high print speeds is not easy with ink jet printing. If the print head has a larger number of nozzles for each color then fewer passes are required. The alignment of each pass however becomes more difficult the wider the line of printing is made. Really fast ink jet printers do not move the print head across the sheet, but rather have a line of nozzles capable of printing all the way across the sheet at the same time. These high speed ink jet printers can be nearly as fast as laser printing, and are sometimes used for production printing of newspapers or cheap magazines. The resolution of these whole sheet print heads is however typically quite low as even 100dpi across an eight inch sheet requires over 3000 individual nozzles.
However the pigment is laid down on paper and however it is glued in position there is ultimately one limitation on printing, and that is the cost of the pigments themselves. The most expensive form of printing is the exposure of photographic paper, often called chromomeric printing. The reason that chromogenic printing is so expensive is that the chemical compounds for the formation of colors must be present in the paper in large quantities. In fact chromogenic paper for color printing has so much pigment material in it that it is capable of being solid black, solid magenta, solid blue or solid yellow. Pigment deposition printing only uses as much pigment as is required to create the desired image on the sheet. For some images with large areas of white and light colors the amount of pigment required is in fact quite low. For darker images quite a bit of black pigment is required, but black is by far the cheapest of the pigment colors. For really good photographic prints of dark and intense colors less black pigment is used and rather large quantities of the color pigments are required. The darkest of photographic prints however still require only a small fraction of the pigment materials that a sheet of chromogenic paper contains.
How the pigment is applied and glued in place also makes a big difference in how much pigment will be required to cover a sheet. Laser printing is the most pigment frugal pigment deposition printing process. The variable dot size allows a lighter image to look much better with excellent color fidelity and smooth color gradients. The process of melting a thin layer of pigment and thermoset resin binder to the surface on the sheet also keeps the pigment use low. Ink jet printing with a liquid ink has the problem of the pigment soaking into the paper, requiring more ink and more pigment. If smooth surfaced paper is used so that the ink does not soak in then much lower quantities of pigment can be used for ink jet printing. Evaporative solvent ink jet printing can potentially produce dark solid colors with the lowest amount of pigment since the reduction in volume of the ink when the solvent evaporates tends to bring the pigment molecules to the surface where they are most easily seen.
The print resolution also has something to do with how much or how little pigment is required. If the print resolution is very low then any areas of the print that are light in color will not have smooth and natural looking color gradients. Printing darker hides problems with extremely low resolution, and allows the color gradients to look smoother and more natural. Print resolution and image resolution should not be confused in regard to pigment use. A high or low resolution image printed on a high resolution printer will have good color fidelity and generally naturally looking color gradients even when printed quite light. It is only the low print resolution that cause problems with color gradients and requires a higher pigment use to compensate.
That is however not to say that the image itself has nothing to do with how much pigment is used, because there are in fact a number of factors involved with image production and modification that can dramatically alter the pigment required to get a good photographic print. Obviously dark images require more pigment than light images, but most dark images can be modified to print lighter and look good with a much lower pigment use. As far as the images themselves go the primary culprit for excessive pigment use is images that are overly light in the light areas and overly dark in the dark areas. These images with unnaturally large differences between light and dark often have to be printed very dark to keep the light areas from becoming "blown out" in the final print. With the overall photograph printed extremely dark the dark areas end up as solid blocks of black and dark color that use a large amount of toner. Sometimes images with excessively light areas and excessively dark areas can be modified by darkening the light areas and lightening the dark areas so that the photograph can be printed lighter overall for a more even look and a lower pigment use.
The final point related to pigment use is that lower resolution printing processes will require larger size prints and correspondingly higher pigment and paper use to represent a full photograph. High resolution 300dpi 6x8 prints can do quite a good job at representing fairly high resolution photographs from 9 to 14mp advertised resolution cameras (4 to 6mp sensor resolution), but the resulting prints have to be viewed up close to see all of what is there. If the same 6mp image is printed on a 150dpi machine then representing the full photograph requires an 18 inch wide print that is quite cumbersome to handle and uses four times as much pigment.