Dieter
Becker and Klaus Kasper
The printing of digital data with electronic non-impact printing technologies has evolved with advances in computer technology. Electrophotography, better known as Xerography, and thermal processes, initially designed for optical/analog copying, have been adapted to print digital information. As computer technology progressed from processing alpha numerics to graphics and finally images, xerographic and thermal printing processes, joined by ink jet, have steadily increased in print quality and speed. The printing processes and materials will be described and data on image stability presented.
Electrophotography, the technology underlying the xerographic process consists of six steps:
The printed toner image is fused to the surface (figure 1) of the substrate, most commonly an uncoated paper, and adhesion is critical. In general, electrophotographic prints remain sensitive to handling. Toner transfer to an adjacent sheet, due to the thermoplastic nature of the print, can occur during storage, especially at elevated temperatures and pressures. Hendriks has summarized the existing literature on "The stability and Preservation of Electrostatic Images". A paper by Inoue on the "Preservation Qualities of Color Electrophotographic Images", especially addresses color prints.
Fig. 1: Cross section of an electrographic print (paper thickness 116 µm). -> Text |
Black electrophotographic toners contain carbon black which assures the light stability of the image thus produced. The light stability of colored toner images depends on the pigment dyes used and can vary widely as shown in table 1.
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Table 1: Xenon accelerated fade test
(Blaszak 1994)
change ÆE (CIELAB) at 24 hours exposure (C =
Cyan, M = Magenta, Y = Yellow). ->
Text |
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C |
M |
Y |
C |
M |
Y |
|||||
|
Xerography |
Thermal Ink |
|||||||||
|
# 1 |
2.3 |
2.0 |
5.0 |
Special Paper |
# 1 |
2.3 |
5.5 |
8.0 |
||
|
# 2 |
2.3 |
15.5 |
1.9 |
Special Paper |
# 2 |
77.6 |
98.4 |
18.6 |
||
|
# 3 |
3.0 |
5.8 |
8.5 |
Glossy Paper |
# 2 |
70.8 |
95.4 |
41.2 |
||
|
# 4 |
3.1 |
4.6 |
9.7 |
Plain Paper |
# 2 |
42.4 |
55.9 |
7.7 |
||
|
# 5 |
1.8 |
15.5 |
2.5 |
|||||||
|
# 6 |
2.3 |
13.9 |
1.3 |
Thermal Wax Transfer |
# 1 |
0.8 |
2.0 |
4.7 |
||
|
# 7 |
1.9 |
1.1 |
0.8 |
Thermal Dye Transfer |
||||||
|
# 8 |
0.6 |
26.4 |
1.7 |
|||||||
|
# 9 |
1.2 |
1.7 |
0.8 |
# 1 |
8.0 |
3.6 |
6.1 |
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Photography |
# 2 |
2.1 |
3.3 |
2.5 |
||||||
|
# 3 |
6.0 |
14.9 |
9.6 |
|||||||
|
# 1 |
1.6 |
1.3 |
2.6 |
|||||||
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# 2 |
2.2 |
1.2 |
2.8 |
|||||||
|
# 3 |
3.1 |
5.9 |
8.5 |
|||||||
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# 4 |
1.1 |
2.5 |
0.7 |
|||||||
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# 5 |
2.0 |
2.2 |
1.1 |
|||||||
|
# 6 |
2.4 |
2.1 |
1.2 |
|||||||
|
# 7 |
1.7 |
1.6 |
2.1 |
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The better images are as light stable as photographic prints. Electrophotographic printing, generally understood as an office copying and printing technology, has sufficiently increased in quality and speed and the recently introduced digital color printing presses are targeting the short run offset printing market.
Print quality is not limited by the laser exposure, but toner particle size and the subsequent transfer and fusing cycle. It has steadily increased from readable copies to color print quality sufficient for a large part of the offset printing market.
Ink jet printing forms an image by depositing ink drops on a substrate. Ink jet printing technologies are usually classified as to their drop generation mechanism, thermal, continuous or piezoelectric, which has a strong bearing on ink formulations, paper requirements and image stability.
Thermal ink jet printers generate drops by activating a heating element inside the print head in rapid succession. The vapor bubble that develops ejects the ink drop (figure 2). Inks are water based and contain low concentrations of soluble dyes for colored inks and pigment for black inks. On uncoated papers the aqueous ink is absorbed into the paper base (figure 3) and tends to spread along the fibers, which results in lower color densities and reduced image sharpness.
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Fig. 2: Thermal ink jet print (source: Hewlet Packard). -> Text |
Fig. 3: Cross section of an ink jet print on uncoated
paper |
Coated papers retain the dyes on the surface (figure 4) and can be formulated to control spreading of the ink, resulting in better image quality and color densities. For photographic quality printing high gloss, resin coated photographic papers, with an ink receptive layer on the surface, are used (figure 5). Additives in the coating impart a degree of water fastness to the water soluble dyes. For outdoor exposure prints are generally overlaminated with film, but combinations of waterproof papers and inks have recently been introduced. The preservation of ink jet prints has been discussed at a Symposium on Photographic Conservation in Tokyo (Koike 1986).
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Fig 4:Cross section on an ink jet print on coated
paper |
Fig. 5: Cross section on an ink jet print on resin
coated paper |
Light stability is worse than for photographic and xerographic prints, although there are large differences between manufacturers, as shown in table 1. The situation should improve with the introduction of pigment dyes, expected in the near future. Pigment dyes tend to be more light stable than water soluble dyes. However, the nature of the drop generation process, which involves very high temperatures, severely limits ink formulation.
Thermal ink jet printers, because of their low cost, have taken over the low end office printing market and have made a new market for low volume, wide format graphics printing economically feasible.
Continuous ink jet printers generate drops by ejecting a continuous, fine stream of ink from a nozzle. Surface tension breaks up the stream into drops, which are electrically charged, with unwanted drops deflected by an electronic image signal. The process generates much smaller drops at high frequencies, making possible high quality printing through variable dot size by placing multiple drops per location. Inks are water based, similar to those for thermal printers, and so are paper requirements and image stability. These printers have found their market niche in direct digital proofing and photorealistic printing, especially for large format images.
The piezoelectric effect, the deflection of an asymmetric crystal under the application of an electric potential, is utilized by piezoelectric printers to eject the ink drop (figure 6). The mechanism is mechanical and no heat is involved, leaving a much wider latitude for ink formulation. Two very different ink formulations have been commercialized.
Fig. 6: Piezoelectric ink jet. -> Text |
One group of printers uses water based inks, similar to thermal ink jet inks. These printers require papers similar to thermal ink jet printers and image stability is comparable to thermal ink jet prints. The other group of printers, referred to as phase change or solid ink jet printers use a solid, wax based ink that is ejected in the molten state and solidifies when it hits the substrate. Solid ink jet printers are almost substrate independent. Adhesion may be a problem and, due to the thermoplastic nature of the ink, handling and storage characteristics should be similar to xerographic prints. Hot melt inks use pigment dyes which show improved light stability over soluble dyes.
Thermal printing forms an image under the application of heat. Images are formed, either directly by a chemical reaction in a layer on a substrate, or by transfer of a colored material from a donor ribbon to a receiver. The transferred material can be a molten, colored wax or dye molecules that diffuses from the donor to the receiver. The three different printing processes are depicted in figure 7. The heat is supplied by a thermal head, containing a linear array of heating elements. Electric energy, supplied by the electronic image signal, selectively energizes the heating elements, triggering the image forming reaction. The image is printed line by line.
Fig. 7: Thermal printing technologies. -> Text |
Direct thermal printing forms an image by melting a heat sensitive coating on the paper, allowing embedded chemicals to react, turning colorless components into a colored material. The process is reversible and the non-imaged areas remain heat sensitive. As a result the image is highly unstable and the background prone to discoloration. In spite of these drawbacks, direct thermal printing has found wide applications where the simplicity and reliability of the printing process are primary objectives and image stability is secondary. Examples are fax machines and the printing of sales receipts or airline boarding passes.
The process is capable of continuous tone printing and has found applications in higher quality printing, as for instance the recording of medical images. While addition of stabilizers and protective overcoatings help, the fact remains that the recording layers stay heat sensitive in the non-imaged areas. The only recommendation there is storage in a dry, cool place.
Fuji Photo has addressed the stability problem with its recently introduced Thermo Autochrome Printing System. This is a color process that prints the yellow, magenta and cyan color layers on the paper in three successive steps at increasing temperatures. The non-imaged areas in the yellow and magenta layers are deactivated after each printing step by exposure to ultra-violet light. This leaves only the unprinted areas in the more stable cyan layer heat sensitive. The photo realistic prints should be considerably more stable than conventional thermal prints, but may be susceptible to yellowing of the background. The system has just been introduced and it is too early to tell how it will be accepted.
Thermal wax transfer printing, also referred to as thermoplastic or mass transfer printing, forms the image through the transfer of a thermoplastic ink from a donor ribbon to a receiver. The ink melts under the application of heat and adheres to the receiver. Color prints require three separate passes. The ink image, as in electrophotographic and solid ink jet prints, consists of a thermoplastic adhered to a substrate (figure 8), most commonly a smooth, uncoated or coated standard paper. The same procedures suggested for the handling and storage of electrophotographic prints would apply. With ink and pigments similar to xerographic prints, light stability, as shown in table 1, is also comparable (Wilson 1992).
Thermal dye transfer printing, also referred to as dye sublimation printing, works like thermal wax transfer, with the exception that dye molecules, rather than a thermoplastic, are transferred from the donor into a receiving layer on the substrate. The amount of dye transferred is proportionate to the amount of energy applied. The energy supplied to each heating element can be varied over a wide range and the better printers print 256 gray levels at 300 dpi (dots per inch). This results in continuous tone photographic quality prints to which thermal dye transfer prints are frequently compared.
Continuous tone is unique to thermal dye transfer, direct thermal printing and photography. Because of the photographic appearance of thermal dye transfer prints they are frequently compared to photography, not only for image quality, but also image stability. In this respect they have been lacking, although recent improved products have addressed the drawbacks. After transfer the dyes are not fixed in the receiving layer. The image remains susceptible to contaminants and retransfer to other materials to which the dyes have an affinity. Polyvinylchloride films, a material frequently used for storing photographs, will accept transferred dyes easily and must be avoided.
Improvements have been made in recent products by adding an additional, protective layer to the three color layers of the print as a last step in the printing process. This layer contains UV absorbing agents that improve light stability as well as provide protection against retransfer, contaminants and fingerprints (Harrison 1994).
The papers used for thermal dye transfer prints are complicated structures. Visually they look like resin coated papers and there is some structural similarity as shown in their cross sections in figure 9 for a thermal dye transfer print and in figure 10 for a resin coated photographic color print. Both have a paper fiber core with resin laminated to both sides. The film on both sides of the thermal dye transfer print is a pigmented, foamed polyolefin film. The foamed structure provides thermal insulation during the thermal printing process by reducing heat loss into the receiver, as well as good compliance between donor and receiver during printing.
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Fig. 8: Cross section of a thermal wax transfer print (paper thickness 116 µm). -> Text |
Fig. 9: Cross section of a thermal dye transfer print (paper thickness 223 µm). -> Text |
Fig. 10: Cross section of resin coated photographic color print (paper thickness 236 µm). -> Text |
A dye receiving layer is applied to the face side. The resin coated photographic color paper has a clear polyethylene coating on the backside and a white pigmented polyethylene coating on the face side. The color emulsion is applied to the faceside.
Electronic non-impact printing technologies supplement and in some areas are starting to challenge established printing processes like offset lithography and photography. As a result, printed information will increasingly originate from these technologies. Most prints are produced by electrophotography, followed by ink jet, with thermal printing occupying specialized niche markets.
Originally designed to produce instant, printed information on demand, scant attention was given to print stability, but as the data presented show, this is changing. Developments are continuing and we can expect entirely new products to appear in the future. Fuji Photo's Autochrome Printing System is one such example. Given Fuji's position in the medical recording market it may find applications there.
The situation is complicated as electrophotographic toners and ink jet inks are available not only from printer manufacturers, but also a large number of refillers. Variations in print quality and image stability can be expected to be much larger than for photographic prints made only by a limited number of large manufacturers.
Blaszak, S. E.; et al. (1994): Lightfastness
in xerography and competitive technologies. In: IS&T's tenth
international congress on advances in non- impact printing
technologies. Proceedings 1994, pp. 566-568. ->
Text
Harrison, D. J.; et al. (1994): Image Stability
advances in thermal dye transfer printing. In: IS&T's tenth
international congress on advances in non- impact printing
technologies. Proceedings 1994, pp. 346-348. ->
Text
Hendriks, Klaus B. (1989): The stability and
preservation of electrostatic images. In: Imaging Processes and
Materials. Neblette's eights edition. New York, N.Y.: Van Nostrand
Reinhold, pp. 668-669.
Inoue, S. (1987): Preservation qualities of
color electrophotographic images. Nippon Shashin Gakkaishi (J. Japan
Soc. Photog.), 50 (3), pp. 244-248.
Koike, S.; et al. (1986): The preservation of
ink jet prints. In: Proceedings of the second symposium on
photographic conservation. Tokyo: Society for Photographic Science
and Technology of Japan, pp. 78-84. ->
Text
Wilson, D. E.; et al. (1992): Pigments and hot
melt inks. In: IS&T's eighths international congress on advances
in non-impact printing technologies. Proceedings 1992, pp.
293-295. -> Text
Dieter Becker studierte Chemie an der Justus Liebig Universität in Giessen. Er promovierte dort und arbeitete anschliessend in der Entwicklung bei Felix Schoeller jr. auf den Gebieten von Papierträgermaterialien für nichtfotografische Bebilderungsverfahren wie Dye-Diffusion-Thermal-Transfer-Papieren, Fotopapieren und gestrichenen Spezialpapieren für Ink-Jet sowie Papierträgern für Analog-Proof-Verfahren. Seit 1995 ist er Geschäftsführer der neu gegründeten Felix Schoeller Digital Imaging GmbH und mit dem Auf- und Ausbau des Geschäftsbereichs Digital Imaging betraut.
Klaus Kasper promovierte am physikalisch-chemischen Institut der Technischen Hochschule Stuttgart. Er hatte, bevor er als Berater für Felix Schoeller auf dem Gebiet der Beurteilung digitaler Bebilderungstechniken tätig wurde, in dieser Firma verschiedene Managementfunktionen inne und war dort langjähriger technischer Direktor. Er lebt seit den 50er Jahren in den USA.
Aus: Rundbrief Fotografie, Sonderheft 3, S. 10-14 (auch in N.F. 12, S. 10*-14*).
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