Photographs
5.2.3 Types of Photographs, part 3: Digital Output Media

Monique Fischer
Senior Photograph Conservator
Northeast Document Conservation Center

introduction

This leaflet introduces the basic types and processes of digital output media and serves as a primer for the identification of these materials. Further readings, such as NEDCC’s preservation leaflets 5.4 Creating Long-lasting Inkjet Prints and 5.5 Storage Enclosures for Photographic Materials, and resources listed below provide more information on the care and identification of digital output media. 

PRINTING TECHNOLOGIES

There are a variety of output devices that can be used to print digital image files. The most common ones include the digital photo process, dye sublimation, electrography, and the inkjet, which came into significant use in 1998.

The photothermographic transfer (examples include the Fuji Pictrography) is a high-end, large-format device used in many photo labs to print snapshots. The process, a combination of photographic and thermal dye diffusion methods, involves exposing a sheet of photosensitive “donor” paper to laser diodes (LD). Small amounts of water or heat are applied to create the dye image on the donor paper, which is then transferred to the “receiving” paper with a combination of heat and pressure. The receiving paper with its transferred dyes is peeled off and separated from the donor paper.

The digital silver halide process (examples include Lambda and Kodak Pegasus) uses light sensitive paper exposed by lasers or LEDs and then chemically processed. There is no negative involved. It is still a wet process and is considered a chromogenic color print.

Dye sublimation, used by desktop units by Canon Selphy, and kiosk printers by Kodak Picture Maker, work with a single-color ribbon containing dye heated by a special head that runs the width of the paper. When the head heats up, it vaporizes (sublimates) the dye in that location. The dye, now in gaseous form, is absorbed into the paper. Since the paper receives the dye layers separately, the print can result in a smooth, seamless image.

Electrography includes laser prints and photocopies. In this process, the toner is transferred to an uncoated paper base then fused into place. The images composed of pigment particles are generally stable but are not often used for photo-quality printing.

Inkjet is the most widely used printing technology for digital fine art prints. Inkjet systems are based on the flow of colored ink from a nozzle that is deposited on a support to form an image. There are two types of technologies for inkjet printers: continuous flow and drop-on-demand or impulse jet. The Iris printer and popular Epson Stylus® Photo printers are the best known of these systems.

Continuous-flow inkjet printers use an electrostatic charge to push ink out of the printhead reservoir. As the ink droplets are released, charged droplets are deflected and recycled while the uncharged particles spray a continuous stream of microscopic ink droplets onto a flat substrate. The Iris printer is an example of the continuous-flow printer.

The other type of printer, drop-on-demand, uses only ink droplets needed to form the image produced. There are two main types: thermal and piezoelectric. The thermal process, used by the Canon Bubble Jet Printer, is based on heating a resister in the printhead. As the printhead heats up, a bubble is produced and the increased pressure inside the printhead chamber forces the ink droplet out. After the bubble collapses, more ink is drawn from the reservoir. The piezoelectric effect (employed by Epson printers) uses a crystalline material inside the printhead reservoir to create an electric field, which produces the pressure instead of heat to release the ink.¹

DEFINITIONS

First, a few useful terms and concepts: 

Coated paper: paper that has coatings (microporous or swellable) which aid in receiving inks, create higher color-range for pigment inks, and create better image quality as well as greater brightness and ink stability. 

Gloss Differential: Variation in gloss across the surface of the print. 

Satellites: Unintended specks or drops of colorant outside the printed dot shape. 

identification

To identify the printing technology used to make a digital print, examine the dot formation or image grain under magnification using a 60-120X LED lighted pocket or hand-held microscope (for example, the Carson MicroBrite Plus). View the print at 30X magnification and compare it to the examples below.

Other distinguishing characteristics include the print’s primary uses and format, and the gloss differential.  

• Aardenburg Imaging Research. https://www.aardenburg-imaging.com/research/
• Image Permanence Institute. Digital Print Preservation Portal. http://www.dp3project.org/
• Image Permanence Institute. Graphics Atlas. http://www.graphicsatlas.org/
• Jürgens, Martin. The Digital Print: Identification and Preservation. The Getty Conservation Institute. 2009.
• NEDCC Preservation Leaflet 5.4 Creating Long-lasting Inkjet Prints.
• NEDCC Preservation Leaflet 5.5 Storage Enclosures for Photographic Materials.
• Wilhelm Imaging Research. http://www.wilhelm-research.com/

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