ID Tips: Offset vs. Liquid-toner Electrophotography

While liquid-toner electrophotography does share similar dot patterns and appearances with offset lithography, there are tell-tale clues to help differentiate the two. The first is the date the print was made. While liquid-toner as a technology had been experimented with even prior to the digital print age, it didn’t really come into its own until the Indigo Digital Press was introduced in 1993. Since then, this technology has been used by the commercial printing industry to fulfill jobs both large and small, and with quality ranging from low-end disposable advertisements to high-end photography and art reproduction books.

If a print in question was created after 1993 or if the date of creation is unknown, then a microscope, along with some practice, should provide all the information necessary to make an accurate ID. Optimal magnification will be from 50-100x.

The second clue to look for is dot morphology. Morphology refers to the shape of the half-tone dot. An offset press compresses the ink first against the surface of a transfer blanket and from there to the paper, giving the dot a squished-looking ragged edge. The liquid-toner dot on the other hand is formed by the pooling of liquid on the blanket and is mostly dried before being transferred to the paper allowing the dot to be more circular in shape.
 

The third clue to look for is the presence of satellites. Satellites are small specks of colorant between the intended imaging forming dots or in white areas of the print. These are rare in offset and common in liquid-toner electrophotography. These appear because the either the photo-conductor blanket was not fully cleaned or discharged before printing the next image or because drops of colorants crossed from the printer’s developer station to the blanket in an uncontrolled way. Note however, that dots in offset prints can sometimes be very small as are the black dots in the micrograph of the offset image above. These small dots are not satellites and will always show a regular pattern.

The last sign to look for is partial dots at image edges. When offset plates are designed on computers, partial dots can be created in order to give a fine edge to pictures that do not take up the entire page. Liquid-toner, when applied to the blanket, will always form round dots, so image boundaries will show a less defined edge under magnification.

While finding any one of these clues in a print should be enough to confidently identify the print as offset or liquid-toner electrophotography, finding two or all three will give you full assurance that you’ve made the right determination.