An Impression by Any Other Name...
How do you define “impression?” Printers have been arguing about the definition of an impression for as long as we’ve had devices that put marks on pieces of paper. Longer, actually.
The earliest known forms of printing date back to 3000 B.C. and involved rolling an “impress” into clay tablets (according to Wikipedia). Modern printing continues to be based on the technology of bringing a substrate into contact with an image and somehow transferring—or pressing—that image onto the sheet. Some printing texts define “impression” as the amount of pressure that the press sheet receives, and controlling the amount of pressure required to create the optimum image is a critical component of press operation.
Somewhere along the way impressions began to be used as a measure of productivity. The amount of work done by a press over a period of time was determined by counting the impressions produced, and the definition an impression became one rotation of the plate cylinder.
Today, presses are rated by the number of impressions per hour they are capable of producing. When one describes the condition of an offset press, the number of impressions may be used as an estimate of the amount of wear and tear the press has experienced.
As copiers and digital print devices evolved into high-quality production devices, some manufacturers have adopted the term impression to measure the performance of digital print engines, and it’s not uncommon to hear a digital vendor measure production in impressions per hour. Businessdictionary.com defines an impression as the number of images (individual copies) produced in a print run. www.businessdictionary.com/definition/print-impressions.html
These definitions are fine as far as they go, and that’s the problem. They don’t go far enough. Is the impression from a 40˝, five-color press the same as the impression from a 29˝, two-color? What about the output from a 12x18˝ digital engine? And if you count a four-color press sheet as four impressions (four revolutions of the plate cylinders), how do you measure the same job if it’s run on a digital press? Ask any three printers to define impression and you’ll probably get three different answers.