Densitometry Basics for Production Control

Densitometers are instruments that determine the amount of light absorbed by a surface. They do this by comparing the intensity of the light that reflects off (or transmits through) a surface to the intensity of the light upon it, and then calculating the density through a mathematical relationship.

Transmission densitometers measure the amount of light that is transmitted through transparent material, such as film. Reflection densitometers measure the amount of light reflected, and thus are a critical aid in quality control in the printing production process.

In printing and publishing, reflection densitometers are used extensively in prepress and pressroom operations by ad agencies, publishers, color separators, and printers. Densitometers allow print professionals to communicate process information through objective numbers rather than seat-of-the-pants reckoning.

Wide-band densitometers usually are preferred to narrow-band versions for most situations. Wide-band densitometers provide functional measurements, which in turn correspond to attributes of the printing system such as ink film thickness, ink laydown, print contrast, and dot gain. This information can be used for statistical process control.

Says David Hultin, Graphic Communications Association (GCA) print media project manager, "In GRACoL, SNAP, and SWOP, ink density, print contrast, and dot gain are critical features in defining quality printing."

In 1984, the American National Standards Institute (ANSI) Committee PH2 (now IT2) finalized a spectral response for graphic arts reflection equipment, naming it Status T. Status T defines the response for the measurement of off-press proofs, press proofs, press sheets, and other graphic arts materials being measured using wide-band equipment.

Says Miles Southworth, industry consultant and professor emeritus at Rochester (N.Y.) Institute of Technology, "The densitometer is one of the most important and widely used instruments in the graphic arts today. Any attempt to control production without it would be hopeless. It has become the lifeblood of the quality control department everywhere. It should be as accurate as possible.

"More importantly, each instrument should provide readings that are able to be compared with readings taken by every other instrument. Printers need to be able to confidently compare readings, exchange information, and know that they are speaking the same language."

To let densitometer users know how close their densitometer comes to true Status T wide-band reflection, and to provide accuracy between equipment, production people, or printing plants, GCA developed the T-Ref reference system.

"It's simple," states Hultin. "You measure some CMYK sample patches on the T-Ref that have previously calibrated to ANSI Status T. If your wide-band densitometer doesn't match the calibrated values, something's up, and you may need to re-calibrate.

"GCA actually provides two categories of references, T-Ref and Lab-Ref, to aid end users in comparing the response of their instruments to the response of instruments which are traceable to accepted standards laboratories."

Says Bob Hutchinson of Pittsford, N.Y.-based In T'Hutch, Ltd., a firm specializing in certifying compliance with standards, "The T-Ref is printed on specified paper stock with SWOP ink by specially selected printers of exacting standards, and is aimed to hit the center point of SWOP ink reflection standards. After lamination, the T-Ref is certified using calibrated and controlled spectrophotometers. The Status T densities are calculated from the spectral data."

Hutchinson continues, "Lab-Ref is designed to allow colorimeter, spectrophotometer, and hybrid color measurement device users to verify that their equipment is capable of measuring colors accurately in terms of the International Commission of Illumination specified values."

Larry Steele, a technologist at RGB Metrology, a Webster, N.Y.-based company that specializes in measurement of process parameters, says, "The GCA T-Ref and Lab-Ref material references are very important in establishing accurate and repeatable measurements by all areas in the graphic arts industry where measurements are made of hard-copy images.

"Those in prepress and production printing environments can make more accurate and repeatable measurements as well as establish agreement between instruments. This is especially important these days as the industry moves towards reducing tolerances and implementing statistical process control."

Hultin concludes, "GCA recommends the use of U.S. standards developed through CGATS, ergo, the development of T-Ref and Lab-Ref. We are working on establishing GRACoL proofing specifications through the CGATS formal report process, similar to SWOP and SNAP, in support of industrywide consensus for quality printing."