Measuring OBAs
By Christopher Smyth, Assistant Professor, Ryerson University; and Gerry Gerlach, Color Management Expert & Member of Idealliance Print Properties Commitee
Several groups focused on printing and publishing responded to this challenge by updating standards and specifications.
The Magazine 34
WINTER 2017
Optical brightening agents (OBAs) are chemical additives used in the production of cosmetics, fab- rics, laundry detergents, papers, and more. They make things appear “whiter and brighter”. Used in paper coatings, they provide an economical and efficient way to help make stock more attractive to both print buyers and end users.
OBAs work by absorbing light energy in the ultra- violet (UV) parts of the electromagnetic spectrum (approximately 340–370 nm) and re-emitting this light energy in the “bluer” region of the spectrum (420–470 nm). This is what lets “yellowish” products appear whiter. On their own, the yellowish colored products reflect less blue light; the brighteners supply this light, and the printing appears whiter to our eyes.
Background The challenge is that conventional color manage- ment workflows did not necessarily take into account the UV portion of the spectrum. Different lamps and measuring devices had varying levels of support for UV and effectively “ignored” it.
ISO: ISO 3664:2009 and ISO 13655:2009 Published in 2009, in effect internationally in 2012, ISO 3664:2009 “Graphic technology and photogra- phy (viewing conditions)” was designed to support viewing conditions that included the UV portion of the industry standard D50 viewing condition. Previously, ISO-compliant lamps were not required to have a specified amount of UV light. As a result, higher-OBA sheets would not appear consistent in the viewing booth as the OBAs were not emitting the same level of light as outside of the booth.
Another change in 2009, ISO 13655:2009 “Graphic technology – spectral measurement and colori- metric computation for graphic arts images” was introduced to include additional measurement approaches in part to help accurately measure and predict the impact of UV light. New measurement classifications included the ability to use legacy unfiltered measurements (M0) and new “D50 full spectrum measurements” (M1).
Together, these changes offered better alignment between the viewing light and the instrument measuring light. However, the higher OBA content in some press sheets meant that the proof to press did not match—either visually or numerically. Press sheets would look blue (or the proofs look too yellow), due to the addition of UV light and the resulting blue re-emitted by the OBAs.
GRACoL 2013/CRPC-6 G7 GRACoL® 2013 (CGATS 21-2, CRPC-6) is a newer print characterization data set that was developed
TECHNOLOGY
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