Optical Brighteners in Paper
ISO 3664:2009 is the international standard for “graphic technology and photography-viewing conditions”. ISO 3664:2009 specifies the requirement of good quality visible energy and UV energy for all visual observations. The standard illuminant D50 — daylight at 5000K with a UV component — was chosen to provide viewing conditions for proofing and on press that simulate real life daylight viewing conditions.
When ISO 3664 was updated in 2009, optical brightening agents (OBA) used in paper to enhance the whiteness of the substrate became a larger issue. High quality and coated proofing paper no longer matched the press paper. And press paper containing different concentrations of OBA could have a bluer or yellower appearance under a light source with UV energy.
The amount of variation could be different from batch to batch, or from manufacturer to manufacturer. The use of ultraviolet energy in the D50 daylight source did not create this problem, but merely exposed a variable that was previously there and never addressed. In addition to the quality of paper, the level of whiteness needs to be evaluated and controlled by selection throughout the specification and incoming inspection process.
The use of optical brightening agents is not new. They have been used in paper, plastics and textiles for years. The UV component in a light source will also excite fluorescent dyes and pigments used in printing inks to enhance the color gamut range of the printing process. OBA’s absorb UV energy in the near UV area of the spectrum, between 365 and 400nm, and re-emit this energy as blue light, enhancing the perceived whiteness of the paper. It is a fact that the human observer prefers bluer whites to yellower whites, and a bluer white is characterized as a brighter or cleaner white. Most brighteners will fade over time with exposure to UV energy and oxygen.
Recycled paper can have varying amounts of OBA. Naturally, if the amount of optically recycled paper is not controlled, then the appearance of the paper under a UV enhanced daylight source can look spotty or vary in brightness. The way to control this variable is to use a viewing booth that has a light source with UV energy, so subtle differences can be seen and evaluated.
The GTI (Booth 2208) ColorMatcher and MiniMatcher have separate UV lamps that exaggerate the presence of optical brighteners, whitening agents, fluorescent dyes and fluorescent pigments. These products are excellent tools for exposing differences in substrates during incoming inspection. UV can also be added as a second source on GTI CVX and EVS viewing stations.
Spectrophotometers have been developed with ultraviolet light sources that allow the UV component to be switched on or off. Some instruments have light sources with a UV component. They use a UV blocking filter that can be switched in or out, and in some cases be partially switched in to not only determine the presence of OBA but also quantify the amount.
The use of an ISO 3664:2009 compliant viewing booth is an important tool to evaluate and control the appearance of optical brighteners in printed products. GTI provides a full range of products including floor models, desk top viewers and luminaires. All include D50 daylight lamps that boast a 95 CRI and a CIE assessment of B/C or better. These lamps utilize a nine phosphor technology developed to exceed the colorimetric requirements of ISO 3664:2009. These “E” series lamps contain a little more non-harmful ultraviolet energy than the previous lamps; this enhances the presence of optical brighteners found in some press papers.