How to check for optical brighteners

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This reserved article originally appeared in CHROMiX ColorNews Issue 53 on October 23, 2013.

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Contents

Optical Brighteners

Printing papers, scorpions and counterfeit wine bottles...optical brighteners have been popping up in more places in recent years. It's a very exciting topic in more ways than one.

Presses are running more media containing optical brighteners than ever before. The ISO 3664 standard just last year required viewing booths to have more ultraviolet light in their lamps, which will bring out more whiteners in the papers. Photographers are choosing to print onto media with more optical brighteners despite the availability of papers without them. Just this week I got a call from a wine merchant who is using ultraviolet lights to look for optical brightening agents in counterfeit wine labels.

With so much activity around this subject, it's useful to know what they are and how to detect them. This article will present a few practical ways to determine when optical brightening agents are present in your media as well as some suggestions about how to determine how much is present.

Definition

Optical brightening agents (OBA's) are additives that paper manufacturers put into paper in order to help a paper look "whiter." These artificial whiteners are chemicals which can take invisible ultraviolet light and cause it to re-emit - or fluoresce - in the nearby blue spectrum, at a wavelength that is just barely within our ability to see, around 430-460 nanometers. While our eyes see this as a brighter, blue-ish white - a light measuring instrument will only see this as a different form of blue. That is why printer profiles made with paper using a lot of optical brighteners can end up printing out images that have a yellow tint to them. The profile is trying to correct for what it sees as too much blue in the paper. If a paper visually appears to be a bright white, it will likely have some OBA's in it. This is not true in every case. Paper without OBA's (or very little) include "silver halide" RA-4 process photographic paper, certain press proofing papers, and specific fine art papers marketed as having a "warm tone".

Especially in the area of inkjet media, a number of high quality art papers look fairly bright to the eye, but actually have no OBA's in them. This is part of what makes these papers high quality and appreciated for the longevity of any image you put on them - and more expensive, since the manufacturers have to come up with other ways to achieve this brightness. Just a few examples are Hahnemuhle Photo Rag, Canson Fine Art Watercolor, and Crane Museo papers. Popular papers like Epson Ultra Premium Photo Luster and Red River Ultra Pro Satin show a very strong white, but have very few OBA's in them.

Black lights

Example of a retail black light flashlight.

In a high volume environment, when you are wanting to make quick decisions about a large variety of papers, a simple black light comes in very handy. It used to be hard to find a small, handheld fluorescent fixture that contained a black light tube. Nowadays, it's easy to pick up flashlights that contain ultraviolet LED's. The lamps are made to produce ultraviolet light while blocking most of the visible light. When this light source comes in close proximity to optical brightening agents, the light will "excite" the OBA's in the paper, and the surface will fluoresce with a bright blue glow. When first trying this, have a sample of known OBA-free paper on hand to compare to some everyday bond copy paper. The copy paper will show the tell-tale glow while the "natural" paper will hardly react at all.

These lights can be readily purchased online. For a local source, try your neighborhood pet supply store. These are often sold as pet urine detectors. Or if you live in Arizona, UV flashlights are sold to hunt down scorpions in the dark, as they fluoresce under UV light. (I'm going to have to try this when I get to Phoenix this December!)

Lab Values

A high negative b number can indicate the presence of OBA's

A more quantifiable method for determining how much OBA's you have is to look at the L*a*b value of the paper white of your stock. It is frequently true that the higher the negative "b" value of your Lab measurement, the higher the amount of OBA's in your paper. This will be true as long as your paper visually looks to be fairly neutral in color, and when you are using a measurement device that uses the full spectrum of light (without a UV-cut filter).

For example, if your paper white measurement returns a Lab value of 97, 1,-1 then you probably don't have OBA's or very few (if your paper looks white and you're not using a UV-cut instrument). If your Lab value is 97, 0, -6 then your measurement instrument is likely picking up the fluorescing of your OBA's and giving you a high -b value.

More Precise Results

A number of instruments can measure with UV light included as well as UV excluded. Among these are the X-Rite iSis, eXact, i1Pro 2, and the Konica Minolta FD-7. Using one of these instruments, measure the paper white both ways, and then compare the two measurements. A paper with no OBA's will have a very similar result regardless of which filter is used. (Some difference will be expected due to normal instrument variation.) For papers with a large delta E difference - especially in the whites, this will be directly related to how much the OBA's have been excited by the UV light.

Spectral Histogram

Spectral Graph of white in MeasureTool Spectral Graph of white in Maxwell - no OBA's Spectral Graph of white in Maxwell - OBA's included


Finally, a sure indicator of the presence of OBA's can be gleaned from spectral analysis of a paper white measurement. Remarkably, paper that has OBA's will show over 100% of the light's reflectance in the blue spectrum. This means the instrument is capturing more light returning from the paper than is being delivered to (shone on) the paper! This is a demonstration of what I mentioned earlier. Light below this wavelength is being converted to a wavelength that we can see, is re-emitting and adding to the light. The online color repository, Maxwell, provides a histogram like this, as well as the discontinued MeasureTool program. Note that the measurement must contain spectral values rather than Lab-only values. Bring up a histogram of the paper white patch, and check out the wavelength at about 440nm. If there is an obvious blue hump there, and especially if it is reporting more than 100% of the reflectance, then you know that white is getting a little extra help.

Whether you need to quickly determine which paper stocks have OBA's in them, or if you want to find out which stock in the past had been printed with OBA's, there are several methods here that will fit your needs in an "exciting" way.

Thanks for reading,

Patrick Herold


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