Measuring fluorescent inks

People email me questions fairly reguarly. If I am in a good mood, I actually read the emails. Once in a while, I actually respond. On rare occasions, I actually try to answer the questions. The following is one actual interchange between me and an adoring fan... I took out all of the embarrassing stuff. 

The question

John,

A recent question came up for which I didn't have an answer. I immediately thought of you so here goes. Do you have any experience with reading florescent colors? I’m not really sure what types of devices might be out there (if any) for reading those really wild colors used in some printing and certainly more and more in fabrics.

I’m not sure how a profile would even be put together because the standard tools (100% maximum colors) don’t seem to apply. 

Do you know of any devices out there targeted toward florescent colors?

Thanks,

Mike

The response

Dear Mike,

This is a very difficult question. Well... the question is easy, but the answer is hard!

I have three different answers:

1. Scientific answer

The characterization of a fluorescent color is not a one-dimensional spectrum, but a two-dimensional one. You illuminate the same with a "monochromator", illuminating it at one wavelength at a time. For each wavelength in, you measure a full spectrum out. This gives you what is called a Donaldson matrix. From this you can predict the CIELAB value for any type of illumination possible.

You pretty much have to build your own spectro if you want to do this. I am sure you could do a decent job of $50K if you had a metrologist to help you. NIST has one and Avian has one. The equipment pretty much needs a technician in a lab to run it, it takes tens of minutes to make a measurement, and you probably have to write special software to interpret the results.

Clearly this is not a good production solution!!

2. Industry answer to a narrower problem

The print industry has faced a similar problem, but limited to one fluorescent pigment. Paper manufacturers currently add stilbene to virtually all paper. It's a cheap way to make paper that is whiter than white, or even just not dingy. Stilbene absorbs UV light and re-emits it in the blue region so as to undo the natural yellow or brown color of paper.

The standards groups huddled together and came out with a solution that is to standardize the UV content in viewing booths and in spectros. Coincidentally, I recently blogged on that topic.

This handles one fluorescent whitener. It was not intended for DayGlo orange or neon green.

3. Perhaps a practical solution

The color of a fluorescent sample will vary depending on the spectrum that it is illuminated with. It will look different under daylight versus incandescent lighting. But, so long as you restrict yourself to one illumination spectrum, there might not be a problem. If the printer and print buyer can agree that they will visually evaluate under a specific illumination and that they will measure with an instrument that has that same illumination, then everything should work.

[Note: Instruments have settings for different illuminants, such as D50, A, F11... This does not change the illumination, just the calculation afterwards. I blogged about that recently, too.]

The tough part (you would think) would be to find a viewing booth that uses the same illumination as a spectro. But actually, that's not so hard because of #2. Theoretically, you should be able to use a relatively new viewing booth (one that complies with the M1 condition in ISO 3664:2009), and a relatively new spectro (one which also complies with the M1 condition, but in ISO 13655:2009). All the stuff with then provide D50 illumination.

In practice, this may not be as easy as it sounds. One issue is that the instruments and viewing booths may simulate D50 in such a way as to have the correct numbers on paper with FWAs - on stilbene - but with somewhat different spectra.

I would suggest sticking just to one make and model of spectro, and one make and model of viewing booth. Unfortunately, I can't tell you which viewing booth and spectro will agree with each other. The vendors don't readily share this information.

I think at the very least, the practical solution might be to do away with any visual matching, and rely completely on measurement. You would measure a color with one of the M1 instruments (XRite eXact, Konica Minolta FD-7, Techkon SpectroDens, or Barbieri SpectroPad) and set that as the standard numbers and instrument.

The other part of you question has to do with profiling... That's a big "yikes"!  I am going to guess that almost all the fluorescent colors are well outside the gamut of all proofing devices, so, what good does it do to proof it?!? The best you could possibly do is use a softproof, and adjust something or other. I think you would scale the spectra of the whole profile to make sure that there are no points where the spectra goes above 100%.

John

Addendum

After I answered this email, I contacted DayGlo to see what they do for quality control. Here is what they said:

Visual light source is Daylight North Illumination (D65).

We measure the Color with a X-Rite Color i5 colorimeter.

We record the L*, a*, b*, DEcmc and De* for each color.

The measurements are done on the primary color test.

These can be either drawdowns or prints depending on the product.

So... they chose the practical solution.