KB73960714: Color Engine Pilot - When to work with a UV filter on your spectrophotometer

Context

• When measuring color profiles in Color Engine Pilot, when can you use a UV-cut filter on your spectrophotometer?
• What is the influence of a UV filter on spectrophotometers?
  It seems that they are used to filter out and minimize the effect of fluorescing brighteners and whiteners used in some proofing and printing media.
• Would these whiteners and brighteners influence our CMS and could it influence the results on the proofer after applying Color Management?

Procedure

Normally, light falling on a substrate at a certain wavelength, is reflected at the same wavelength.

If the substrate contains optical whiteners then the UV-component (below 380nm) of the incoming light is reflected at a higher wavelength (typically in the blue region). The same happens with fluorescent inks.

If you put a UV filter on the spectrophotometer, then all incoming light below 400nm is filtered out and only light without the UV component will fall on the substrate. In the case of a fluorescent sample this will influence the measurement a great deal.

Now, what will the effect be on color matching?

When measuring an overprint chart (typically CMYK) it doesn't matter if you apply a UV-filter or not as long as you know what you are doing and don't mix.

If you measure a spot color without UV-filter, measure the prooferspace with an overprint chart and without using a UV-filter. Apply CMS, get a simulation of the targetcolor on the proofer and evaluate the proof in a lightbox with UV component ON then everything will be OK. It has no importance here if the substrate is fluorescent or not.

If you measure a press-colorspace by an overprint chart and with a UV filter, then too, apply a UV filter on the proofercalibration and do not select UV in the lightbox.

For the way the Esko Color Management system calculates overprints of inks on top of each other, it is important to rule-out fluorescence and to apply the UV filter in case of fluorescence. The Esko Color Management System is a spectral CMS. The absorption and scattering behavior of the inks is derived from the spectral data. If there is a higher reflection in the blue region because of fluorescence, then our CMS believes the ink is less absorbing in the blue region and the CMS is mislead. In some cases, the high reflection in the blue area can be wrongly interpreted as opacity.

Recommendation:

Always use a UV filter if you have fluorescent substrates or inks. After you start using it, you can even use it for the non-fluorescent substrates. This makes life easier. The only case where you should NOT use a UV filter for fluorescent material is, when you want a match under real daylight.