UV vs Non-UV Spectrometers

Opinions on how color measurement should be performed with or without a UV-cut filter on a spectrophotometer differ greatly in the market.

Optical Brightener Agent (OBA)
Paper manufacturers add UV brighteners or fluorescent materials to their paper to increase the apparent brightness of the media and the range of colors that can be produced when ink is applied to it.

Fluorescent materials have the unique characteristic in that it absorbs invisible ultraviolet light and emits the light into the visible region. When measured with a spectrophotometer, the effect is that some wavelengths will be unusually high—in some cases resulting in more than 100% reflectance, which is higher than expected. Although fluorescent brighteners may improve the appearance of printed materials, they present a problem for color measurement and color matching. The light source (usually tungsten) used in color measurement devices such as spectrophotometer does not contain the same amount of ultraviolet light as the light source used to view the printed material (fluorescent light bulbs simulating D50). This introduces a discrepancy between measured color and viewed color, even under laboratory conditions.

Spectrometer with UV-cut Filter

The use of UV-cut filters in the spectrometer is to remove the UV component of the light source in the instrument so that the effect of the fluorescent brighteners or colorants is excluded from the color measurements. The above charts show the effect of measuring a fluorescent media both with and without UV filtration. The two measurements shown are 4.34 dE2000 apart. The UV-filtered measurement data suggests that the paper is more neutral; the unfiltered measurement data suggests that the paper is cyan-bluish in hue. When viewed in a light booth with the proper light source, the paper appears bright and neutral, not cyan-bluish.

Color Calibration
Fluorescent brighteners present a challenge for color calibration. Some fluorescent brighteners can become unstable over time and with exposure to light, if stored improperly. Careful storage maintains the level of fluorescent brighteners and ensures measurements for color calibration are accurate.

If the fluorescent brightener shifts over time, the longer the paper sits on the shelf, the darker it becomes (brighteners can lose their effectiveness with time). So, if your color measurement device does not have a UV filter, it will measure the colors of the output device as darker. The software may try to lower the amount of ink in order to compensate for this—or produce an erroneous “out-of-tolerance” message as a result.

If the fluorescent brightener shifts from batch to batch (or with different lots of paper) this would affect calibration as well. Without UV filtration in the color measurements, some batches may measure brighter than others. Again, the software may try to compensate for this even though the output device has not shifted.

Compensation for UV Brighteners
There are two ways to compensate for UV brighteners and get a normal-looking profile. One is to attach a UV filter to the spectrophotometer. The i1, DTP41, DTP45, and Pulse spectrophotometer are available with a UV filter, but they must be ordered with this feature and cannot be changed by the user. The Spectrolino spectrophotometer has an accessory UV filter that can be changed by the user. The DTP70 and iSis both have user switchable UV options.

Alternatively, effect of UV brighteners can be corrected by using a profiling program with built-in UV compensation feature. Both ProfileMaker 5 and i1Match 2 include software compensation for UV brighteners. With these programs it is not necessary to use a UV filter.