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Color Measurement With a UV Filter
Introduction
Opinions on how color measurement should be performed with or without a UV-cut filter on a spectrophotometer differ greatly in the market. This document explains how ultraviolet (UV) light interacts with ink and paper to affect the accuracy of color measurements. It concludes with recommendations on best practices for day-to-day application.
Ultraviolet Radiant Energy
Visible light is a type of energy that can be described by the length of its waves. The human eye is sensitive to wavelengths between 380 and 750 nanometers (nm). Ultraviolet light is invisible and has wavelengths less than 380 nm. Various levels of ultraviolet light are emitted from most sources of light, such as the sun, tungsten lamps, fluorescent bulbs, and so on. The standard light sources such as D50 and D65 are defined by their spectral power distribution, and cannot be artificially produced exactly. Different manufactures of light sources designed to copy D50 and D65 vary in the amount of ultraviolet light that they emit.
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Paper and Ink
Although ultraviolet light itself is invisible, its presence can be seen when it interacts with a surface that contains fluorescent material (chromatic fluorescent colorants, optical brighteners, or fluorescent whitening agents) such as paper and ink. Fluorescent materials have the unique ability to absorb invisible ultraviolet light and emit 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.
Paper manufacturers add fluorescent materials to their paper in order to increase the apparent brightness or look of the media, and to increase the range of colors that can be produced when ink is applied to it. Most, if not all, of the bright white type 1 or type 2 stocks used in printing contain these agents. Many proofing stocks contain them as well. The following chart shows spectral measurements of two different papers: one with and one without UV brighteners. The paper with UV brighteners is shown with the red line. It has spectral values that exceed a reflectance of 1. This makes the paper appear to reflect more light than it is actually taking in. The “extra” light comes from the invisible UV region and is “converted” to visible light at around 430 nm by the UV brighteners. The paper without the UV brighteners (shown with the dotted line) has a relatively flat spectral response and never exceeds a reflectance of 1.
Spectral ReflectanceMedia with(red line) and without(gray dots) Fluorescent Brighteners
Ink manufactures add fluorescent agents to their inks in order to increase the brightness or chroma of a particular colorant. Many of the high-color or large-gamut ink sets used on printing presses contain these agents.
Color Measurement
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 spectrophotometers 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. Even though the media shown in the previous chart with the red line has a bright white appearance to the human eye, the measured color has a CIE b* value of –5.50. This would indicate that the color appears blue. If you try to create a profile with measurements on this paper, the profile will add a cyan tint to the paper white value in an attempt to match this color. The result would be far from ideal. The media without fluorescent brighteners in the previous chart (shown with the dotted line) appears to be yellow. Its measured CIE b* value is 3.76, which agrees with the visual appearance of the media. Profiling a printer using this media produces good visual results.
Another problem is that different light booths can contain slightly different light sources, which can use different levels of ultraviolet light. This means that the appearance of fluorescent colors will change in different light booths, creating a problem for color matching. The fluorescent media in the previous example would change appearance in a light booth with more or less ultraviolet light; whereas, the non-fluorescent media would not change appearance with more or less ultraviolet light.
Some proofing manufacturers have chosen to solve the challenge of fluorescence using the following techniques in combination:
Technique One: Eliminate Fluorescence:
1. Filter the UV light in the light booth with a UV-absorbing material such as Plexiglas®.
2. Select proofing media that do not contain fluorescent brighteners.
3. Formulate inks that do not contain fluorescent brighteners or colorants.
Although this method is effective, it is not always practical.
Technique Two: Use Fluorescent Media and Inks but Remove the Effect With a Software Algorithm:
1. Allow UV light in the light booth to vary.
2. Allow media to contain fluorescent brighteners.
3. Allow inks to contain fluorescent brighteners or colorants.
4. Estimate the effect of fluorescence and remove it from the spectral measurements with a software algorithm.
This method is convenient for the user but still has its challenges. The appearance of the proof may vary in different light booths, depending on the light tube manufacturer. The algorithms are only accurate if they are designed for specific ink and media combinations, because some fluorescing media and ink react in unpredictable ways to UV light. These challenges may introduce measurement errors that can produce poor profile results.
The best solution would be to accurately measure the effect of fluorescence with a bi-spectral spectrophotometer. This device can measure the spectral reflectance at a sampling of visible wavelengths with different light sources (each light source can output a specific range of wavelengths from the electromagnetic spectrum). Using a bi-spectral spectrophotometer, one can predict how the human eye actually perceives the fluorescent material under different lighting conditions. Unfortunately, this device is currently no longer commercially available.
Technique Three: Use Fluorescent Media and Inks but Remove the Effect With a UV Filter
1. Allow UV light in the light booth to vary.
2. Allow media to contain fluorescent brighteners.
3. Allow inks to contain fluorescent brighteners or colorants.
4. Use a measurement device with a UV filter to remove the effect of fluorescence from the spectral measurements.
2. Allow media to contain fluorescent brighteners.
3. Allow inks to contain fluorescent brighteners or colorants.
4. Use a measurement device with a UV filter to remove the effect of fluorescence from the spectral measurements.
UV-cut filters 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 following chart shows the effect of measuring a fluorescent media both with and without UV filtration. The two measurements shown are 5.8 dE*(ab) units 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.
Spectral ReflectanceFluorescing media measured with and without UV Filtration
Measurements of a paper that did not contain fluorescent brighteners, both with and without UV filtration, would be much closer—less than or equal to 1 dE*(ab).
Technique Four: Specially Formulate Fluorescent Media and Inks to Match Source Paper Stocks
1. Allow UV light in the light booth to vary.
2. Specially formulate media to contain the same fluorescent brighteners found in the paper stocks that the profile will be targeted to simulate. Select brighteners that are very stable over long periods of time.
3. Specially formulate inks to contain the same fluorescent brighteners or colorants used in the target color device. Select brighteners that are very stable over long periods of time.
4. Use a measurement device with or without a UV filter to measure color.
In this method, the fluorescent brighteners will look alike under any lighting situation because both papers and/or media contain the same type and levels of brightener. An example of this specially formulated media is the Kodak® Pro PressWhite media. This method may be costly for the manufacturer, but can provide good profile results.
Color Management
An ICC profile made using the fluorescent paper in the previous chart and UV-filtered color measurements would be significantly different than one made with non-filtered color measurements. The profile made from unfiltered measurements would produce a simulation with an overall cyan-bluish cast. The one made from the filtered measurements would produce a simulation that was closer to the correct visual appearance.
Non-fluorescent papers and papers that contain the same type and amount of stable fluorescent brighteners do not have this problem. These papers would produce visually similar profiles from both UV-filtered and non-filtered color measurements.
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.
Kodak manufactures its Pro PressWhite proofing paper according to strict guidelines, which ensures the same level of fluorescent brighteners from batch to batch. Storage instructions describe how to maintain the fluorescent brighteners over time.
Recommendations From Kodak
None of the above solutions completely solves the problem that fluorescent brighteners introduce. Avoiding the use of fluorescent brighteners is not always practical. Software correction can be accurate only when it is closely correlated with specific media. UV-filtered measurements are another option.
In order to obtain the best results, Kodak strives to remove the effects of fluorescence from color measurement and viewing, both for calibration and profile making. With the technology available today, the best means of accomplishing this is by doing the following:
1. Filter the UV light in all your light booths with a UV-absorbing material such as Plexiglas.
2. Perform all color measurements with a UV-filtered color measurement device.
In specific cases where proofing standards require unfiltered measurements, Kodak uses a smart algorithm—designed for the specific media—to remove the effects of the UV-filtered color measurement.