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Tuesday, November 17, 2015

Color Measurement: What is UV Level (or source)?

When making color measurements it is important to understand the terms Illuminant, Observer and UV-Level (or source). There will be three separate blogs addressing each of these points.

UV Level (or source)

When making measurements on a spectrophotometer, many times different UV Levels or sources may be selected.  The UV Level determines the amount of UV that will physically strike the sample when the measurement is made. This is generally calibrated to one of three levels, D65, C or UV-EX. In most cases a pulsed xenon lamp us used and there is a cutoff filter which is adjusted in the calibration procedure to mimic the amount of UV from the appropriate source.

UV Level D65
As seen in the chart (right), D65 has a lot of UV energy, similar to outdoor daylight.

UV Level C
C has a moderate amount of UV energy, similar to office lighting e.g. some energy from fixed lights in the office and additional UV energy from light coming in through the windows.
Note: UV Level C is nearly identical to what is referred to as QTH.

UV-EX is UV-excluded, therefore, there is no UV energy when using this UV Level.

Ideally, the UV Level should be selected which matches the Illuminant that is being used, e.g. D65 Illuminant & D65 UV Level. However, some people choose different combinations.  The UV Level is important when measuring things with fluorescence. The UV energy will excite the fluorescing agent and provide a boost in the visible spectrum.

Since a change in UV Level is a physical change in the amount of UV energy striking the sample, multiple measurement must be made to get the results of a single specimen under different UV Level conditions. This cannot simply be calculated. It is important to understand, though, that color values obtained under one UV Level will be different than those obtained under a different UV Level, The one exception is where the sample is totally non-fluorescent. In that case measurements under D65, C and UV-EX will all yield the same results.

In the Paper Industry, UV Level D65 and C are very common. Typically, D65 is used to make process changes on the machine as it expands the measurement scale and allows for more fine tuning of chemicals. However, C is more commonly used in final specification setting since it relates to more common UV conditions that someone may encounter.  Finally, there are still instances where specifications are based on D65 and UV-EX, primarily for specialty products.

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