The ColorFlex L2 uses controlled UV illumination to improve measurement consistency when samples contain optical brighteners or other fluorescent materials. This article explains how UV energy affects fluorescence and color measurement, how UV cutoff filters are used, and the difference between UV handling in the ColorFlex L2 and the previous-generation ColorFlex EZ.
 

Important Notes:

Why UV Energy Matters in Color Measurement

Color measurement instruments measure reflected visible light, which can include visible fluorescence produced by the sample. When ultraviolet (UV) energy from the instrument's light source reaches a material containing optical brighteners or fluorescent agents, the material can absorb that UV energy and re-emit part of it in the visible spectrum.

For many traditional optical brighteners, this fluorescence occurs strongly in the blue region of the visible spectrum, around 450 nm. As a result, fluorescence can affect colorimetric values and indices sensitive to this region, including CIE b*.

For samples containing little or no fluorescent material, differences in source UV energy generally have little or no effect on the measured color. For fluorescent samples, however, the amount of UV energy illuminating the sample can directly affect the amount of fluorescence produced.

If two instruments illuminate the same fluorescent sample with different levels of UV energy, the sample may fluoresce differently on each instrument and produce different colorimetric results.

What Does a UV Cutoff Filter Do?

A UV cutoff filter controls the amount of ultraviolet energy from the instrument's light source that reaches the sample.

A UV cutoff filter, such as a nominal 400 nm or 420 nm high-pass filter, can be used to substantially attenuate UV energy while transmitting the visible portion of the source spectrum. This reduces or eliminates excitation of optical brighteners and therefore reduces their fluorescent contribution to the measured color.

UV filtering can also be used to provide a controlled amount of UV rather than completely excluding it. In systems designed for adjustable UV control, the UV level can be established using a fluorescent reference standard with known properties. The resulting measurement condition may be identified as UV

Calibrated or UV Controlled.

Measurements made under controlled UV conditions can then be compared with measurements made under UV-excluded conditions when the objective is to evaluate the effect of optical brighteners on measured color.

ColorFlex EZ and UV Control

The standard ColorFlex EZ optical system does not actively control the UV content of its xenon source illumination. The lamp output is delivered through the source optics to the sample without an adjustable UV-control mechanism.

For nonfluorescent samples, this normally presents little concern because variations in UV energy have little effect on the measured color.

For samples containing optical brighteners, however, differences in UV output between instruments can produce different levels of fluorescence and therefore different colorimetric results.

For applications requiring UV exclusion, ColorFlex EZ could be used with an optional port plate incorporating a UV cutoff filter. This provided a means of substantially excluding UV excitation when measuring fluorescent materials, but it did not provide calibrated, controlled UV illumination.

How ColorFlex L2 Controls UV Energy

ColorFlex L2 addresses this limitation by incorporating a fixed UV filter into the instrument's optical path between the xenon source lamp and the illumination optics.

The UV filter is established at the factory to attenuate the source UV energy to a controlled level based on measurement of a known fluorescent reference standard. This provides a defined balance of UV and visible illumination rather than allowing the sample to be exposed to the uncontrolled UV output of the xenon lamp.

Because the UV condition is controlled, ColorFlex L2 instruments can provide improved inter-instrument consistency when measuring materials containing optical brighteners while also providing consistent measurement of nonfluorescent materials.

The ColorFlex L2 measurement condition is identified as UV Calibrated or UV Nominal. The UV setting is established at the factory and is not adjustable by the end user.

Why This Matters

The importance of UV control depends on the sample.

For materials without optical brighteners or other fluorescent components, UV-source differences generally have minimal influence on color measurement.

For fluorescent materials—including an increasing number of recycled plastics and other materials that may contain optical brighteners—UV control can be important for obtaining consistent measurements between instruments.

The ColorFlex L2 fixed UV-control system provides a defined illumination condition so that fluorescence is excited consistently rather than being dependent on uncontrolled variations in lamp UV output.

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To learn more about Color and Color Science in industrial QC applications, click here: Fundamentals of Color and Appearance