The application of FWA’s (Fluorescence Whitening Agents) is a common practice in some textile applications where the need to achieve “high” whiteness is a requirement. Other applications include the use of fluorescent colorants to dye textile substrates in order to accomplish high chromatic colors for style or high visibility apparel. The visual and instrumental evaluation of such materials requires the simulation of the ultra-violet component found in daylight.

In order to achieve this simulation it is critical to define the specific qualities of the illumination. The sample illumination should be the same for which colormetric values have been calculated. The common use in today’s visual and instrumental measurement is simulation of illuminant D65.

Additional considerations in instrumental measurement relate to geometry, measurement area, UV content of the light source, and UV calibration of the light source.

The objective of this paper is to discuss the practical uses of instrumentation for those applications that use FWA’s in their process. We need to realize that there are many complex issues regarding instrument measurement and our purpose here is to outline those that are most important, from a practical viewpoint of the average instrument user.

Light and Color
The human eye has the ability to see colors that fall between 400nm to 700nm (nanometers) on the visible spectrum. At the lower end of the spectrum (400nm) the color we see is violet; the hue we recognize as blue is around 480nm; green is approx. between 480nm and 560nm; yellow between 560-590nm; and orange between 590-630nm. We see the color red at those wavelengths longer than 630nm. The energy below 400nm is categorized as ultra violet light and is “invisible” to the eye just as light energy above 700nm is near infra-red light and also invisible to the eye.

When light strikes an object there are two things that can happen relative to color. Either the light transmits through the object or reflects from the object. It is possible that some of the light may pass through, some may reflect, some may scatter, and some do all of the above but for the purpose of our discussion the general categories of transmittance or reflectance will suffice.
With the above basics in mind it is now that we approach the issue of fluorescence, what it is and its applications within the textile industry.

Fluorescence
The use of FWA’s (Fluorescent Whitening Agents) or optical brightners has been used in the textile industry for those applications where a high degree of whiteness or chromatic color was required. In the case of white fabrics, the nature of these chemicals is to absorb light in the invisible, or near ultra-violet, region of the spectrum and then re-emit this light as fluorescence in the visible region of the spectrum. This re-emitted light generally occurs between 420-500nm. The effect is a greater degree of reflectance in the blue region of the spectrum, therefore a “bluer” white. In effect, this process produced much whiter whites! Shades that are white have a high degree of lightness and a low chroma. More important, the perception of white is generally caused by a high lightness with a very low amount of yellowness. Yellowness causes a white fabric to look faded or degraded over time. Therefore, a fabric that has high lightness with a lack of yellow direction (or toward the blue region of the spectrum) will appear whiter to the eye.

Prior to the use of FWA’s, the bleaching process was used to achieve a higher degree of white. As an example, cotton is gray in its unbleached state and the bleaching process is used to increase its level of whiteness. Bleaching does increase the level of lightness over the entire visible spectrum (400nm-700nm) with a somewhat higher level in the blue region. Therefore, after the bleaching process the cotton is “perceived” to be whiter. In order to further increase this perception of a whiter effect, FWA’s were added to the process. As described above, these chemicals absorb light in the near UV and re-emit by fluorescence as visible blue light. This effect gives a white fabric a higher degree of blueness therefore a greater “perceived” whiteness. (See Figure 1)

Instrumental Measurement
Most color measurement instruments have not been designed to measure the effects of fluorescence. Color instruments found in most textile companies are spectrophotometers that are either 45º0 or sphere diffuse 8º geometry instruments. They are what we refer to as single monchromator instruments where light reflected or transmitted from the sample is broken into its respective wavelengths and presented to the analyzer. The analyzer calculates percentages of this light at its designated bandwidths and displays the color curve. The light from the sample is referred to as polychromatic (white light) and the instrument as a polychromatic illuminating spectrophotometer. This is an efficient, accurate, and repeatable method of measuring non-fluorescent samples.

As we know, fluorescent samples absorb light in other regions of the spectrum and then re-emit in the visible area of the spectrum. Ideal instruments for measuring this effect would be those that have the ability to separate the base reflectance from the amount of reflectance caused by the fluorescence.

Such instruments do exist but they are expensive and not common to the industry. They are dual (2) monchromator systems where the first monchromator breaks the illuminant into its individual wavelengths and the second monchromator receives the light from the sample (at its specific wavelength) and presents it to the analyzer. Output data can separate the base reflectance from the fluorescent reflectance and allow better analysis of the effects of optical brightners and fluorescent dyes and/or pigments. Again, these instruments are expensive, slow in measurement time, and not common to the other applications within the textile industry.

When using conventional spectrophotometers for fluorescent measurements and comparing their results to other instruments, the following conditions should be considered:

• Geometry of Instrument.
• Measurement Area
• Light Source of Instrument
• Illumination Filtered to D65
• Calibration of the UV Component

Instrument Geometry: Measurement Area
Two types of geometry found in conventional spectrophotometers are sphere (8º) and 45º/0 or 0/45º. Both geometries can be used to measure fluorescent samples and if comparing data from two (2) instruments it is important to insure that they have identical geometry. If not, measurement data may be different and may not be compatible. This is true not only with fluorescent samples but also with any measured samples.

Another variable required for consistency is that of aperture or “area” of measurement on the instrument. As an example, an 8mm circular aperture will provide reflectance energy from an 8mm area of the sample. When measuring fluorescent standards/samples the data collected is a result of the measuring area. If the measuring area is changed to 25.4mm the data collected may not correlate to the data collected from the smaller measuring area. Reflectance from the fluorescent sample may be higher or lower in UV content relative to the area of measurement.

Light Source: Illuminant D65
Two important specifications are necessary relative to the light source of the spectrophotometer. One, in order to meet CIE specifications for the measurement of fluorescent samples it is necessary for the illuminant to contain ample levels of UV energy. Secondly, it is necessary for the light source illuminating the sample to be filtered to D65. Some instruments use quartz or tungsten light sources that lack ample UV energy to excite the whitening or fluorescent agents within the sample. Whatever UV energy they do contain begins to diminish early into the life cycle of the lamp. Because of the low amount of UV energy emitted by these type lamps they are more difficult to filter to D65.

Instruments that have pulsed-xenon light sources contain a high amount of UV energy and they are easier to filter to D65. Therefore, this type source provides the UV power to simulate D65 daylight.

Calibration of the UV Component
It has been found that the Xenon light source provides an excess of UV energy and that the UV content of these lamps degrades with use. This was discovered in the early seventies by work performed by Griesser and Gartner relating to the UV content of new xenon lamps and xenon lamps after a period of aging.

Their work resulted in the creation of the UV calibrator used within some makes of spectrophotometers to monitor and provide consistent amounts of UV energy to the sample.

Figure 1. UV Calibrating Device (Griesser/Gartner)

The basic issue is that xenon flash lamps are not consistent in regard to the amounts of UV energy they output. We are aware that they contain an excess amount of UV energy but from lamp to lamp they may contain “uneven” amounts of this energy. We have also indicated that the UV energy contained in these lamps will degrade as the lamp ages and changes occur within the instrument due to aging. The effects of these changes upon the sample indicate that measurements taken of a fluorescent standard over a period of time will show differed measured results even though the fluorescent standard
Has not changed. These changes occur on the same instrument, the same type of instrument, and with a light source that has ample UV energy and excellent simulation to D65 daylight. Take into consideration the differences that can occur when results of these measurements are compared to measurements of the same sample taken on instruments with different geometry containing insufficient UV energy with the illumination source to the sample. What can occur are measurements taken with too low or too high UV energy as opposed to measurements taken with the right amount of UV energy to simulate D65 daylight.

Reflectance curves of a fluorescent sample illuminated with too little UV energy (A), too much UV energy (C), and UV energy according to daylight.

In summary, the best spectrophotometer to use for measurement of fluorescent samples is one that has a pulsed-xenon light source that is properly filtered to daylight D65 and that has the ability to filter or calibrate the UV component to a fluorescent standard. Calibration of the UV component was based on results obtained by measuring fluorescent plastic standards obtainable from Ciba-Geigy. These fluorescent standards are no longer available from Ciba but are now available from Frederick T. Simon (FTS, Inc.) of Clemson, SC.

The result that you want to achieve is the right balance between the visible amount of UV present and the amount of UV emitted by the source in the instrument. To achieve this you need to have a light source, like the pulsed-xenon, that emits more UV energy than you need. You need to be careful because some instruments that use a pulsed-xenon light source may provide too high an intensity of pulsed light and this can cause an effect that we refer to as the triplet effect. Triplet effect results in false readings that are low at some wavelengths that will lead to numeric values that do not correlate to the eye. In the past this was a problem but most manufacturers of pulsed-xenon instruments are aware of this effect and have taken steps to correct this problem within their instruments.

I think we can assume that most instrumental measurements of fluorescent samples are provided by the traditional spectrophotometers found in the QC or formulation lab. These instruments are used for many different textile color measurement applications and most likely no special calibration routine is provided relating to the measurement of fluorescent samples. In order to improve the accuracy and repeatability of your color measurements I recommend that you consider the following procedures:

• Insure that the spectrophotometer used for this measurement contain a pulsed-xenon light source.
• Make sure that the instrument provides for a method of calibrating the UV component.
• Obtain a set of plastic fluorescent standards and regularly measure these standards to calibrate the UV source according to instrument procedures.
• Provide proper maintenance to the optics of the instrument in regards to dirt and dust and provide regularly scheduled factory service to the instrument.
  

Conclusion
We have dealt mainly with the hardware issue relating to color measurement of fluorescent samples. This was our purpose. We need to be aware that the software issue can be just as critical. It has been stated that the best procedure for measuring whiteness is use of the whiteness index. A whiteness index maintains a “the perfect” white standard to which all your white samples are compared against. This standard has the value of 100 as the perfect white. It is up to the user to determine which whiteness index is best for their application. Some indexes use two dimensions of color space while some use three. Some formulas place more value on lightness while some place more value on yellowness as a deterrent to whiteness.

Regardless of the approach one may take relative to the mathematics of calculating the formats of your measurement, proper instrument and instrument calibration is crucial.