SENAI/CETIQT Colour Institute, Brazil
Originally presented in AIC 2004, Interim Meeting of the International Color Association
The instrumental evaluation of white objects treated with fluorescent whitening agents, such as commonly found on substrates like textiles, plastics and paper, is a task not as straightforward as it might seem (ASTM 1992, Hayhurst and Smith 1995). One of the main reasons is the fluorescence of the fluorescent whitening agents (FWA) that is influenced by the amount of UV radiation in relation to the amount of radiation in the visible range of the spectrum available in the sample illumination. This makes it necessary to perform a UV adjustment on the light source in reflectance spectrophotometers to establish the adequate ratio of UV and visible radiation. This study compares whiteness measurements of textile samples treated with fluorescent whitening agents made on 4 different industrial reflectance spectrophotometers, some using a traditional method of adjusting a UV filter position and others performing a numerical and virtual UV control. Two sets of standards were used for the calibration and the measurement results obtained after the different calibrations are compared.
1. UV Adjustment and UV Standards
For instrumental evaluation with reflectance spectrophotometers the spectral power distribution of the light source, usually a pulsed Xenon lamp, is rarely known to the industrial user, who generally simply adjusts the UV content of the flash during a so-called UV calibration. There are two types of UV adjustment, the traditional filter method introduced by Grtner in the 1970s (Grtner and Griesser 1975) and the numerical UV control patented by Imura (Imura 1997, USPTO 1977, 2000).
For this UV adjustment it is necessary to use a fluorescent standard with known calibration values, and at present there are different standards available: paper, Teflon, plastic and textile standards. The textile standards used in this study were produced and calibrated by the Textilforschungsinstitut Thringen-Vogtland (TITV), Germany. Two sets each of the cotton (CO) and polyester (PES) were used. The calibration data available in the certificate are the CIE whiteness value, the Ganz-Griesser whiteness value (Ganz 1972) and the spectral radiance factors for the five standards. This allows for the UV adjustment to be done according to any of the three types of calibration values.
2. Samples, Instruments and Settings
The samples measured in this study are lightweight woven textile samples with a very similar structure: one sample is a polyester/cotton mixture treated with a FWA, one sample is a 100% cotton bleached fabric without FWA, six samples use the fabric of the previous sample as substrate to which two different kind of FWAs and fractions of four different dyes where applied to produce different tints and five standards each of the cotton and the polyester standard scale were also measured as samples.
Four industrial spectrophotometers, produced by Konica Minolta, were used for the measurement of the textile samples, all of them with sphere geometry: two bench-top instruments that make use of the traditional filter adjustment for the UV setting, two CM- 3720d spectrophotometers, and two instruments with numerical UV control, a portable CM- 2600d and a bench-top CM-3600d.
The 18 samples were measured on the 4 industrial instruments after having performed the adjustment of their UV setting. On the two filter instruments the calibration was done according to the CIE whiteness value (W-CIE) of the whitest standard of the scale and also according to the Ganz-Griesser whiteness value (W-GG) of the same sample. As mentioned previously, four calibration scales were used, a cotton and a polyester scale that were purchased 8 months before being used for this study (identified as original scales) and another cotton and polyester scale that were used immediately after their arrival (identified as new scales). All the measurements were done during the same week. Table 1 shows the different percentages of UV obtained as result after the different types of calibration.