The wool scale ( WS for short ) is a lightfastness measure (blue scale) that is used to determine the lightfastness of a color . Lightfastness describes the resistance of a color to the effects of light without the direct influence of weathering . The wool scale ranges from level 1 to 8. Level 1 corresponds to the lowest resistance to light, level 8 to the highest. The main area of application of the scale is in the printing and textile industries .
Determination of the lightfastness
The test sample of the material to be tested is exposed to the light of a light fastness tester together with a light fastness scale (wool scale with eight different blue colored wool fabric strips). This is usually a xenon high pressure lamp as the radiation source. This light is very similar to daylight . The lightfastness is determined by comparing the change in the sample and the wool scale. On the wool sample, the outer left square is WS 1 and the outer right square is WS 8. Determining the lightfastness can be a very time-consuming process. The determination for samples with lightfastness 1 takes a few hours, whereas for lightfastness 7 and 8 it takes about two weeks. In practical use, there is no longer any difference between lightfastness 7 and 8.
In order to obtain realistic results, the lightfastness levels are determined depending on the season.
|3||4-8 days||2-4 weeks|
|4th||2-3 weeks||2-3 months|
|5||3–5 weeks||4–5 months|
|6th||6-8 weeks||5-6 months|
|7th||3-4 months||7–9 months|
|8th||Over 1.5 years|
Evaluation of the wool scale
The classification in the scale can be done visually and by measurement. In the visual assessment, the respective wool strip is irradiated until a clear change in color compared to the non-irradiated strip can be seen. This applies to levels WS 1 to WS 6. For levels WS 7 and WS 8, the color difference must just be recognizable. In the metrological assessment, a distinction is made between the measurement of the irradiation and the color measurement. The irradiation measurement is carried out automatically when the test is carried out using a radiation measuring device in the wavelength range between 300 and 400 nm over the entire irradiation time. The color measurement is carried out with a spectrophotometer . This measuring device shows the respective color value with the help of the Lab or Lch values. The Delta E value serves as a yardstick for the color difference between the two wool strips .
Practical importance in the printing industry
With certain printed products it is necessary to achieve a high level of lightfastness. By hanging them outdoors, posters are exposed to a high level of daylight. Therefore, only colors with a correspondingly high resistance should be used for this. The same applies to high quality art prints and high quality folding boxes. An early fading of the colors has a considerable influence on the product quality. The so-called fluorescent colors (fluo colors) fade the fastest under the influence of light (daylight and artificial light). With these special colors, the printed products only have a lightfastness of WS 1. In the case of printing inks, the lightfastness is always indicated on the can label according to the wool scale. In practical use, it should be noted that with mixed colors, the lowest lightfastness value for the color always applies. If, for example, a ruby red WS 7 is mixed with a yellow WS 5, the result is no average value of WS 6, but actually a color with WS 5, since the yellow component fades before the red component. The lightening of the colors with transparent white reduces the resistance of a color to the influence of light. This reduction depends on the amount of transparent white added, i.e. H. the more a color is stained, the faster it fades. The wool scale differentiates between the old DIN 54003 (daylight) and DIN 54004 (xenon light). The wool scale corresponds to the following levels of light fastness:
- Joachim Böhringer, Peter Bühler, Patrick Schlaich: Compendium of media design for digital and print media (X.Media.Press) . Springer, 2005, ISBN 3-540-24258-9 .
- Dr. Artur Rosenberg, Angela Diwisch: Light and weather fastness of offset printed products. Forschungsgesellschaft Druck, July 1998, accessed April 7, 2015 . (PDF; 413 kB)
- Light resistance of offset inks. (No longer available online.) Hubergroup, January 19, 2002, archived from the original on March 4, 2016 ; Retrieved April 7, 2015 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 433 kB)