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64 PCB007 MAGAZINE I JULY 2018 is calculated as a spacial Pythagoras from the three values L 1 * – L 2 *, a 1 * – a 2 * and b 1 * – b 2 *: In view of describing a white surface one can either use a reflectivity graph (see Figure 9 which shows various reflectivity levels de- pending on LPiSM type and thermal stress ap- plied) or the brightness value L* from the CIE lab system. According to the definition, a per- fectly white surface would have the coordi- nates L* = 100, a* = 0 and b* = 0. As an ad- vantage, the CIE lab system restricts the values to be measured to no more than three figures and provides the option of forming differenc- es that largely correlate with the colour per- ceived. This approach is also very helpful in the case of white LPiSM for the electronics/printed cir- cuit board field. Besides an evaluation in ac- cordance with the CIE lab system, there is also the possibility of defining colour changes— i.e., yellowing—as it is practiced, for example, by the paper industry according to DIN 6167. Based on DIN 6167:1980-01, one speaks about yellowing if a non-desired yellowing value is recognised with the material observed, in con- sequence of its handling. From the primarily calculated X, Y and Z values a yellow value G is obtained. The change of the yellowing de- gree between an untreated (G 0 ) and a treated specimen (G 1 ) is calculated and referred to as the yellowing value, V. The colour coordinates of the CIE lab system (i.e., the values L*, a* and b*), can be direct- ly measured by means of colorimetric devices. Visible differences are indicated as follows: Δb* = 0–1: not visible in general Δb* = 1–2: low yellowing, only visible by the trained eye Δb* = 2–3.5: medium difference, visible by the untrained eye Δb* = 3.5–5: distinct difference Δb* > 5: strong difference This assessment can also be transferred, in good approximation, to the interpretation of Figure 9: Reflectivity graphs of a white "classical" and new-generation LPiSM.