MFT data on the Scream (1910?)

In the course of a MFT measurement colorimetric values are obtained and values of color change are calculated as CIE ΔE, being a difference between the initial color for a selected spot at the beginning of the test and a color resulting from the accelerated light ageing.  

The color change values (CIE ΔE) were calculated in two different ways - according to the formula proposed by CIE in 1976 and with the most recent colorimetric formula, proposed by CIE in 2000. Reporting ΔE76 values for color change is more common in the conservation literature, although ΔE2000 better reflects the observer's experience. The results table shows the final color change calculated with both methods as well as Δa, Δb and ΔL values. It's worth noting that when ΔL has negative value, the tested color darkens under illumination, when ΔL is positive – the color lightens up (fades). The measurement time for a single point on the object was 6 minutes (360 seconds) and corresponds to 0.50 MLxh (megaluxhours) of illumination.  

The conversion of MFT test results into years of exposure with the assumed lighting requires adoption of many simplifying assumptions that make such uncertain. The results of MFT tests indicate weakest points and points that have practically not changed their color under the test. Such data allows to plan illumination for a given collection even without accurate calculations of expected color changes after the planned exposure. In order to relate the obtained results of color changes to the materials that are accepted as a standard reference materials, lightfastness measurements were taken also for the Blue Wool Standards No. 1, 2 and 3, obtained from James Heal  (Halifax, UK) and produced in accordance with the European standard ISO 105-B08. In conservation practice, it is assumed that objects with photo stability corresponding to BWStd 1 require special protection. The Blue Wool scale was development several decades ago with the intention of regular progression of their lightfastness in consecutive order - BWStd 1 being twice as sensitive as BWStd 2, BWStd 2 twice sensitive as BWStd 3, and so on.