Laboratory setup. The QTH lamp and the monochromator sit outside of the dark enclosure and are not drawn. The light from the monochromator exit slit is conveyed through a liquid light guide (Newport 77639 Liquid Light Guide, 2 m long, 8 mm diameter) that feeds the integrating sphere. Exposure time is controlled through a mechanical shutter placed between the end of the light guide and the sphere entrance port. Our cooled photodiode (“CLAP”) is attached on the dark box wall just above the dewar window, and allows us to measure the effective exposure time (see Fig. 2 below).