A fluorimeter illuminates a cell through one window and collects emission through the window beside it, from a narrow slice of solution. The distance from the illuminated window to that slice is called the collection depth (d, in mm). The signal recorded from the slice is proportional to the dye concentration in the slice and to the excitation intensity reaching it. Dye in front of the slice absorbs some of the excitation on its way in. Figure 1 shows the recorded signal (S, in arbitrary units) against dye concentration (c, in µmol/L) at one collection depth.