FCCS models for 3D diffusing interacting particles A and B with concentrations c_a and c_b. The CCF (gray) and both ACFs are shown in dependence on the bound fraction as determined by the concentration c_ab. The concentrations of the particles varied according to c_a = c_b = 5 nM-c_ab, c_ab = 0 … 5 nM. We assumed identical detection volumes for both channels with axial radii of 260 nm, and axial radii of 1.3 μm, corresponding to observation volumes of 0.5 fl; the brightness of the two particles was set to 1500 counts/second in their respective channel, and 10% cross-talk from particle a into the detection channel B and 1% cross-talk of particle b into detection channel A; the diffusion times for the two particles were set to τ_Da = 3 ms, τ_Db = 0.3 ms.