(a)–(c) Illustration of the imaging configurations for 2PM, 3PM, and ND-2PM, respectively. (d)–(f) Fluorescence intensity profile I(z) for the three types of MPM systems. (g)–(i) Lateral and axial PSF widths as functions of focal depth zf for the three types of MPM systems. (j)–(l) Signal, background, and background that comes only from the ballistic light as functions of zf for the three types of MPM systems. The maximum imaging depths we have simulated are 1000, 2000, and 1500 μm for 2PM, 3PM and ND-2PM, respectively, which covers the maximal imaging depth demonstrated or expected for these types of systems.