Table 1:

Current and potential limits of depth penetration capabilities of one-, two-, three-photon excited fluorescence microscopy. Visible range of light: 380 – 700 nm; near-infrared I light: 700 – 900 nm.

Fluorescence microscopy	Demonstrated depths so far with high spatial resolution (close to diffraction-limited)	Potential estimated depth limits with high spatial resolution (close to diffraction-limited)
Excitation: One-photon excited Detection: Widefield	0.1 ~ 0.2 mm123 (visible range of light)	0.6 ~ 0.8 mm124,125 (*near-infrared II or short-wave infrared light)
Excitation: One-photon excited Detection: Confocal	0.3 ~ 0.4 mm126 (visible range of light)	1.5 ~ 2 mm124,125 (*near-infrared II or short-wave infrared light)
Excitation: Two-photon excited (temporal focusing) Detection: Wiefield	0.3 ~ 0.4 mm127 (near-infrared I light)	0.6 ~ 0.8 mm124,125,128 (*near-infrared II or short-wave infrared light)
Excitation: Two-photon excited Detection: single-element detector (e.g. photomultiplier tube, PMT)	0.6 ~ 0.8 mm7,129 (near-infrared I light)	1.5 ~ 2 mm125,128 (*near-infrared II or short-wave infrared light)
Excitation: Three-photon excited Detection: single-element detector (e.g. photomultiplier tube, PMT)	1.2 ~ 2.1 mm129,130 (near-infrared II or short-wave infrared light)	3 ~ 4 mm125,131 (*near-infrared II or short-wave infrared light)

^*Indicates optimal imaging windows around 1300 nm and 1700 nm (in the region of near-Infrared II between 1000–1700 nm also called the short-wave infrared range in similar or even broader ranges in some definitions.