Table 2.

Intrinsic Fluorophores

Endogenous chromophores	Single-photon excitation [λmax (nm)]	Single-photon emission [λmax (nm)]	Two-photon excitation [λmax (nm)]	Two-photon emission [λmax (nm)]	Single-photon cross section (cm2 s)/photon	Two-photon cross section (cm4 s)/photon
FAD	375,84 45084	52084	73084	52584	1.29 × 10−21 (85,86)a	8.55 × 10−52(84)
Flavins	375,86 45086	520–55086	700–73087	52587	1.29 × 10−20a	1–8 × 10−52 (87)
NADH	35088	45088	690–73088	45088	4.5 × 10−22 (89,90)a	9 × 10−52 (87)
Tyrosine	28391	30591	566–70087,91	30591	5.35 × 10−22(90)a	1 × 10−84(3PE)87
Tryptophan	28090	35090			1.86 × 10−21(90)a	1 × 10−84(3PE)87
Retinol	325	400	700–83088	49088	4.76 × 10−21(92,93)a	7 × 10−52(87)
Collagen type I	280–32094,b	350–42094,b	730–800 (SHG strongest)87	44058
				400 (SHG)87,b
Elastin	300–34095	420–460	730–80059	420–46059
				49558

^aCalculated value.

^bEmission varies with excitation λ.