Poor detection limits of two-photon excited fluorescence in cylindrical capillaries are attributed to photothermal expansion and beam astigmatism. Photothermal expansion is demonstrated for excitation in a 1 cm cell and is inferred for the larger diameter capillaries. Astigmatism is caused by focal differences between rays in a plane longitudinal to the capillary and rays in a plane transverse to the capillary. Data were obtained by integrating the fluorescence from variously sized cylindrical and square capillaries, and by photographing the fluorescence within a 1 cm cell.

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