Light scattering in TIRF microscopy: A theoretical study of the limits to surface selectivity

Area of Science:

• Biophysics
• Optical Microscopy
• Cell Biology

Background:

• Total internal reflection (TIR) microscopy relies on an evanescent field that ideally decays exponentially.
• Sample refractive index (RI) inhomogeneities and system imperfections create scattered light, degrading TIR field purity.
• These RI variations are common in biological samples, such as living cells.

Purpose of the Study:

• To quantify the optical degradation caused by sample RI inhomogeneities in TIR microscopy.
• To approximate the electric field strength of light scattered by various RI patterns under evanescent illumination.

Main Methods:

• Derived a first-order perturbative approximation from Maxwell's equations for scattered light.
• Analyzed scattering from arbitrary RI inhomogeneity patterns under coherent evanescent field illumination.
• Focused on excitation light scattering and assumed insignificant RI variations in the z-direction.

Main Results:

• TIR scattering increases effective illumination thickness by only ~50% for typical cell culture RI variations.
• Scattering introduces a slower decaying background, deviating from pure exponential decay at shorter distances.
• Qualitative surface selectivity of TIR fluorescence is largely preserved despite scattering.

Conclusions:

• Sample RI inhomogeneities cause minor alterations to TIR illumination depth, retaining surface selectivity.
• Calculations assuming pure exponential decay are approximations; scattering effects should be considered.
• TIR scattering is weakly polarization-dependent but significantly reduced at higher incidence angles.