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FRET efficiency

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FRET efficiency diagram
FRET transfers excitation energy from one molecule to another without a need for fluorescence and re-absorption.
This calc determines the Forster (or fluorescence) resonance energy transfer efficiency and critical distance from the spectral overlap and other known parameters.
Donor φ
Refractive index

Critical distance
FRET efficiency
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The orientation factor (k²) is 2/3 in quickly reorientating media (e.g. free moleculaes in a low-viscosity solution), but may be between 0 and 4, depending of conditions. The donor quantum yield (φ) is a scaling factor.
The overlap function is a value derived from the integration of spectra of the donor and acceptor, and ranges approximately between 10^8 and 10^16 (consider the 4th power of wavelength in nm multiplied by the molar absorption coefficient).