= Secondary inner filter effect
A <secondary inner filter effect> occurs when emitted <fluorescence> is absorbed within the sample before reaching the detector. The absorption changes the collected signal and can distort its <emission spectrum>, especially when absorption differs strongly between emission <wavelengths>. It concerns the emitted-light path, whereas a <primary inner filter effect> concerns the excitation path.<Automatic Correction of Fluorescence Spectra for Primary and Secondary Inner-filter Effects (IFE) with Duetta by HORIBA>[PDF p.1, secondary inner-filter effect and overlap of absorption/emission spectra]{ref}
For a small emitting region followed by a purely attenuating detection path, let $A_{\rm em}(\lambda)$ be the path <absorbance> at emission <wavelength> $\lambda$. The <Beer-Lambert law> gives the simple model
$$
F_{\rm obs}(\lambda)=F_{\rm emit}(\lambda)\,10^{-A_{\rm em}(\lambda)}.
$$
The path geometry is part of this statement; a whole-cell absorbance must be scaled to the path actually traversed.
For a synthetic two-band example, suppose equal intrinsic signals of $100$ units face path absorbances $0.1$ and $0.5$. The collected signals become
$$
100\,10^{-0.1}\simeq79.4,\qquad
100\,10^{-0.5}\simeq31.6.
$$
Their ratio is now $10^{-0.4}\simeq0.398$, although the intrinsic ratio was one. A distorted band ratio can thus arise without a change in the relative emission initially produced.
Within this attenuation-only model, multiplying each observed band by $10^{A_{\rm em}(\lambda)}$ recovers its intrinsic value. Real collection samples an extended volume, and scattering or absorption followed by re-emission can require a richer model. An absorbance-based correction is consequently meaningful only with its geometry and assumptions stated.<Automatic Correction of Fluorescence Spectra for Primary and Secondary Inner-filter Effects (IFE) with Duetta by HORIBA>[PDF p.1, absorbance measurement and inner-filter correction discussion]{ref}
A practical comparison measures an <absorption spectrum> along with fluorescence and repeats the measurement with a shorter detection path or suitable dilution. Record raw intensities as well as normalized spectra: normalizing peaks can hide a large loss of detected light. A <fluorescence blank> removes additive background under matched conditions, but does not by itself undo this wavelength-dependent attenuation.
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