= Primary inner filter effect
A <primary inner filter effect> occurs when absorption within a sample weakens the excitation reaching the region from which <fluorescence> is observed. The measured intensity then depends on the excitation path as well as the emitting material. This differs from a <secondary inner filter effect>, which attenuates emitted light on its way to the detector.<Automatic Correction of Fluorescence Spectra for Primary and Secondary Inner-filter Effects (IFE) with Duetta by HORIBA>[PDF p.1, Introduction and Fig. 1, excitation attenuation before the viewed region]{ref}
Let $q_0$ be the incident <photon> rate and $A_{\rm path}$ the <absorbance> from the entry face to a small observed region. The <Beer-Lambert law> gives the excitation reaching that region as
$$
q_{\rm view}=q_0\,10^{-A_{\rm path}}.
$$
For $A_{\rm path}=0.1$ and $0.5$, the fractions reaching the region are approximately $0.794$ and $0.316$. These absorbances describe the actual excitation path to the observed region, not automatically the entire sample-cell width.
A local model explains why more fluorescent material can even give less measured light. Suppose emitter <concentration> is $c$, local fluorescence is linear in both $c$ and excitation, and path absorption is proportional to $c$. Then
$$
F(c)=Kc\,e^{-\alpha c},\qquad
F'(c)=K e^{-\alpha c}(1-\alpha c),
$$
with fixed positive $K$ and $\alpha$. The model rises to a maximum at $c=1/\alpha$ and then falls, solely because the viewed region receives less excitation. With $\alpha=1$ in compatible concentration units, raising $c$ from one to two changes the signal by $F(2)/F(1)=2/e\simeq0.736$. This is an illustrative small-region model, not a universal whole-cuvette response.
A useful diagnostic holds instrument settings fixed while recording both <absorbance> and fluorescence across a dilution or excitation-path-length series. A shorter path can reduce optical attenuation, but dilution can also change other sample properties. A declining signal alone therefore does not establish <fluorescence quenching> or a change of <fluorescence quantum yield>. JASCO distinguishes concentration quenching from this internal shielding mechanism.<Principles of fluorescence spectroscopy (4) - Notes on fluorescence measurements by JASCO>[Concentration quenching versus Internal shielding effect]{ref}
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