= Detector saturation
\b[Detector saturation] occurs when increasing the input no longer produces the response expected from the detector's lower-input calibration. The limitation may arise in the sensing element, its load circuit, an <amplifier> or the digitizing stage of the <measuring chain>. A nearly flat output then represents many different inputs, so the lost amplitude information cannot be recovered by applying the original linear conversion.
A primary physical example is Thorlabs' silicon-<photodiode> saturation study. It varies reverse bias and load resistance and examines the resulting saturation limit and noise floor. Its stated limitations include testing one FDS100 photodiode with a continuous-wave source; those observations do not establish a universal limit for every detector or pulsed illumination condition.<Photodiode Saturation and Noise Floor by Thorlabs>[Experimental Limitations, p11/12; indexed primary-document text]{ref}
For an original idealized output-limit model, let incident <optical power> be $P\geq0$ and
$$
V(P)=\min(aP,V_{\max}),\qquad
a=5000\,\mathrm{V\,W^{-1}},\quad V_{\max}=2.5\,\mathrm V.
$$
The linear branch reaches its limit at $P=0.5\,\mathrm{mW}$. Inputs $0.5$, $1$ and $2\,\mathrm{mW}$ all produce $2.5\,\mathrm V$ in this model. Interpreting that output as $P=V/a$ therefore reports $0.5\,\mathrm{mW}$ for each, hiding the differences. The abrupt limit is a teaching model; real response can depart from linearity gradually.
An <optical attenuator> placed before the detector can provide a check while retaining usable input information. For transmission $T=1/4$ and original $P=1\,\mathrm{mW}$, the detector receives $0.25\,\mathrm{mW}$ and gives $1.25\,\mathrm V$ on the linear branch. Correcting the known attenuation recovers $P=V/(aT)=1\,\mathrm{mW}$. Scaling a clipped stored value after detection cannot perform the same recovery.
To test <detector linearity>, vary calibrated attenuation or source power while monitoring stability and keeping geometry and wavelength fixed. Compare output ratios, including the background contribution, rather than assuming every output plateau is a constant source. Thorlabs also identifies reverse bias, load and bandwidth as relevant system conditions.<Photodiodes by Thorlabs>[Lab Facts, Photodiode Saturation Limit and Noise Floor, Figures1–3 and accompanying discussion]{ref} In the flat part of the ideal response, <transducer sensitivity> is zero, and the inverse measurement model is no longer unique.
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