= Photoelectric saturation current
{title2=$I_{\rm sat}=e\eta P/(h\nu)$}
The <photoelectric saturation current> is the <photocurrent> reached when increasing the collecting <electric potential difference> no longer increases the fraction of available emitted <Electrons> collected. It is saturation with collecting voltage at fixed illumination. It does not mean that increasing incident <light intensity> cannot increase emission.
If $P$ is incident <radiant flux> on the emitting area and $\eta$ is the external <quantum efficiency> for producing escaping <Electrons>, then the incident <photon> arrival rate is $P/(h\nu)$ and complete collection gives $I_{\rm sat}=e\eta P/(h\nu)$. For the example's $\nu=8.00\times10^{14}\,\mathrm{Hz}$, choose $\eta=0.10$ and $P=1.00\,\mathrm{\mu W}$. The calculated <photon> arrival rate is $1.886488\times10^{12}\,\mathrm{s}^{-1}$ and $I_{\rm sat}=30.224866\,\mathrm{nA}$. Doubling $P$ doubles this current to $60.449731\,\mathrm{nA}$ while leaving $V_s$ fixed. At fixed illuminated area this doubles the <irradiance>. Changing <frequency> at fixed $P$ changes the arrival rate as well as the <photon> <energy>; $\eta$ can also change, so there is no general rule that a higher <frequency> gives a larger <photocurrent>.
A missing plateau can indicate incomplete collection or an <electric field> altered by the emitted <electric charge>. Heating, surface changes and additional emission channels can invalidate a constant $\eta$. Even a small amount of unwanted higher-<frequency> radiation can move an apparent endpoint. A <stopping potential> inferred from zero net current additionally needs background subtraction and a check for emission from the collector itself.
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