= Irradiance
{title2=$E=d\Phi_{\rm incident}/dA$}
{wiki}
<Irradiance> measures incident <radiant flux> per actual receiving <area>. Its units $\mathrm{W\,m^{-2}}$ match <radiant exitance>, but it counts the incoming side of the radiation exchange. Measure incoming <angles> from the <unit normal> pointing toward the incident hemisphere. Then
$$
E=\int_{\Omega_{\rm incident}}L_{\rm incoming}(\theta,\varphi)\cos\theta\,d\Omega.
$$
For $2\,\mathrm W$ distributed uniformly over $0.010\,\mathrm{m^2}$, $E=200\,\mathrm{W\,m^{-2}}$. If the receiving plane is tilted $60^\circ$ in the same broad uniform parallel beam, the incident <power> per actual <area> is halved to $100\,\mathrm{W\,m^{-2}}$. This receiving-surface <cosine> factor is geometric and does not assume <Lambertian reflectance>. Together, <irradiance> and <radiant exitance> describe how radiation supplies and removes <energy> at a surface, while <radiance> preserves the directional information.
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