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==== Irradiance ====
**Definition:** The radiant flux (power) incident on a surface per unit area, denoted by the symbol $E$.\\
**Unit:** $W / m^{2}$\\
**Mathematical Expression:** $$E = \frac{d\Phi}{dA}$$

**Relationship Between Irradiance and Radiant Intensity for a Point Source on a Differential Surface Element**\\
For an isotropic point source with a radiant intensity of $I$ (unit: $W/sr$) in a given direction, there is a differential surface element $dA$ perpendicular to the direction of incidence at a distance $r$ from the point source.\\
  * The solid angle subtended by the surface element $dA$ at the point source is: $d\Omega = \frac{dA}{r^2}$
  * The radiant flux emitted by the point source into this solid angle is: $d\Phi = I \cdot d\Omega = I \cdot \frac{dA}{r^2}$
  * Substituting the above equation into the definition of irradiance $E = \dfrac{d\Phi}{dA}$, we obtain:

$$E = \frac{d\Phi}{dA} = \frac{I \cdot \dfrac{dA}{r^2}}{dA} = \frac{I}{r^2}$$

**Summary of Relationship:** Under conditions of normal incidence, the irradiance on a differential surface element from a point source is directly proportional to the radiant intensity of the source and inversely proportional to the square of the distance from the source to the element, thus following the **inverse-square law of irradiance**.\\

**See Also**\\
[[yanding:成像基础知识:光学:辐射度学与光度学:照度|Illuminance]], [[yanding:成像基础知识:光学:辐射度学与光度学:辐射通量|Radiant Flux]], [[yanding:成像基础知识:光学:辐射度学与光度学:辐射强度|Radiant Intensity]]