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# 1.8: Normal Flux Density F

The rate of passage of energy per unit area normal to the direction of energy flow is the normal flux density, expressed in $$\text{W m}^{-2}$$.

If a point source of radiation is radiating isotropically, the radiant flux being $$\Phi$$, the normal flux density at a distance $$r$$ will be $$\Phi$$ divided by the area of a sphere of radius $$r$$. That is

$F= \Phi / (4 \pi r^2) \label{1.8.1}$

If the source of radiation is not isotropic (or even if it is) we can express the normal flux density in some direction at distance $$r$$ in terms of the intensity in that direction:

$F = I/r^2 \label{1.8.2}$

That is, the normal flux density from a point source falls off inversely with the square of the distance.