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I am studying radiometric measurement concepts and found out (on lecture notes), that the cosine term (from the irradiance formula) is in the denominator.

I could not grasp it, as it would imply that irradiance goes to infinity when $θ_d$ goes to 90 degrees to the normal.

I could not find another online source about this particular formula and do not know whether it is valid.

Does irradiance go to infinity when light is sent parallel to the surface?

From my study material

Brendan Darrer
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Gunay Karaoglu
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1 Answers1

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The case when $\theta_d = 90^o$ is best considered by rearranging the formula to get

$\displaystyle E \cos \theta_d = \frac {d\Phi} {d A_d}$

So when $\theta_d = 90^o$ (i.e. the angle of incidence of the radiation is parallel to the surface) then $\frac {d\Phi} {d A_d} = 0$, no matter how large $E$ is.

gandalf61
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