I have an IR LED emitter.

It has a 20 deg. beam width.

The total power radiated is 70 mW.

I want to calculate the power density (mW/cm^2) at a range of 30 cm.

How do I calculate exactly, the area of a sphere that is intercepted by the IR
beam at 30 cm?

I have done an approximate calculation by assuming that the radiation is a cone, and I have calculated the area of the base of that cone, but this is approximate and I would like to calculate the exact surface area of the sphere, intercepted by the beam.

I did find one post of a similar problem, but the person who kindly answered the post simply gave the resulting number and did not mention the formulas he used.

If I have posted in the wrong forum, would you kindly tell me which is correct forum?

Thank you.

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