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?