All LEDs have a certain radiation pattern or far-field pattern. The intensity, measured in W/cm2, depends on the longitudinal and azimuth angle and the distance from the LED. The total optical power emitted by the LED is obtained by integration over the area of a sphere.
P = jA lx IWdX dA (5.25)
where I(X) is the spectral light intensity (measured in W per nm per cm2) and A is the surface area of the sphere. The integration is carried out over the entire surface area.