OK. you are measuring the effective power of the transmitted beam by deriving it from some measurement taken from the IR receiver which you will present in uW or nW.
The IR emitter led will have in its specification an 'angle of view' and the effective received power will also be dependent on the distance between the emitter and receiver.
It is also possible the beam from the IR led is not a true cone, symetrical in all planes.
You'll also have to minimise the effect of reflected IR emissions.
But, anyway, you could start with the IR receiver at a fixed point and mount the IR emitter a fixed distance away, say one meter from the receiver. Manually vary the angle of the beam direction from the emitter away from the receiver in degrees (say from 0 to 90 horizontally) using a protractor and say every 5 degrees take a power measurement at the IR receiver.
Maybe repeat the exercise for another fixed distance between the emitter and the receiver.
Maybe repeat the exercise this time altering the angle in the vertical plane.