Using Ronja Audiofire

Interconnection

Interconnect the components according to the picture.
Connect selected LED in the Ledholder and install the Ledholder upright into a vice.
Install selected lens into a vice
Check that the LED points right towards the center of the lens
Power up Benchpress

Focus the lens on a distant wall, at least 3m is recommended, according to the drawing
Check the lens is perpendicular to the direction the light travels

Take a video camcorder with a night vision function (possibly also with a zoom function) and turn the night vision feature on. B/W CCD camera works too. Digital still cameras seem to have built-in filter that drastically reduces their IR sensitivity (however you can still see the LED shine with a digital still camera).
Focus the lens on a distant wall, at least 3m is recommended, according to the drawing
Check the lens is perpendicular to the direction the light travels

The figure read from AC multimeter is proportional to rate of modulated light photons impinging onto the photodiode.
If there were no lens, Ul would be (Us*s^2)/(l^2)
Numerical optical gain (how many times more photons are received with the lens compared to without lens) is therefore (Ul*l^2)/(Us*s`^2)
Lens transmitter optical gain in decibels is therefore 10*log_10((Ul*l^2)/(Us*s^2))