Toshiba TLSH180P LEDs from Maplins. Today I did some tests without optics using these. Still using just my NE555 1kHz oscillator output directly driving the LED the signal is considerably brighter. It should be some 10dB better than the high brightness LED used before. Using the Toshiba LED both as TX and (unbiased) RX detector the signal could be detected over a range of around 20m in daylight. The beamwidth of the LEDs is narrower (8 degrees) so aiming is quite critical. Further may well be possible with very careful beam alignment.
So, with 100mm lenses at each end (24dB minimum gain each) 5km should be possible and maybe up to 20km. Progress on the full FM/SSB systems has slowed as Christmas approaches with other activities taking priorities in the family, but the 481THz work is my main priority ham radio-wise.