The  worlds brightest LED bike light is 9 km away.  60 years ago, John Glenn saw the lights of  Perth City close to where I live. Calculations show similarly that at 10 km the light should be as bright as a half moon (apparent magnitude of full moon – 12.92) . So let’s test this. For more details and photos:

 The three photos above are from my first distance shot. 2.5 km along the straight stretch of Lillydale Rd out of Bunbury. The left photo shows the 1/4 moon for brightness reference and a car heading down that stretch about 100m away. My son’s car taillights are in the foreground. The center photo shows the flashlight firing the white LEDs 1200 W from 2.5 km. The moon has moved to be a bit obscured by trees (long wait for a car free period). The right photo shows the green light alone to confirm the source. In fact this green brightness (about 1/16th total power) would be similar to a 10 km brightness ({10 km/2.5 km} ² =16 ) for the whole array and indeed looks like it would be similar to a half moon intensity. So the maths seems to be broadly holding up to experiment. I need to find longer distances to do further testing. Here’s the light seen from 9 km over a good part of the Australind Estuary (Bunbury, Western Australia).   The left photo shows my light pointing towards the suburb of Australind across a long estuary. The center photo shows how the light appears from 9 km away with car tail lights and streetlights for comparison. Beam width would be around 1.5 km. If shone at the ISS at 370km, the beam would be 1700 times less bright [(370/9)² ] but with a dark field and dark adaption of the eyes this should be easily visible. The right photo shows the observation path from Google Earth. I also did a 12 km shot but got partially obscured by the geography.