IMHO, this cannot work. Sunlight comes with approx. 1kW/m^2, the human eye detect changes in brightness if they exceed 30%, i.e. we'd need 300W/m^2 to be visible at daytime. OTOH, looking directly at the sun is harmful even with the sun having a diameter of 0.5 degrees. A light source with a third of the sun's brightness, but point-like would probably burn a hole right through the retina before the blink reflex can kick in. Even the discussed 10W/m^2 (absolutely invisible unless one looks directly at the right point in the sky) would be dangerous (the eye's resolution is one angular minute, i.e. light from a point source would cover 1/900 of the sun's image's area, giving a retina burning power density of 9 suns).
Short wording: you can't see it unless it makes you blind.
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IMHO, this cannot work. Sunlight comes with approx. 1kW/m^2, the human eye detect changes in brightness if they exceed 30%, i.e. we'd need 300W/m^2 to be visible at daytime. OTOH, looking directly at the sun is harmful even with the sun having a diameter of 0.5 degrees. A light source with a third of the sun's brightness, but point-like would probably burn a hole right through the retina before the blink reflex can kick in.
Even the discussed 10W/m^2 (absolutely invisible unless one looks directly at the right point in the sky) would be dangerous (the eye's resolution is one angular minute, i.e. light from a point source would cover 1/900 of the sun's image's area, giving a retina burning power density of 9 suns).
Short wording: you can't see it unless it makes you blind.