# Is long distance visible light transmission possible?

 P: 8 Hi Lets say i need to transmit a lot of visible light from one place to another. Is this possible to do on long (50~100 miles) distance?
 Mentor P: 12,111 What is the purpose of the transmission? Data transmission? Possible, but radio frequencies might be better. With light, you need a direct line of sight*, and clouds, dust, temperature gradients and so on can absorb/deflect/scatter your signal. Power transmission? I don't think that is practical. *this requires some sort of mountain or high tower(s) for 50-100 miles.
P: 8
 Quote by mfb What is the purpose of the transmission?
The purpose is to transmit light to photoreactor, to grow plants

 Mentor P: 12,111 Is long distance visible light transmission possible? I don't think this is useful. Grow plants where the light is.
P: 8
 Quote by mfb I don't think this is useful. Grow plants where the light is.
 P: 661 So your collector is big and the target as well? Then it must be possible. On smaller items diffraction at the concentrator would limit the distance, and atmospheric turbulence would as well (it makes data transmission impossible) but with BIG items on both sides this improves. Look, Sunlight reflection on a single window is powerful at 1km range and it illuminates only a few metres across, so if the reflector and the target are 100m wide, 100km look possible. Will you grow bananas in Alberta?
 Mentor P: 12,111 As seen from earth, the sun has an angular diameter of ~0.5° or ~9mrad. Over a distance of 100km, this corresponds to a minimal (!) spot size of about 1km. It is possible to reduce the size, but this does not give an increased intensity. If your reflector has a diameter of ~100m, the final spot will have a diameter of ~1km and get an intensity of 1% of the solar flux - even if the reflector is in bright sunlight and orthogonal to it, and without any atmospheric losses, this corresponds to just ~10W/m. A deflector with a diameter of ~1km could mimic a second sun, but even there we have atmospheric losses. And a mirror of that diameter is a serious engineering problem.
 P: 661 I'd say that 20km distance, from one side of a valley to the next, is a better goal. Very few places offer a line of sight of 100km. So let's have a field of mirrors of 200m dimensions, and the field of vegetals as well, and the Sun's apparent diameter gets acceptable. For diffraction to add only 40m spread at 20km, each individual mirror must be larger than 0.3mm so 1m-5m size is just fine for the individual mirrors. At such a distance, but with far less area, a town in Bavaria (or Austria?) has done it. They had no Sunlight for the whole winter due to the moutains; mirrors on the opposite side were the remedy. What I doubt is that a field of steered mirrors can be paid by the production of vegetals on the same area.
Mentor
P: 12,111
 Quote by Enthalpy At such a distance, but with far less area, a town in Bavaria (or Austria?) has done it. They had no Sunlight for the whole winter due to the moutains; mirrors on the opposite side were the remedy.
Viganella in Italy. 100 000 € for 40m2 movable mirrors according to Wikipedia. The german version has more details about the mirrors. I can help translating if automatic tools don't work.
 P: 2 I would like to transmit sunlight, not to grow bananas in Alberta, but to iluminate tunnels. In a sunny day, we have to illuminate the entry and exit sectors ( 1/3 mile ) with 400w lamps installed every 5/10 meters with a huge electric power cost. So, an efficient solution could be to collect sunligth from outside and transmit it into the tunnel, working as auto-regulation system. Has anybody any idea or experience?
 P: 270 No, visible light's intensity will weaken too fast. To reach the end of the tunnel you'd have to use a massive laser(and kill everyone).
Engineering