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# Focusing light

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I want to make something like a solar powered landscape light visible from several (2 to 5?) miles away, but only from one location. The one location part seems relatively easy, I can just put it in a box and make the opening a long tube, pointed at the location. I am less sure about it being visible from that far away. To make it easier to spot, I was wondering if it is possible/practical to direct more of the light down the tube to increase the intensity. My first thought was to coat the inside of the box (sphere?) with something reflective, so light from the source bounces around until it eventually goes out the right way, rather than just being absorbed. next I was wondering about properly shaped lenses and reflectors. Like if i were to put a parabolic reflector on the side opposite the opening, am I gaining anything significant? Would a dome shaped lens at the opening refract enough down the tube to make a difference? If you can't tell, I don't have much experience with optics. Not sure what the best reward/effort option might be. Generally restricted to the low tech/DIY options, though not against spending 50 bucks or so on a lens if it would make a difference. ##### Link to comment ##### Share on other sites There are several things you can do or you need to think about. - The brightness you'll be able to see will depend on the ambient light. It'll need to be much brighter if you're near a city than miles from anywhere on a moonless night. - if you want to use a traditional spherical point source bulb then your best bet is an integrating sphere and some optics. The sphere gathers the light and puts it out a single output and you can then try and use optics to collimate the light. This won't be cheap, the collimation will be limited and it'll suffer from alignment changes. - use a directional light source. You can get directional LEDs or go all the way and get a laser diode, a relatively low power one that is eye safe should be your stating point. They're pretty cheap but might mean you need to build some electronics to power it. The beam will spread out but you'll need to work out how much to see whether it'll need to be very accurately lined up. - a decent torch, they often have optics to do some limited collimaton. It's all but and ready to go... Again you might need to build some electronics to power from the solar panel. ##### Link to comment ##### Share on other sites There are several things you can do or you need to think about. - The brightness you'll be able to see will depend on the ambient light. It'll need to be much brighter if you're near a city than miles from anywhere on a moonless night. - if you want to use a traditional spherical point source bulb then your best bet is an integrating sphere and some optics. The sphere gathers the light and puts it out a single output and you can then try and use optics to collimate the light. This won't be cheap, the collimation will be limited and it'll suffer from alignment changes. - use a directional light source. You can get directional LEDs or go all the way and get a laser diode, a relatively low power one that is eye safe should be your stating point. They're pretty cheap but might mean you need to build some electronics to power it. The beam will spread out but you'll need to work out how much to see whether it'll need to be very accurately lined up. - a decent torch, they often have optics to do some limited collimaton. It's all but and ready to go... Again you might need to build some electronics to power from the solar panel. Wouldn't a decent LED torch and a long enough tube, slightly wider than the torch to restrict the spread, do the job? Pulse the light so it's identifiable. A ship's navigation light is 90 lumens or candelas and can be seen at 6 miles, so I don't think you would need that bright an LED torch. Edited by StringJunky ##### Link to comment ##### Share on other sites Look at Heliograph on Wikipedia. "The record distance was established by a detachment of U.S. signal sergeants by the inter-operation of stations on Mount Ellen, Utah, and Mount Uncompahgre, Colorado, 183 miles (295 km) apart on September 17, 1894, with Signal Corps heliographs carrying mirrors only 8 inches square." ##### Link to comment ##### Share on other sites Wouldn't a decent LED torch and a long enough tube, slightly wider than the torch to restrict the spread, do the job? Pulse the light so it's identifiable. A ship's navigation light is 90 lumens or candelas and can be seen at 6 miles, so I don't think you would need that bright an LED torch. Yep. Depends what it's for and whether it needs to integrate with anything else. If it was me and manual control I'd use a torch. ##### Link to comment ##### Share on other sites Yep. Depends what it's for and whether it needs to integrate with anything else. If it was me and manual control I'd use a torch. I've got a programmable LED torch but it wasn't within the OP's budget though. Edited by StringJunky ##### Link to comment ##### Share on other sites I want to make something like a solar powered landscape light visible from several (2 to 5?) miles away, but only from one location. The one location part seems relatively easy, I can just put it in a box and make the opening a long tube, pointed at the location. I am less sure about it being visible from that far away. To make it easier to spot, I was wondering if it is possible/practical to direct more of the light down the tube to increase the intensity. My first thought was to coat the inside of the box (sphere?) with something reflective, so light from the source bounces around until it eventually goes out the right way, rather than just being absorbed. next I was wondering about properly shaped lenses and reflectors. Like if i were to put a parabolic reflector on the side opposite the opening, am I gaining anything significant? Would a dome shaped lens at the opening refract enough down the tube to make a difference? If you can't tell, I don't have much experience with optics. Not sure what the best reward/effort option might be. Generally restricted to the low tech/DIY options, though not against spending50 bucks or so on a lens if it would make a difference.

I build high powered flashlights myself for many years as a hobby, I'm starting to turn it into a business now.

There are several factors you need to consider. First off you need to decide what "visible from several miles 2-5" means. Stringjunky has a tiny, pocketable flashlight which I built (124mm x 24mm - 95 grams) which spits out around 1600 lumens through a tiny reflector (20mm) which is enough light to be visible from half a mile away on a dark night. You can setup a stationary, multi emitter light with a large reflector which will actually be able to illiuminate objects at 2 miles and will be visible for 5+ miles. Options are just endless. What I would suggest would be the easiest is just buy a long range spotlight with a large diameter reflector which will give you focused beam and range and mod it to be powered off your solar panels. HiD's need high voltage balast to exite the emiiter (several thousand volts) With todays LED technology it is possible to build a fairly small light (say the size of a couple of coke cans but a little wider at the head) to be capable of being seen from 2 miles away. If you need serious power and beam focus you need a High intensity discharge (HiD) emitter light which has far greater surface brightness than any LED due to tiny emitter size. You can buy these ready made too and just modify it to accept power from your solar panel accu's. Drop me a private mesaage and I will direct you to where you can buy what.

Edited by koti
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Defining visibility:

The idea is for it to be visible at night. The location is in the hills about 5 or 6 miles from the nearest town. That being said, its about 5 or 6 miles from several towns, sort of nestled in the dead space between them. The goal is certainly not to illuminate any surface from 2 miles away, more like make it look like there's an unusually low star in the middle of one of the hills across the valley. I was reading that the human eye can detect candle light from something like a couple of miles, but I think that was more of a theoretical limitation than a practical one. If it could appear a little brighter than any actual star that would be cool, but I wouldn't want anything more than that.

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Defining visibility:

The idea is for it to be visible at night. The location is in the hills about 5 or 6 miles from the nearest town. That being said, its about 5 or 6 miles from several towns, sort of nestled in the dead space between them. The goal is certainly not to illuminate any surface from 2 miles away, more like make it look like there's an unusually low star in the middle of one of the hills across the valley. I was reading that the human eye can detect candle light from something like a couple of miles, but I think that was more of a theoretical limitation than a practical one. If it could appear a little brighter than any actual star that would be cool, but I wouldn't want anything more than that.

Does it have to be visible from the whole radius of those 5,6 miles or will a directional beam be sufficient? If it doesnt have to be a lighthouse style light a strong, directional focused beam from a single powerful LED should do the job. I got a waterproof light for you that you can setup on a fixture and it will look like a supernova amongst the other stars around. Its powered by 3,7v Li-Ion cell and could be modified for external power supply - I'd have to know the details though.
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Does it have to be visible from the whole radius of those 5,6 miles or will a directional beam be sufficient? If it doesnt have to be a lighthouse style light a strong, directional focused beam from a single powerful LED should do the job. I got a waterproof light for you that you can setup on a fixture and it will look like a supernova amongst the other stars around. Its powered by 3,7v Li-Ion cell and could be modified for external power supply - I'd have to know the details though.

All the torch needs to do is excite a single rod on the retina; that's all the image of a star is in our vision. Apparently, we can be aware a single photon has been emitted, if not actually see it because it's too brief in duration.

Edited by StringJunky
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Welcome back, Callipygous. Nice optics topic, because long time no see.

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All the torch needs to do is excite a single rod on the retina; that's all the image of a star is in our vision. Apparently, we can be aware a single photon has been emitted, if not actually see it because it's too brief in duration.

I have to disagree Stringy...torches need to obliterate, burn and blind with ridiculous power

Welcome back, Callipygous. Nice optics topic, because long time no see.

Yes, more efficient optics, less heat, more power - no see !

Edited by koti
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Does it have to be visible from the whole radius of those 5,6 miles or will a directional beam be sufficient?

No, I specifically want it to only be visible from one spot. think geocaching, ideally I want you to have to stand in a particular 5-10ft circle (at the previous cache) and the light is your only method of finding the next cache. I think my challenge here is going to be restricting it enough over that distance while still having it bright enough. Like, if I could just put a landscape light in the dirt at the right location, I think that might be visible across the valley, but if you're seeing through a half inch tube, most of that light is blocked. Im assuming I need to direct more of the light down the tube than a half inch circle would normally get without my help.

And long time is right, Phi. I dropped in a while ago for one post, but I think even that was years ago.

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No, I specifically want it to only be visible from one spot. think geocaching, ideally I want you to have to stand in a particular 5-10ft circle (at the previous cache) and the light is your only method of finding the next cache. I think my challenge here is going to be restricting it enough over that distance while still having it bright enough. Like, if I could just put a landscape light in the dirt at the right location, I think that might be visible across the valley, but if you're seeing through a half inch tube, most of that light is blocked. Im assuming I need to direct more of the light down the tube than a half inch circle would normally get without my help.

And long time is right, Phi. I dropped in a while ago for one post, but I think even that was years ago.

That changes things a little. The only source of light which will be capable of delivering such a divergence at such a distance is a laser. There is no way you can focus any emitter even with a several feet diameter reflector to be able to achieve 5-10ft visible area on a 5 mile beam travel. You should be looking at green lasers well above 50mW power (more like 200mW) which depending on where you live might be legal or not. You're also looking at significant \$ when buying a laser with optics capable of keeping a beam at such minimal divergence as 5-10ft beam circumference after a 5 mile beam travel.

If you can live with a spot more than 5-10ft of area, more like 50-80 feet, you can go for a powerful flashlight which will be significantly cheaper.

Edited by koti
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Ok. The exact radius is negotiable. Kinda why I posted the question, don't know whats practical. : )

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All the torch needs to do is excite a single rod on the retina; that's all the image of a star is in our vision. Apparently, we can be aware a single photon has been emitted, if not actually see it because it's too brief in duration.

They eye is a nonlinear detector with some crazy image processing. It's a very different problem doing this in a dark tunnel compared to a moonlit night.

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Ok. The exact radius is negotiable. Kinda why I posted the question, don't know whats practical. : )

I never had a chance to geocache (I have to fix that when my kid gets a little bigger) so I'm not sure what would be practical. If the geocachers can be permited to see this light well outside of the mentioned 5-10ft radius than a focused LED torch is the way to go. I can send you some beam shots if you want.

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Honestly, on the scale of this project, if the radius went up to like 200 feet it wouldnt be tragic. Were talking about caches that are several miles apart, and you wont know to look for the light until you read the clue at the previous cache anyway.

I dont want you to be able to follow the light all the way to it, and I want to minimize how much it disturbs other park users and wildlife, so I would like it as restricted as is reasonable.

Edited by Callipygous
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Honestly, on the scale of this project, if the radius went up to like 200 feet it wouldnt be tragic. Were talking about caches that are several miles apart, and you wont know to look for the light until you read the clue at the previous cache anyway.

I dont want you to be able to follow the light all the way to it, and I want to minimize how much it disturbs other park users and wildlife, so I would like it as restricted as is reasonable.

A well focused search light type torch with an XP-L Hi emitter running a healthy 3,5 amps with good regulation is what you need then.

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I dont know what many of those words mean, but it sounds good.

To give me a feel for scale... 3.5 amps, if I want the light to stay on all night, lets say 10-12 hours to account for winter, that means I need 35-42 amp hours. Seems like a fairly substantial battery pack, isnt it? What sort of voltage?

Looks like minimum daylight in my area is actually about 9.5 hours. Suppose we could set the sights lower and assume they wont be out looking for it past 2am.

Edited by Callipygous
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I dont know what many of those words mean, but it sounds good.

To give me a feel for scale... 3.5 amps, if I want the light to stay on all night, lets say 10-12 hours to account for winter, that means I need 35-42 amp hours. Seems like a fairly substantial battery pack, isnt it? What sort of voltage?

3,7V.

3,5A would be the amperage at max output. You could easily run it at a lower mode at say 1,5A and it will still do it's job.

Check your PM, I sent specs.

Edited by koti
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I dont know what many of those words mean, but it sounds good.

To give me a feel for scale... 3.5 amps, if I want the light to stay on all night, lets say 10-12 hours to account for winter, that means I need 35-42 amp hours. Seems like a fairly substantial battery pack, isnt it? What sort of voltage?

Looks like minimum daylight in my area is actually about 9.5 hours. Suppose we could set the sights lower and assume they wont be out looking for it past 2am.

If you use a torch in beacon mode a single 3000mah battery will last way past your target duration.

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3,7V.

3,5A would be the amperage at max output. You could easily run it at a lower mode at say 1,5A and it will still do it's job.

Check your PM, I sent specs.

I make a bit of a habit of posting silly-ass ideas like this on various forums. Fairly often people who seem more informed than me tell me that its not going to work the way I want it to and "heres what you would really need to make that happen". I usually do my best to respect the fact that they seem to know what they are talking about, and adjust my plans closer to what they say. But if im being honest, the fact that you are both the dude selling the flashlights, and the dude saying the flashlights are the solution, has damaged my trust a bit, and my reading elsewhere is leading me to believe it shouldnt take that much tech or that many lumens.

So I'm going to try to direct this back towards the physics of the question. If I build a box, sphere, whatever shape enclosure from whatever seems appropriate down at the ol' hardware store (read:dirt cheap) and pick up a pile of whatever solar powered landscape lights I can find (read:dirt cheap) and plug them all into said enclosure somehow...

I figure I now have a somewhat respectable light source. However, its all bouncing around at weird angles, and most of it is going to end up absorbed by the enclosure rather than directed down my aperture toward the target. I realize with passive optics its not really going to be possible to direct all those weird angles where I want them, but are there means to significantly improve the amount that ends up going the right direction? Via a properly shaped reflector and/or lens setup, could I increase the output through the aperture 3, 4, 10 fold?

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I make a bit of a habit of posting silly-ass ideas like this on various forums. Fairly often people who seem more informed than me tell me that its not going to work the way I want it to and "heres what you would really need to make that happen". I usually do my best to respect the fact that they seem to know what they are talking about, and adjust my plans closer to what they say. But if im being honest, the fact that you are both the dude selling the flashlights, and the dude saying the flashlights are the solution, has damaged my trust a bit, and my reading elsewhere is leading me to believe it shouldnt take that much tech or that many lumens.

So I'm going to try to direct this back towards the physics of the question. If I build a box, sphere, whatever shape enclosure from whatever seems appropriate down at the ol' hardware store (read:dirt cheap) and pick up a pile of whatever solar powered landscape lights I can find (read:dirt cheap) and plug them all into said enclosure somehow...

I figure I now have a somewhat respectable light source. However, its all bouncing around at weird angles, and most of it is going to end up absorbed by the enclosure rather than directed down my aperture toward the target. I realize with passive optics its not really going to be possible to direct all those weird angles where I want them, but are there means to significantly improve the amount that ends up going the right direction? Via a properly shaped reflector and/or lens setup, could I increase the output through the aperture 3, 4, 10 fold?

Read about ship beacons and aeroplane beacons and you'll get an idea of the requirements to be seen at certain distances.

You can get a T6 emitter torch for about 6 dollars from China. Here's one but don't believe the 6000 lumen figure; more like 500. Stick that on the end of a tube to limit the spread - play with the length - and some kind of stand, you are done. Note that the demands of seeing with a light and seeing a light source are different; the latter requires much less.

https://www.aliexpress.com/item/Promotions-Ultrafire-E17-2000-Lumens-5-Mode-CREE-XM-L-T6-LED-Flashlight-Zoomable-Focus-Torch/703354679.html?ws_ab_test=searchweb0_0,searchweb201602_1_10152_10065_10151_10130_10068_436_10136_10137_10157_10060_10138_10155_10062_10156_10154_10056_10055_10054_10059_10099_10103_10102_10096_10147_10052_10053_10142_10107_10050_10051_10084_10083_10080_10082_10081_10110_10111_10112_10113_10114_10179_10181_10037_10183_10182_10032_10078_10079_10077_10073_10070_10123-10000902,searchweb201603_9,ppcSwitch_3&btsid=50f3b995-26a9-40b5-8669-eb00a60b940e&algo_expid=88030fe1-a8c9-4317-a8a7-19d56f86b3c9-2&algo_pvid=88030fe1-a8c9-4317-a8a7-19d56f86b3c9

Edited to add: The maximum distance due to curvature of the Earth, on flat ground you can see is 2.9 miles.

For an observer on the ground with eye level at h = 5 ft 7 in (1.70 m), the horizon is at a distance of 2.9 miles (4.7 km). For an observer standing on a hill or tower 100 feet (30 m) in height, the horizon is at a distance of 12.2 miles (19.6 km). https://en.wikipedia.org/wiki/Horizon

Edited by StringJunky
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I make a bit of a habit of posting silly-ass ideas like this on various forums. Fairly often people who seem more informed than me tell me that its not going to work the way I want it to and "heres what you would really need to make that happen". I usually do my best to respect the fact that they seem to know what they are talking about, and adjust my plans closer to what they say. But if im being honest, the fact that you are both the dude selling the flashlights, and the dude saying the flashlights are the solution, has damaged my trust a bit, and my reading elsewhere is leading me to believe it shouldnt take that much tech or that many lumens.

So I'm going to try to direct this back towards the physics of the question. If I build a box, sphere, whatever shape enclosure from whatever seems appropriate down at the ol' hardware store (read:dirt cheap) and pick up a pile of whatever solar powered landscape lights I can find (read:dirt cheap) and plug them all into said enclosure somehow...

I figure I now have a somewhat respectable light source. However, its all bouncing around at weird angles, and most of it is going to end up absorbed by the enclosure rather than directed down my aperture toward the target. I realize with passive optics its not really going to be possible to direct all those weird angles where I want them, but are there means to significantly improve the amount that ends up going the right direction? Via a properly shaped reflector and/or lens setup, could I increase the output through the aperture 3, 4, 10 fold?

No worries, there are multiple ways of achieving what you need and a DIY setup which you are leaning towards might be better for your needs. In case you end up with not enough light at those 5,6 miles range you can upgrade. As for me, yes - I'm a flashaholic

Just one correction, I'm also the dude who makes them.

Edited by koti

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