Low tech telecommunications (light)

[Vir27]

I'm a historian. I've struck on a curiosity about telecommunication technology in pre-modern times. I've read about a number of methods, and I'd like to ask some physics questions that got beyond my ken. This information is for my curiosity and creativity, not for any rigorous study, so I may be mentioning Wikipedia.

Thank you for humoring me, and I hope I'm not asking too many questions :) I've now separated them into multiple threads by subject; this one ought to be about light. To conform with PF custom, I'll be adding the other questions to the threads after the first ones have been answered. Let anyone feel free to answer only those questions he prefers to.

Light
Signal beacons are really long established, as are smoke signals (inadvertant and otherwise). Heliography (flashing mirrors) and, later, semaphor towers were other low tech options. I'm certainly aware of the "hydraulic semaphor" of the ancient Mediterranean, but that's a mechanism for increasing the data in the transmission, where I'm curious about range.

Q1. I wonder what range one would expect for fire/smoke visibility, assuming the message was simple.

 

[russ_watters]

The range of the smoke signal would depend on the horizon distance. You'd need to know how high the smoke signal could be visible, then use a method like this to see how far it can be seen over the horizon:
http://www.ringbell.co.uk/info/hdist.htm

For example, if the puffs of smoke are coherent for 100m, you can see them from 35 km.

 

[berkeman]

Light
Signal beacons are really long established, as are smoke signals (inadvertant and otherwise). Heliography (flashing mirrors) and, later, semaphor towers were other low tech options. I'm certainly aware of the "hydraulic semaphor" of the ancient Mediterranean, but that's a mechanism for increasing the data in the transmission, where I'm curious about range.

Q1. I wonder what range one would expect for fire/smoke visibility, assuming the message was simple.

Certainly it needs to be a line of sight distance, but with a mountain, you can stretch to many 10s of miles. Beyond that, it's up to how sensitive the human eye is to light, and how bright the light source or sun reflector is. What have you found in your research into the sensitivity of the human eye?

 

[jedishrfu]

Some info is here though not the distance:

http://www.telcomhistory.org/vm/LHSmokeSignals.shtml

and US Army hand signaling:

http://armypubs.army.mil/doctrine/dr_pubs/dr_a/pdf/fm21_60.pdf

and this seems to answer based on weather conditions:

http://www.worldweatheronline.com/Smoke-Signal-weather-text/Arizona/US.aspx

 

[Vir27]

Alright, thanks. I think with all that I can move onto the second question in this vein.

Q2. Clearly this one is limited to daytime, but would flashing highly polished metal mirrors (because not everyone might have good glass) be better than fire beacons/smoke signals?

From the information above, I'm imagining if you're at a height great enough for the mirror and to compete with the fire, then it's a question of does the bright, focused mirror beam carry as far as the larger, more consistent fire light?

 

[jedishrfu]

Signalling mirrors and morse code signaling beats smoke signaling speedwise:

http://www.adventuresportsonline.com/signal.htm

The topography and general curvature of the Earth's surface limits the distance.

This article on the heliograph get more specific on mirror size and distances seen:

http://en.wikipedia.org/wiki/Heliograph

 

[Vir27]

Oh thanks. I know I read part of that Wikipedia page, but I had many tabs open and clearly didn't read far enough down before closing it for whatever reason.
Edit: Errr, sorry. I forgot I was also asking about polished metal mirrors. Do you think if I find numbers for reflectivity of polished copper and mirror glass, for example, I can reckon a proportional relationship between those numbers and the distance?

 

[mfb]

The limiting factor is not reflectivity, it is the angular spread of your reflections and the quality of the parabolic surface.
You need about 1nW/m^2 for the receiver to have some visible signal at night (that would look a bit similar to a star of magnitude 3 to 4). A telescope or something similar for the receiver helps. Should give a nice distance.
During daylight, you will need orders of magnitude more light.

 

[Vir27]

I see. Thanks for the equation. I googled it to find out if the m was meters, and found this site, in case anyone's interested. For the sun, you can switch in kilowatts, specifically. From other things on that site, I think it is meters. So shiny metal would work, assuming it were as good a parabola, is what I'm hearing.

 

[mfb]

Meters, sure.

With sunshine, a flat mirror is interesting - reflections from glass windows can be seen kilometers away.
The worst case are clouds during daytime.Where do you want to "use" those low-tech communication systems? Electricity is easy to produce if you know how, and gives much more reliable communication methods.

 

[Vir27]

Oh, I'm not putting it to use. Sometimes when I realize how simple the prerequisites of a technology are, I'm very interested in the idea that this or that ancient people whose resources and inventions I do know about could have done it. Mind you, I'm not talking about it might have happened in fact--just it's a neat idea that the ancients were one bright idea away from hot air balloons, for another example. It's like a fun game of historical fiction engineering, except without writing a novel or being a qualified engineer :)

This time around I somehow happened on a picture of British acoustic mirrors built to listen for incoming planes between the World Wars, and reading what they were for I found that smaller ones (and historical whispering galleries) are used to project sound between such acoustical mirrors, too, for fun physics displays at science museums. Well, ancient people all over the world loved shaping giant pieces of stone as well as learning magic tricks! They probably wouldn't have hated long distance communication, either. So I wanted to know if there's any special acoustical property of the modern materials, and in my other topic they told me there's not! I'm very interested to discuss the potential of relays of big stone acoustical mirrors communicating simple sound over long distances(presumably with sentries repeating the sound at stages of the relay: by default I don't assume the sound would ping pong :D ).

I made the other two topics about light and water just because those were other ideas I brainstormed or found for low tech telecommunication. I'm going to try to revive the sound topic later tonight to hope for more leads on that. The light and water are cool, but I guess the acoustical mirrors have an added element of mystery for me.

 

[mfb]

Lewis Dartnell wrote a great book with a similar focus: "The Knowledge: How to Rebuild Our World from Scratch"
There are tons of things that can be done with materials available to the ancient Romans and Greeks, if you have the right idea (or do enough experiments).

 

[Vir27]

I'll make a mental note about that book, thanks.

 

[anorlunda]

I don't think you need highly polished mirrors, or parabolic shape. Just go outside many evenings and witness an "iridium flare" with your own eyes. The iridium satellites are 485 miles up. The "flare" is a reflection from their solar panels. The best ones are almost as bright as Venus.I use an android app called Predisat that tells me where and when to look for iridium flares.

 

[mfb]

I don't think you need highly polished mirrors, or parabolic shape. Just go outside many evenings and witness an "iridium flare" with your own eyes. The iridium satellites are 485 miles up. The "flare" is a reflection from their solar panels. The best ones are almost as bright as Venus.

They can get much brighter than Venus - magnitude -7 is not uncommon, -9 is possible under ideal conditions. Venus does not exceed -5. Each step is a factor of 2.5 in intensity so we have a factor ~6 to ~40 more light.

They have large solar panels and antennas - and they have sunlight (~1kW/m^2) while the ground does not have that any more. In addition, their light crosses the atmosphere just once, vertically, so conditions for those are basically ideal. It will be hard to reproduce that for ground/ground communications.

 

[rootone]

In reply to the original post, and considering that you are a historian.
That original idea of low tech light communication technology developed over time into our best yet high tech communication technology.
If we hadn't started playing with mirrors we would not have fiber optics.

 

[Vir27]

That's certainly true, rootone. Did I say something to suggest it wasn't so?

 

[rootone]

No you did not say otherwise, I was just saying that some ancient technology can (and has )been developed in useful ways.
Smoke signals, a good idea at the time, they can't really be updated though

 

[Vir27]

Sure. In this case I'd have been doing the opposite of improving, replacing perfectly good mirrors designed for the purpose with bits of polished metal. I just was curious to what extent the earlier technology (polished metal) might have accomplished the task of the mirrors--in some world where the same idea had struck earlier--so I was seeking the practical limits on such a scheme. Just a game of what if.

 

[rootone]

The proposed James Web telescope looks to be as far as we can get right now with bits of polished metal.
That will be fun or something.

 

[russ_watters]

I don't think you need highly polished mirrors, or parabolic shape. Just go outside many evenings and witness an "iridium flare" with your own eyes. The iridium satellites are 485 miles up. The "flare" is a reflection from their solar panels. The best ones are almost as bright as Venus.

I use an android app called Predisat that tells me where and when to look for iridium flares.

The brightest are an order of magnitude brighter than Venus! Good example.

 

[NascentOxygen]

A mirror needs to be accurately aimed to the location of the observer, a fire doesn't need aiming at all. A mirror's communication can be relatively private, a fire is for all to see.

 

[mfb]

The proposed James Web telescope looks to be as far as we can get right now with bits of polished metal.
That will be fun or something.

The E-ELT will have a similar quality but a diameter of 39 meters instead of 6.5. Space telescopes are always worse than telescopes on Earth - their advantage is the lack of an atmosphere.

A mirror needs to be accurately aimed to the location of the observer, a fire doesn't need aiming at all. A mirror's communication can be relatively private, a fire is for all to see.

Aiming is not hard, you have to establish the correct orientation once and then you can switch between "aligned" and "pointing somewhere else" with a few minor adjustments over time.

 

[anorlunda]

A mirror needs to be accurately aimed to the location of the observer, a fire doesn't need aiming at all. A mirror's communication can be relatively private, a fire is for all to see.

You don't need accurate aim for low baud rates. You can pan the mirror from side to side. The recipient need only see a flash as the beam passes by him.

You could also pivot the mirror 360 degrees, broadcasting your signal in all directions to multiple recipients on the surface. I think a flash from a signal mirror will beat a fire every time. Life vests carry flashing lights rather than steady lights.

 

[rootone]

The E-ELT will have a similar quality but a diameter of 39 meters instead of 6.5.

Yes that project is very exciting too, and being ground based it will be a lot more versatile.
Interestingly it seems that the material to be used for the mirrors is not yet decided.
(For JWST it will be gold coated beryllium).