# How far can a flashlight be seen?



## Kitchen Panda (Feb 17, 2012)

So I was wondering how far a flashlight beam could be seen in good conditions. The ANSI FL1 standard rates working distance to 0.25 lux, the illumination produced by the full moon. 

Astronomers have been measuring brightness of objects for centuries, so it's not hard to find a table showing the moon has "magnitude" -12.5 (1 lux corresponds to magnitude -14, and 5 magnitude steps correspond to a 100-fold change in illumination; the dimmest stars visible by someone with excellent vision and really dark sky is magnitude +6). 

Also according to the table, the brightest stars in the night sky are magnitude 0. So, that's 12.5 magnitude steps, cranking the algebra, that means you can easily see an object that is providing 99,000 times less light than the full moon. The illumination drops off as the square of the distance and the square root of 99,000 is 315, nearly. 

So, all this rigamarole says that if you take the ANSI FL1 working distance and multiply it by 315, you'll have a range at which the light will appear about as bright as the brightest stars. 

According to that, the TK70 should be visible up to 130 miles! There's probably some additional atmospheric factors that crop up over such great distances, but it's kind of cool to imagine this. That doesn't quite reach the International Space Station. 


Bill
( But if I'm forced down somewhere in Northern Ontario at night, you can bet if I can see *your* lights, I'll be flashing all of mine in your direction!)


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## StarHalo (Feb 17, 2012)

The catch is always atmosphere and divergence - I know that on a perfectly clear night, my 35W HID will visibly light an object half a mile away, but on dusty or moist nights, that figure is literally halved. And I've brought up the idea of pointing a 1W Wicked Arctic laser directly at the International Space Station; it was calculated that by the time the beam reached the Station, it would be many hundreds of feet across, far too diverted to see at all.


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## Norm (Feb 17, 2012)

Slightly different application but it will give you some idea of how far an LED is visible.

VK7 OPTICAL COMMUNICATIONS EXPERIMENTATION

http://reast.asn.au/optical.php


Norm


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## Quiksilver (Feb 17, 2012)

50 miles in clear atmosphere for a sufficiently bright and/or focused light.


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## gcbryan (Feb 18, 2012)

Stars and flashlights are two different things. You can see magnitude +6 (as mentioned with good eyes and dark conditions) but you can't see a +6 magnitude nebula because of the way magnitude is figured. It takes into account area so unless your flashlight has a pinpoint light similar to a star your conclusions aren't going to apply.

Maybe I've missed something however


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## StarHalo (Feb 18, 2012)

gcbryan said:


> but you can't see a +6 magnitude nebula because of the way magnitude is figured. It takes into account area



Nope, it's "apparent magnitude" meaning how bright it appears to the eye as-is. And you'd have to be somewhere extremely remote to see a +6 object; from out in the desert I can just make out the galactic plane, and I can't make out anything over roughly +4.5..


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## gcbryan (Feb 18, 2012)

StarHalo said:


> Nope, it's "apparent magnitude" meaning how bright it appears to the eye as-is. And you'd have to be somewhere extremely remote to see a +6 object; from out in the desert I can just make out the galactic plane, and I can't make out anything over roughly +4.5..



I'm not sure what "nope" is in answer to but I get apparent magnitude but what I what I'm talking about is how magnitude (apparent or absolute) is applied to something that is not a star. A +6 magnitude nebula is like a +6 star only if you "squeezed" the nebula to a point like a star.

However, I agree the atmosphere is what limits the kind of distance the OP is talking about
Sure +6 is under perfect conditions but I'm not arguing whether the standard is +6 or +5 or whatever. When I live (in the city) I can barely see +3.

I may be wrong or missing some point but that point isn't related to absolute vs apparent magnitude or whether anyone other than someone in the desert with perfect eyes can see it +6 naked eye.


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## StarHalo (Feb 18, 2012)

gcbryan said:


> what I'm talking about is how magnitude (apparent or absolute) is applied to something that is not a star. A +6 magnitude nebula is like a +6 star only if you "squeezed" the nebula to a point like a star.



I'm saying "nope" to that - the basic apparent magnitude system is rather crude and predates Ancient Rome; the idea is how bright the thing you're looking at appears to you, and that's it. It has since been refined to exclude the effect of atmosphere, but is otherwise still the same. The star Vega is roughly 0, the Moon is -13, the Sun is -27, etc. The math used to reach these numbers is a bit complex, but the overall concept is just the amount of light reaching your eye.


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## gcbryan (Feb 18, 2012)

StarHalo said:


> I'm saying "nope" to that - the basic apparent magnitude system is rather crude and predates Ancient Rome; the idea is how bright the thing you're looking at appears to you, and that's it. It has since been refined to exclude the effect of atmosphere, but is otherwise still the same. The star Vega is roughly 0, the Moon is -13, the Sun is -27, etc. The math used to reach these numbers is a bit complex, but the overall concept is just the amount of light reaching your eye.



Let me rephrase it. In a dark environment (rare for many people) you can see a +6 nebula just as you can a +6 star. You either get it all in a bright point (from the star) or you get it dimmer (more spread out) from a larger nebula but in aggregate your eye gets the same amount of light.

The light pollution washes out the larger, dimmer nebula so it's larger size is no help visually. The concentrated point of the star does get through.

In real world conditions with light pollution it is common to be able to see, for instance, a +3 star and not be able to see a +3 large nebula since the light pollution makes it impossible to see the +3 nebula but maybe not regarding the +3 star.

From my location I can see the star Albireo and I cannot see the Andromeda Galaxy (naked eye) even though they are both roughtly +3 relative mag.


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## StarHalo (Feb 18, 2012)

You're over-complicating it; the apparent brightness idea predates telescopes by nearly two millennia, so there was no concept of galaxies or nebula, just the brightness of the object compared to other objects. If a galaxy appears to be as bright as another star in the sky, those two objects will have the same magnitude number, that's it. You have an exactly equal chance to see either object in a perfectly even unlit sky (which is essentially impossible unless you're well out to sea; some portion of the sky is always going to have some light pollution.)


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## Swedpat (Feb 18, 2012)

Interesting matter! Astronomy is another of my big interests, and I have thought a lot about how stars can bee seen from such a huge distances. I am pretty sure a candle light can be seen by the naked eye from at least a few miles distance at dark conditions, so a flashlight has to be visible from much longer distance...
Actually I have planned to do some tests about it.


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## Norm (Feb 18, 2012)

Swedpat said:


> I am pretty sure a candle light can be seen by the naked eye from at least a few miles distance at dark conditions, so a flashlight has to be visible from much longer distance...
> Actually I have planned to do some tests about it.



Has any one bothered to check my link above, post #3?

Norm


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## StarHalo (Feb 18, 2012)

Norm said:


> Has any one bothered to check my link above, post #3?



I can't find the original discussion thread about that, but I still wonder how a DEFT would do in that application; I picture a kit where the flashlight has a headset jack on it..


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## yifu (Feb 20, 2012)

StarHalo said:


> I can't find the original discussion thread about that, but I still wonder how a DEFT would do in that application; I picture a kit where the flashlight has a headset jack on it..


Yep, a DEFT would be very useful, especially in the wild for signalling etc.


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## StarHalo (Feb 20, 2012)

yifu said:


> Yep, a DEFT would be very useful, especially in the wild for signalling etc.



150+ miles using the Weak Signal Communicator setup, as in Norm's link..


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## yifu (Feb 20, 2012)

StarHalo said:


> 150+ miles using the Weak Signal Communicator setup, as in Norm's link..


I can see that happening easily, as signal mirrors have achieved that kind of range as well 100 years ago.


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## StarHalo (Feb 20, 2012)

yifu said:


> I can see that happening easily, as signal mirrors have achieved that kind of range as well 100 years ago.



This would be at night, bouncing light off clouds..


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## AnAppleSnail (Feb 20, 2012)

StarHalo said:


> This would be at night, bouncing light off clouds..


150 miles is well over the horizon, which is impressive for light-based communication.

Broadly speaking, from height in meters, the distance to the horizon in km is : 3.57 * sqrt(h)
in feet and miles, 1.323 * sqrt(h). These are from wikipedia: Horizon

Quite a bit better than a signal mirror without using the Rocky Mountains as a tower...


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## jirik_cz (Feb 20, 2012)

Some radioamateurs reported on our local forum, that they've tried TK70 on 49 kilometres (30 miles) distance and it was easily visible. They've also tried TK45 but it wasn't visible at that distance.


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## bobbagum (Feb 20, 2012)

I've always wondered how the size of reflector and/or lighting element affects the visibility, ie: a cluster of dim LEDs Vs one bright emitter, eventhough the power of multiple lights can be considered additive, but I feels the bigger but dimmer might be more visible further away.


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## bobbagum (Feb 20, 2012)

Now in that link I see amateur radio operators transmitting light, and since light is an electromagnetic wave, would this be considered operating in unauthorized/licensed band? :naughty:


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## AnAppleSnail (Feb 20, 2012)

bobbagum said:


> Now in that link I see amateur radio operators transmitting light, and since light is an electromagnetic wave, would this be considered operating in unauthorized/licensed band? :naughty:


I think that the FCC has an unspoken exception for unlimited broadcast on the 400-790 THz band except where such interferes with the safety of transportation and emergency services. How's that for a new flashlight warning?

"This device has been tested and found to comply with the limits for a Class B digital device, pursuant to Part 15 of the FCC Rules. These standards are designed to provide reasonable protection against harmful interference when these devices are operated in a commercial environment. These devices generate, use, and can radiate visible frequency energy and may cause harmful interference to visual operations unless used in accordance with this User’s Guide. Operation of these devices outside confined areas is likely to cause harmful interference which will make the user responsible for the appropriate remedial action at his / her own expense."


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## StarHalo (Feb 20, 2012)

bobbagum said:


> I've always wondered how the size of reflector and/or lighting element affects the visibility



A smaller, more intense source of light focused into a compact beam is the key to throw; a bigger reflector is better, but the DEFT takes it a step further by using a large aspheric lens, so you get a super-tight "fat laser" beam.



AnAppleSnail said:


> unlimited broadcast on the 400-790 THz band


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## Limey Johnson (Feb 20, 2012)

A bit off topic, but similar...this reminds me of a true story.

While standing in line at the grocery store, I was in front of two women arguing about how far the human eye can see. One girl said she can easily see a mountain range that's almost 100 miles from her relatives property.

The other girl swore she was full of ****, and that maybe could see 20 miles at the most.

This, unfortunately was a slow moving line, and when I had enough, I ended the argument with one question..."excuse me ladies...have either of you ever seen the moon?"

can't make this stuff up.


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## StarHalo (Feb 20, 2012)

And on a clear night, you can see the Andromeda galaxy, 150,000,000,000,000,000,000 miles away


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## Cataract (Feb 21, 2012)

hehe, it's not even about how far our eyes can see, it's about how far a sufficient amount of light can reach


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## Kestrel (Feb 21, 2012)

> How far can a flashlight be seen?


If Mrs. Kestrel last used it, it quite often can't be seen at all, particularly in the last place I had left it.


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