HDS Systems        
Results 1 to 19 of 19

Thread: Flashlight visible on a clear Night

  1. #1
    Unenlightened
    Join Date
    Jun 2013
    Posts
    1

    Default Flashlight visible on a clear Night

    My 13 year old daughter has been tasked to ascertain the visible distance of a flashlight of 200 lumens on a clear night. The flashlight is a led lenser model number 8407 that is able to focus a narrow beam. I would be grateful if anyone could point me in the right direction that will help me understand at what distance the flashlight would be visible.

  2. #2
    *Flashaholic* kj2's Avatar
    Join Date
    May 2010
    Location
    The Netherlands
    Posts
    8,052

    Default Re: Flashlight visible on a clear Night

    The distance that a flashlight is visible, is the distance you still see that something (the flashlight) is producing light.
    For example; a led road-flare only shines his light 2-5 meters, but it's visible from 500 meters (on a clear dry night).
    Mostly reading.

  3. #3
    Flashaholic* subwoofer's Avatar
    Join Date
    May 2010
    Location
    Hove, UK
    Posts
    2,494

    Default Re: Flashlight visible on a clear Night

    According to Led Lenser's website the 8407 is the P7 model which has a beam range of approx 200m.

    You need to understand beam ranges (search CPF for this) and the specifications for this as Led Lenser do not state if this is an ANSI beam range or not.

    There is also a big difference between the range you can see with a light, and the distance from which the light can be seen. So if you shine the light you might be able to see 200m ahead, but you (or more precisely the flashlight itself) can probably be seen from 1 mile away as a pinpoint of light that you could use for signalling. Have a think about that.
    Tactical Reviews by Subwoofer
    Latest Reviews - @TacticalReviews and Facebook
    CandlePower Forums "Trusted Product Tester / Reviewer"

  4. #4

    Default Re: Flashlight visible on a clear Night

    I'd expect the led lensor P7 (8407) to be good for about 150-175 feet of usable light. It should reach dimly to about 250.

    If she wants a similar light that has a tight spot, a Fenix E40 will shine further, but doesn't focus.

    If memory serves my E40 is good for dim light to about 500 feet, usable for a good 300.
    Last edited by Ezeriel; 06-19-2013 at 03:43 AM.
    If you ask me what light to buy, I have an uncontrollable urge to say "Fenix"

  5. #5
    Flashaholic* Launch Mini's Avatar
    Join Date
    Oct 2009
    Location
    Vancouver, BC
    Posts
    1,542

    Default Re: Flashlight visible on a clear Night

    His question is not how far the throw would be, but how far the light would be visible ( I assume this would be from a person LOOKING at the light from a distance).
    I don't know the answer, BUT, when I used my E1B at the lake last fall, a fellow across the lake ( probably 2km or so away), was flashing, I flashed back & he flashed back. So I am pretty sure he could see my light

  6. #6

    Default Re: Flashlight visible on a clear Night

    How far you can see with a light and how far your light can be seen/detected by someone at a distance are 2 different things.

    From an airplane flying at 37,000 feet above in a clear night, I can see lights from the ground.

  7. #7
    Enlightened
    Join Date
    Oct 2010
    Location
    Northern VA.
    Posts
    25

    Default Re: Flashlight visible on a clear Night

    The distance the light will travel is only limited by the height above the horizon.

    This is a geometry problem, not a light problem.

    Any light can be seen at any distance it can be physically seen (line of sight).

    As for the ability of a human to see the light, there are too many variables to calculate an answer. Ambient light, atmospheric refraction due to pollution, the eyesight of the individual, etc.

    For marine navigation lights, vessels over 50 meters need lights that can be seen at 6 miles. Based on standard assumptions that requires a light that puts out 94 candellas. That is not that bright of a light.

    I can tell you from personal experience you can see nav lights on ships from much further away than 6 miles, if its clear and dark, your eyes are adjusted, and you are high enough off the water.

    So the answer is, if nothing is in the way: infinity.
    Last edited by bfayer; 06-19-2013 at 08:15 AM.

  8. #8
    *Flashaholic*
    Join Date
    Jan 2012
    Location
    NJ
    Posts
    7,491

    Default Re: Flashlight visible on a clear Night

    Quote Originally Posted by coppies View Post
    My 13 year old daughter has been tasked to ascertain the visible distance of a flashlight of 200 lumens on a clear night. The flashlight is a led lenser model number 8407 that is able to focus a narrow beam. I would be grateful if anyone could point me in the right direction that will help me understand at what distance the flashlight would be visible.
    As the question asks at what distance the FLASHLIGHT is visible....not what the flashlight can be used to SEE, etc, the ANSI range of the light is not directly usable.

    Obviously, a light with a higher cd rating will be MORE visible...but 200 lumens, in of itself is not that usable. For example, the LED Lenser might have about a 1" diameter lens.

    That's ~ 0.78 square inches or about 500 mm2.

    That means the thing the viewer is trying to resolve is a target about 500 mm2 is size.

    There is a point at which 500 mm2 will be too small to resolve, and that will be a practical limit. A point light source though is more easily seen than an object in broad daylight...much as a mote of dust passing through a window's sunbeam becomes quite visible when the light reflects off of it, etc.

    For the nautical lights mentioned, there are also minimum size specifications, and part of their purpose is to ensure that the light is large enough to be seen.

    Another factor involves the night adapted vision issue.

    When night adapted, you lose the use of the fovea, your sharpest field of vision, as it has very little in the way of rods, being primarily cone-based. That means that you may look right AT a distant light, and not see it...but, if your line of sight was a few degrees to the side of the same light, you might be able to see it peripherally. (Peripheral vision has rods, and, most of your night vision capability)

    So, for maximum range, we are now talking about the trade off in resolution of a dim target, vs an increase in night vision with a loss in resolution.


    At a certain point, there will be a maximum range for this...a point that is the culmination of these factor's effects.

    My GUESS is that as a glowing cigarette can be seen a mile away, we are talking about miles not meters.


  9. #9

    Default Re: Flashlight visible on a clear Night

    Bfayer is closer to the answer on this. 13 years old would be about 8th grade? G/T Program?

    This is mostly an intellectual exercise in open-ended scientific thinking (a great way to really stretch a smart brain btw). There are several ways to approach the problem and the first question I'd want to know is "what have they been studying recently before this question was proposed?" as that is usually an indicator of the direction the teacher wants you to go (given 8th/9th grade I'd figure they might have recently covered optics, maybe some geometry?). Having her parents do this for her isn't the point of the exercise the teachers can usually figure out whose parents helped them I had a friend who went so far as to get help from NASA, they gave him the right answer, but they used math that we wouldn't study or learn about, or understand for *years* think it was obvious to the teacher? So again it is important to know what they've studied recently. I doubt that at age 13 she'd have much knowledge of biology but if you really wanted to make this "inter-disciplinary" you could include some biology in terms of how the human eye works and how sensitive it is too.

    The old Photon lights were advertised as being visible up to a mile and they're running what 4 lumens? The brightness of the flashlight has *very* little to do with the answer to this question.

    Since this is (probably) an exercise in scientific thinking, the test here is how good an experiment can she come up with, how well can she synthesize the things she's learned in Math and Physics. One of the goals of scientific design is "isolating variables". The biggest variable in this sort of experiment is terrain. Obviously the visible distance is greater if the light is shining from the top of a mountain, and lower if it is shining behind a mountain (or inside a house!). So how can you eliminate terrain as a variable? Put the light on a boat in the ocean at a fixed height (say 5-6 ft). The light will (obviously) be visibile from further away at night, than during the day, and (marginally) visible from further away when its a dark night. To add some biology into the mix, it will be visible from further if the person's eyes are sharp and perform well at night (people who laser-eye surgery typically lose some night vision efficiency). After she's done all the "experimental design" and come up with a good hypothesis "I predict the 200lm light will be visible for a distance of 5 km, when held at a height of 5 ft over perfectly level terrain and observed by someone with normal human eyes that are at a heigh of 5 ft when observed on the night of a new moon." Once the hypothesis is constructed, it is up to her to design an experiment that can test it, so that involves putting someone on a boat (with a GPS) and someone on the beach (with the light) row out a mile or so and every few hundred feet record whether the light is visible or not and record the GPS location. Once you row out far enough that you can't see the light anymore, record the location from the GPS and then row back home. If the results of the experiment, differ from the hypothesis, then she speculates as to why and makes recommendations for further improving the experiment.

    Good source of the geometry equations needed: https://en.wikipedia.org/wiki/Horizon

    To conclude here, the goal isn't necessarily that she reach the right answer, its a means for the teacher to gauge exactly how much each of the students has *really* learned (at the end of the year), or how smart they *really* are by challenging them beyond their abilities. Tests where everyone gets 100% don't differentiate amongst the test subjects. Its like examining lights where your measuring device maxes out at 10 lumens. All of your lights show 100% on your measurable scale, and obviously some are brighter than others or project further than others so your test doesn't tell you anything useful. Give 10 x 13 year old kids this assignment and maybe one of'em comes to the right answer but you see how far each kid gets in the process and you can learn a lot about them and how they work (how easily frustrated are they? How good are they at synthesizing information? How much do their parents help? how much do they know about the scientific method? How good an experiment can they design? will they cheat? How good are they at thinking in a multi-disciplinary way? How well can they visualize the problem? How well do they reason?)

    -Morgan

  10. #10

    Default Re: Flashlight visible on a clear Night

    If you have the flashlight, do you have an area flat enough to test it? Might be a fun night time experiment.

  11. #11

    Default Re: Flashlight visible on a clear Night

    CaesarI has the best answer so far. Figure out if the assignment is a gauge or if its truly a 'simple' answer assignment (if it turns out to be a simple answer assignment for a 13 year old, that says more about the teacher than the students).

    As folks have mentioned, there are a whole lot of variables to this question (which is why it had better be a gauging/assessment question and not a simple answer question. Otherwise that teacher is...) :
    - first off, you mentioned that the question is "the visible distance of a 200 lumen focusable light on a clear night". There is no mention of whether this is with the naked eye or not, so an assumption would be with the naked eye.
    - the question as you worded it also does not appear to be asking about estimating the usable throw of the light, but about how far away the light would be visible to an observer
    - clear night isn't particularly descriptive. It could mean clear skies, but even that is vague. How well a light is visible is dependant on the human eye's response. If there's a lot of ambient light in the vicinity of the viewer and/or the flashlight, the light from the flashlight is harder to see/pick out. Maximum visibility would be attained on a dark night with a lunar eclipse (no moonlight), high altitude cloud cover (no stars), low humidity (no moisture or dust, and in a region with no ambient light pollution (maximum dark adaptation, and as TEEJ, mentioned, maximal scotopic vision is achieved by looking s few degrees off center of the target).
    - we'd have to make another assumption on whether 'clear night' means 'clear air', as any dust/moisture in the air will reduce the visibility range compared to clear air.

    There are even more variables, but taking even a few of these into account, it's obvious it isn't a simple question unless the teacher simply wants students to take a flashlight, go out, with the help of a parent, see how far away you can be and still see the light. This seems too simple for 13 year olds, with the exception of the scientific process and noting all the important factors and testing conditions down.


    Max

    P.S. as noted by other folks, under decent conditions, the light would be visible to an observer miles away

  12. #12
    Flashaholic* bshanahan14rulz's Avatar
    Join Date
    Jan 2009
    Location
    Tennessee
    Posts
    2,789

    Default Re: Flashlight visible on a clear Night

    Quote Originally Posted by aau007 View Post
    From an airplane flying at 37,000 feet above in a clear night, I can see lights from the ground.
    Quote Originally Posted by bfayer View Post
    The distance the light will travel is only limited by the height above the horizon.

    This is a geometry problem, not a light problem.
    Quote Originally Posted by CaesarI View Post
    This is mostly an intellectual exercise in open-ended scientific thinking (a great way to really stretch a smart brain btw). ... The old Photon lights were advertised as being visible up to a mile and they're running what 4 lumens? The brightness of the flashlight has *very* little to do with the answer to this question.
    I think these people are onto something.

    Sounds like she's got a good teacher too :-)

  13. #13
    Flashaholic* Lou Minescence's Avatar
    Join Date
    Mar 2011
    Location
    New England US
    Posts
    1,154

    Default Flashlight visible on a clear Night

    A bunch of great answers.
    Would the visible distance of the flashlight be for the flashlight user or visible to another person ?
    My thought would be visibility to the flashlight user. Probably the ANSI rating would be the answer. Maybe the OP could clarify.
    Kata Ton Δaimona Eaytoy

  14. #14
    *Flashaholic*
    Join Date
    Jan 2012
    Location
    NJ
    Posts
    7,491

    Default Re: Flashlight visible on a clear Night

    LOL I would like to also point out that the teacher will not know the answer either, unless the lesson plan included it as a data point, etc. There IS NO " right: answer due to all of the variables. Heck, given the educational system, it would not shock me if it were worded wrong anyway. I see that all the time.

  15. #15
    Flashaholic
    Join Date
    Sep 2010
    Location
    North Carolina
    Posts
    242

    Default Re: Flashlight visible on a clear Night

    Assuming flat terrain such as at sea, the light would disappear at the horizon. As the earth curves away. Say about 20 miles?

  16. #16

    Default Re: Flashlight visible on a clear Night

    I'd wonder from time to time while watching the jets / planes at night in regards to how far are they when the beacons are no longer visible (to my eyes). I'd see head lights in a mountain miles from my location (I mean miles).

  17. #17

    Default Re: Flashlight visible on a clear Night

    BTW, just for amusement's sake and on-topic,

    http://mparam.s3-website-us-east-1.a...beamshots.html

    That link has a very interesting slideshow of beamshots of Spark brand lights vs several other fairly common ones for comparison (including things like the Nitecore EA4, Princeton Tec Apex and Eos, 2xAA and AAA lights etc.). The on-topic part is the last section of the slides (you can skip all the way to the end and go backwards from there).

    They show what the various lights look like when viewed from a mountain about 5km away (the lights are in the valley and there's ambient lighting in the valley).


    Max

  18. #18
    Flashaholic Slazmo's Avatar
    Join Date
    May 2012
    Location
    Gold Coast - Australia
    Posts
    225

    Default Re: Flashlight visible on a clear Night

    Answer = How long is a piece of string...

    I guess it all depends on how good the receivers eyes can focus on the light being shone. I bet I could see a light being shon from the Southport Seaway looking from Kingscliff... With my Nikon Prostaff 7's of course

    I guess thats why I go outside and shin my torches into the air - one point is having a lightsaber effect - second maybe someone's watching from the outside in??? One could only hope!!!


  19. #19
    Flashaholic
    Join Date
    Mar 2007
    Location
    56° 24' N
    Posts
    322

    Default Re: Flashlight visible on a clear Night

    This question has been asked before on here. One poster came up with a figure of 315 times the ANSI FL1 distance.

    http://www.candlepowerforums.com/vb/...hlight-be-seen

    Rob
    Last edited by Scourie; 06-21-2013 at 12:38 PM.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •