The Real Reason for Throw - an in depth examination

[saabluster]

Do you want a definition that enables one to objectively predict the capabilities of a flashlight in the field or do you just want a philosophical concept that has no practical value?

But it does have value. Some people think their light can throw x amount of distance because they can see a faint glimmer of light. The definition helps keep people on track about what it really means to say throw. It is about being able to see.

Thanks to Lux and wapkil for bringing up the new standard. That figure of distance at which a target is illuminated with 0.25 lux is exactly the kind of adjustment to the old CPF standard of distance for 1 lux that I have been wanting. Using the 0.25 lux standard should bring measurements more in line with real world use and also with throw values reported by manufacturers.

Yes it's all wonderful to have it nailed down. That figure represents about the same amount of lux as moonlight. Do you really think that moonlight is enough to see things with recognition at a great distance? I mean it's great to see things right in front of you and all but come on. You really think that figure has real world use as it pertains to throw?

 

[wapkil]

Thanks to Lux and wapkil for bringing up the new standard. That figure of distance at which a target is illuminated with 0.25 lux is exactly the kind of adjustment to the old CPF standard of distance for 1 lux that I have been wanting. Using the 0.25 lux standard should bring measurements more in line with real world use and also with throw values reported by manufacturers.

I like the definition in the standard but I agree with saabluster that it can be misleading. I think no one previously mentioned that measurements in the definition performed at the target may be problematic. The light has to go back to a person holding the flashlight so as I understand it the longer the beam distance, the less bright (inversely proportional to the square of the distance) the subject.

IMHO it doesn't make the definition less useful or invalid but it may be counter intuitive that the light with twice the throw doesn't let you see equally well at twice the distance.

 

[mudman cj]

Yes it's all wonderful to have it nailed down. That figure represents about the same amount of lux as moonlight. Do you really think that moonlight is enough to see things with recognition at a great distance? I mean it's great to see things right in front of you and all but come on. You really think that figure has real world use as it pertains to throw?

I think it is more in line with real world use than the figure of 1 lux. For example, I measured the lux of my Fenix TK20 vs distance and computed a throw of about 75 meters given the cutoff of 1 lux. Fenix rates the TK20 for over 150 meters throw. Using the 0.25 lux standard would result in a throw of 150 meters from the lux measurements I took (which BTW were confirmed to follow the inverse square law). There is somehow reassuring to me since I have been bothered by the discrepancy between my previous result of 75 meters and the claim of over 150 meters.

Does that mean that Fenix is right to say the TK20 can throw over 150 meters? Maybe, maybe not. How did Fenix decide the TK20 was useful to at least 150 meters? I don't know nor have I confirmed to what extent it is, but I suspect that they at least tried it out before making that claim because they are trying to build a brand name and want to earn the respect of professionals that may purchase their lights. Perhaps someone can report their unbaised experience of such a trial.

I also don't know if the 0.25 lux standard results in increasingly inadequate light intensity with increasing distance. You pose a very interesting question that deserves investigation.

 

[mudman cj]

:thinking:

I like the definition in the standard but I agree with saabluster that it can be misleading. I think no one previously mentioned that measurements in the definition performed at the target may be problematic. The light has to go back to a person holding the flashlight so as I understand it the longer the beam distance, the less bright (inversely proportional to the square of the distance) the subject.

IMHO it doesn't make the definition less useful or invalid but it may be counter intuitive that the light with twice the throw doesn't let you see equally well at twice the distance.

Ah yes, I see the argument clearly now. I also agree. The required lux for illumination must also increase with the distance to the target in order for the same intensity of light to return to the observer at the point of original light emission. The adjustment is probably negligible at 150 meters, but at distances illuminated by the DEFT this would result in quite an error I suspect. This thread is just great. We will need to write our own standard when this is done.

Edit: After thinking about this a little more I thought it seemed that a good place to start would be to relate the amount of light needed to that which would be equivalent to 0.25 lux if the target were 100 meters away. This could be written as 0.25 lux = source lux / ((2*target distance)-100 meters)^2. If we had a target 500 meters away, the light source would have to emit 202,500 lux. Without correcting for diminishing light intensity between the 500 meter target and chosen 100 meter reference point, the source would have been required to emit 62,500 lux. Clearly, this is an important factor!

Furthermore, this assumes the target reflects 100% of incident light, and as has been pointed out already, this is unlikely to be the case and is not a constant. We could expect something like 10% for vegetation or 70% for a white wall. Snow could even be as high as 90%. This does make things more complicated and cannot be ignored. Sorry for all of the editing here, I had to work through this a few times to get it to make sense.

 

[dymonite69]

Using the 0.25 lux standard should bring measurements more in line with real world use and also with throw values reported by manufacturers.

Practically speaking, one would like to know the maximal distance that an illuminated object can be usefully visualised by the naked eye of the observer - and not the distance at which light levels drop to 0.25 lux.

0.25 lux may be useful at 50m but not at 500m.

 

[easilyled]

I realize this is very time consuming but I'd still love to see some ray diagrams representing the total distribution of light in situations such as the following:-

1) An SST-90 in a very large reflector shaped like the one in the Olight-SR90-Intimidator.
2) An SST-90 in a much smaller reflector like that of a P60.
3) An XRE in the Olight SR-90 reflector
4) An XRE in a P60 sized reflector.

The much smaller die size of the XRE and the narrower beam angle would have a profound effect on the distribution and it would be very interesting to see how this works in different reflectors.

McGizmo has always used relatively narrow, deep reflectors to address the narrow beam-angle of XREs because in a shallow reflector, some of the more central diverging rays would just escape out the top and be lost in a wide spill that is useless to throw.

Reflector design needs to be allied to the beam divergence angle and this is something that has not yet been addressed here.

 

[McGizmo]

When the LED's first hit the scene, the industry was confronted with a light source quite different than a hot wire. For one, the LED being a constant current device required more than just driving it directly from a battery source. The other significant difference was in regards to the distribution pattern of light from the source. To keep it simple, the LED has a directional distribution pattern whereas the hot wire emits light in all directions. Both have images of size and not an infinitely small point of light origin and in this regard, the size of optic matters. The larger the optic and larger its focal length, the smaller, relatively speaking, the image size and the less divergent the beam, from true collimation.

I believe all will agree that throw requires collimation unless the object is close at hand or the flux of the light source is simply overwhelming in its intensity. To my thinking, throw is synonymous with collimation. If the subject of this thread were to substitute terms and it read; "The Real Reason for Collimation.." I think the discussion would follow on a very similar path.

In terms of real world where the OP wants us to focus (and for good reason) there are two aspects of far field illumination that effect our ability to perceive an object at distance. One is the level of light reflected back from the object and this is primarily what has been under discussion here. However there is also the consideration of field of view. In particular, how much of the distant object is being bathed in similar intensity from our light source.

Much of the discussion and focus of portable illumination has been in the realm of tactical illumination and whether stated or implied, the object of illumination is often a person. I recall a beam size discussion some years back with a CPF member who also was a government employee of unknown agency and he had a parameter of beam angle based on upper torso illlumination at a specified distance. A 3' diameter circle of light at x feet away will define an optimal beam angle for instance. If one were to set a required lux on target at this distance in addition to the 3' field of view then you could have a reasonably well defined minimum or throw qualification. The actual beam from a light might have greater intensity than the required lux and it might also have a larger beam angle or field of view.

I think we would all agree that you can't come up with a better throwing illumination device than a laser but for all intents and purposes, its field of view (or viewing angle if you are the target) is way to small to be of any value. A truly useful measure or definition of throw should disqualify a laser inherently and without additional qualifications.

To try to wrap this rambling up, I would propose that a definition or measure standard for throw might consist of the maximum distance from the light source in which the minimum lux measured within a circle of diameter y would be x. To be clear here, your target would be a lux meter that is moved around with a defined circle (diameter of y) on the same plane perpendicular to the z axis of your light source and with its center in the center of the light's beam. You move further and further from the light source until you find you have the minimum level of lux (x) being measured somewhere within that circle.

As to an appropriate circle diameter, obviously if you are hunting humming birds, humans or elephants you might have a differing opinion of optimal diameter. :shrug:

If the industry standard that has been mentioned but not given in detail above encompasses this consideration then forgive my ramble. From the little I gather about the standard as discussed here in this thread, it does not seem to address a field of view consideration.

 

[jtr1962]

To try to wrap this rambling up, I would propose that a definition or measure standard for throw might consist of the maximum distance from the light source in which the minimum lux measured within a circle of diameter y would be x. To be clear here, your target would be a lux meter that is moved around with a defined circle (diameter of y) on the same plane perpendicular to the z axis of your light source and with its center in the center of the light's beam. You move further and further from the light source until you find you have the minimum level of lux (x) being measured somewhere within that circle.

That's a good definition although the flaw with using that ( and also the 0.25 lux from LuxLuthor's source ) is that it doesn't account for the distance of the target from the viewer. I submit that this minimum lux value varies as the inverse square of the distance from the user. 0.25 lux falling on a target 1 meter away might be sufficient but you'll need 25 lux on a target 10 meters away ( and 2500 lux on a target 100 meters away! ) to have the same intensity of light returned to your eye by the target. The end result of this reasoning then would be a plot of minimum lux versus distance which can then be used to determine the throw. The point at which a light source falls under this curve is the limits of its throw.

 

[saabluster]

That's a good definition although the flaw with using that ( and also the 0.25 lux from LuxLuthor's source ) is that it doesn't account for the distance of the target from the viewer. I submit that this minimum lux value varies as the inverse square of the distance from the user. 0.25 lux falling on a target 1 meter away might be sufficient but you'll need 25 lux on a target 10 meters away ( and 2500 lux on a target 100 meters away! ) to have the same intensity of light returned to your eye by the target. The end result of this reasoning then would be a plot of minimum lux versus distance which can then be used to determine the throw. The point at which a light source falls under this curve is the limits of its throw.

Glad to see people are finally starting to see my point. Continuing down this line of reasoning will be far more beneficial for our community.

 

[waddup]

'throw' in a flashlight is only as useful as a persons eye sight is good, and the intended use.

after that its just physics.


99% of my flashlight use is somewhere between 10 inches and 60 feet



read my sig.

 

[wapkil]

That's a good definition although the flaw with using that ( and also the 0.25 lux from LuxLuthor's source ) is that it doesn't account for the distance of the target from the viewer. I submit that this minimum lux value varies as the inverse square of the distance from the user. 0.25 lux falling on a target 1 meter away might be sufficient but you'll need 25 lux on a target 10 meters away ( and 2500 lux on a target 100 meters away! ) to have the same intensity of light returned to your eye by the target. The end result of this reasoning then would be a plot of minimum lux versus distance which can then be used to determine the throw. The point at which a light source falls under this curve is the limits of its throw.

I'd have to check but it looks like what you propose doesn't really need any plot. If we define 0.25lx at 1m as the reference, the throw definition you propose would be simply a square root of the beam distance. If for example the beam distance is 100m the throw is 10m - at 100m the target illumination is 0.25lx, at 10m it is 100 times higher so 25lx but since the distance is also 10 times longer (10m vs. 1m for 0.25lx in our reference) the amount of light that arrives to the observer eye is equal to the reference.

In general the throw would be defined as the square root of the beam distance multiplied by a constant. The constant value depends on on where we set our reference point (which happens to be 1 for 1m because srtq(1)=1).

 

[copperfox]

You guys seem hung up on Lux on a surface at distance X. But that's not a measure of throw, that's a measure of illumination at a distance. You can't consider a flashlight a good thrower unless a lot (most?) of its light is being projected in a narrow beam. That's why the definition I gave takes into the percentage of a light's total output. Read my post on page 3.

 

[saabluster]

I believe all will agree that throw requires collimation unless the object is close at hand or the flux of the light source is simply overwhelming in its intensity. To my thinking, throw is synonymous with collimation. If the subject of this thread were to substitute terms and it read; "The Real Reason for Collimation.." I think the discussion would follow on a very similar path.

The reason that thread title would be unacceptable in my opinion is that collimation is only one battle in the end goal of throw. You can have a highly collimated beam but no intensity and therefore no real throw. That will fall under the "Optical System Efficiency" part of the main post.

I think we would all agree that you can't come up with a better throwing illumination device than a laser but for all intents and purposes, its field of view (or viewing angle if you are the target) is way to small to be of any value. A truly useful measure or definition of throw should disqualify a laser inherently and without additional qualifications.

But my definition does disqualify lasers by default. Lasers do not allow you to see targets with clarity. Seeing a spot of light does not count so it is right out. With even the tightest of tight flashlight beams that have ever been created you can even wave the light back and forth to make out the target if it happens to be of some girth no amount of waving a laser will ever allow you to see.
To make it extra clear that lasers are not what's being talked about the definition could be altered like this.
Throw- A concentrated beam of non-coherent light that reaches or exceeds the lower threshold of human vision to see a distant target with clarity relative to the user and intended target.

I do not think this is necessary however as it is understood as is. It is a small change and if people see a real need and can explain why that change is deserved then I will add that.

 

[wapkil]

The FL1 standard discusses the distance at which the target is illuminated with the predefined illumination (0.25lx) so it is not the illumination which is measured but the distance. What I was writing about was still the distance but taking into consideration also the amount of light that goes back to the observer - it would be the distance proportional to the square root of the FL1's beam distance.

I agree with McGizmo that this definition is artificial because it measures only the peak intensity, ignoring the illuminated area. I'm not sure though if measuring the minimum illumination as he proposed is the ideal solution. It would in turn ignore the way the light is distributed on the target (e.g. the flashlight illuminating the target with the bright hot spot and the dim corona would have the same throw as the light illuminating it with the hot spot equally bright as the corona). It could be solved by selecting the distance where the illumination integrated over the illuminated area is equal to some predefined constant. It would ensure that the lights with the same throw "on average" illuminate the target with the same amount of light. Defining the throw as proportional to the square root of this distance would also take into account what the observer sees. It is becoming a little bit more complicated though

I don't think that the definition of throw that discusses only the percentage of light, ignoring its intensity, is what the OP wanted to get. It is a good definition of the "throwiness" i.e. how much of the emitted light is thrown straight forward compared to how much of it is spread around but it doesn't define the throw understood as the distance at which "we can see our target well".

 

[IMSabbel]

Glad to see people are finally starting to see my point. Continuing down this line of reasoning will be far more beneficial for our community.

Well, it gets even worse if you take into acount backscatter / airglow.

So if a distance is "X"
You require a throw proportional to X^2 to illuminate the target with the same intensity.

For the same amount of light to come back to your eyes, you need X^4 throw.

But then you get a backscattering.
Lets assume its Rayleight scattering, and the air is homogenous.
This will result in a fake illumination, proportional to the light intensity and its distance from the viewer. An element at the position Y will scatter an amount of light proportional to X^4, from with only 1/Y^2 fraction will go back to the observer.
Thus total backscattered light is: Integral (0...X) X^4/Y^2 dY.
This results in 1/3 X^4/X^3 = 1/3*X illumination from scattered light.

All this is just the O() notation, but the result is clear (and obvious if you think about it):

If you increase your throw so that you actually can see the target area, then with increasing distance your contrast will drop no matter what you do. The ultimate limit to throw will always be athmospheric conditions, and no matter how much you increase output or reduce beam angle it will only get worse.

 

[saabluster]

You guys seem hung up on Lux on a surface at distance X. But that's not a measure of throw, that's a measure of illumination at a distance. You can't consider a flashlight a good thrower unless a lot (most?) of its light is being projected in a narrow beam. That's why the definition I gave takes into the percentage of a light's total output. Read my post on page 3.

This is starting to head in the right direction and I believe I have the entire quantification issue solved but I would like to see people come to this conclusion on their own. Your suggestion for a definition based on percentages of output turned into throw is fallacious and creates more problems than it solves while not addressing the real issue of target recognition.
I came to similar conclusions as you as regards perceived throw and used my decidedly non-flashaholic wife as a guinea-pig. I did not tell her exactly what I was doing but I had her come outside and compare two flashlights. A 007 recoil and a Tiablo A9. I asked her to tell me which light threw the farthest. After going back and forth between the two for a bit she decided that the 007 threw farther. Now it is very clear to my eyes that the A9 throws farther as you can see objects at a distance much more clearly and had she been looking for that she might have gotten it right. So yes I agree a light with a higher percentage of its output collimated can appear to throw farther but that does not mean it does.

 

[saabluster]

Well, it gets even worse if you take into acount backscatter / airglow.

So if a distance is "X"
You require a throw proportional to X^2 to illuminate the target with the same intensity.

For the same amount of light to come back to your eyes, you need X^4 throw.

But then you get a backscattering.
Lets assume its Rayleight scattering, and the air is homogenous.
This will result in a fake illumination, proportional to the light intensity and its distance from the viewer. An element at the position Y will scatter an amount of light proportional to X^4, from with only 1/Y^2 fraction will go back to the observer.
Thus total backscattered light is: Integral (0...X) X^4/Y^2 dY.
This results in 1/3 X^4/X^3 = 1/3*X illumination from scattered light.

All this is just the O() notation, but the result is clear (and obvious if you think about it):

If you increase your throw so that you actually can see the target area, then with increasing distance your contrast will drop no matter what you do. The ultimate limit to throw will always be athmospheric conditions, and no matter how much you increase output or reduce beam angle it will only get worse.

Yes I considered this issue as well. One could introduce a mathematical correction factor based on "normal" atmospheric conditions. As you say you will eventually come up against a virtual wall, not just visually but from a power standpoint, but we are talking about distances that begin not to really have any relevance to a humans actual ability to see as far as distance is concerned.

Getting closer.

 

[dymonite69]

You guys seem hung up on Lux on a surface at distance X. But that's not a measure of throw, that's a measure of illumination at a distance. You can't consider a flashlight a good thrower unless a lot (most?) of its light is being projected in a narrow beam. That's why the definition I gave takes into the percentage of a light's total output. Read my post on page 3.

The sun is completely omnidirectional and I would prefer that illumination and range over any flashlight you can throw against it (pun intended)

 

[dymonite69]

For the same amount of light to come back to your eyes, you need X^4 throw.

But then you get a backscattering.

I'm glad someone is properly addressing the physics. Just extending an extra 10 metres of equivalent illumination takes a whole lot more power than most people realise. So you can quotes about major increaes in surface brightness and lumen output but the net effect on 'throw' is not as much you would hope for.

 

[jtr1962]

Well, it gets even worse if you take into acount backscatter / airglow.

So if a distance is "X"
You require a throw proportional to X^2 to illuminate the target with the same intensity.

For the same amount of light to come back to your eyes, you need X^4 throw.

That only applies to an uncollimated light source. A collimated light source doesn't follow the inverse square law. You still need an intensity of X² falling on the target of course. However, a collimated light source's intensity doesn't fall off with the square of distance ( the extreme example of a perfectly collimated light source has the same intensity regardless of distance ). A real world example of an imperfectly collimated source might have 10000 lux at 1 meter, 2000 lux at 10 meters, 400 lux at 100 meters, etc. The distance at which the light source throws under a certain minimum lux at the target is the limits of its throw. If we take 0.25 lux at 1 meter as a benchmark, then you'll need 25 lux on the target at 10 meters, or 2500 lux at 100 meters. The example light source here would throw further than 10 but less than 100 meters.