Quantifying urban throw (the "washout effect")

Quantifying urban throw (the \"washout effect\")

There have been some recent posts about the superior performance of incandescent lights--versus most LED lights--in urban environments where ambient lighting is a factor. So I started thinking about how to measure a flashlight's throw when there is ambient light to deal with.

When people use flashlights in completely dark environments, the issue of throw seems to come up less often. When there are no other lights around, the human eye adapts to whatever flashlight you are using, and even low-output and diffuse beams can be used to identify objects quite a distance away. But when there are streetlights, your eyes adapt to the background illumination and not to the flashlight. So the absolute brightness of the projected beam becomes important. A diffuse beam in these circumstances will not be useful at long distances even if it puts out a lot of lumens.

I suggest taking Quickbeam's throw chart and recalibrating the distances to 20 lux (fairly standard street illumination) rather than 1 lux. By setting the reference intensity to 20 lux, we do two things. First, we are setting the condition that if the flashlight beam is aimed at a shadowed area near a streetlight, then any objects under the shadow will receive at least as much illumination as the surrounding area and will therefore be easy for the eye to pick out. Second, any objects within the street illumination will receive double illumination (20 lux + 20 lux = 40 lux), and a doubling of illumination is easy to notice.

All we have to do is to take Quickbeam's throw numbers and divide by sqrt(20) to obtain a useful range in meters. Multiply by 3.28 if you want to convert that number to feet. I did this and constructed the graph below for some popular flashlights:



I haven't done any extensive testing, but I took some mid-range LEDs and incandescents out into a parking lot and took a look at how much of a useful reach their beams have. The new estimates seem to work pretty well. For example, I eyeballed the reach of my TT-3C's incandescent beam as about 30 feet, and the numbers predict 24 feet. And the Surefire L4 really does reach only about 16 feet when streetlights are present.

The Streamlight ProPolymer Luxeon is the best LED thrower that I have, and it really does seem to have a "punch" outdoors that even lights like the KL1 lack. So there may be a natural cutoff around 40-50 feet for what constitutes a far-throwing urban flashlight.

Re: Quantifying urban throw (the \"washout effect\")

This subject is of great interest to me. In the city at night with street lights and security lights present I find it necessary to have a high powered incandescent handy.

The best throwing LED light I have is due to a recent delivery of the Mr. Bulk/Otokoyama Big Bulk Head for the VIP. It really does punch even under a street light. However, I still prefer something that commands the scene for night time urban activities. For now, only an incandesent can meet that specification.

Your theory is interesting. For the test area being a lit parking lot, it seems pretty accurate to me. I'll have to make this a favorite thread.

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- Jeff

Re: Quantifying urban throw (the \"washout effect\")

Good observation asdalton. This washout also interferes with indoor maintenance. Often wiring, valves, pipes etc. will be up in a dark recess, yet the ambient lighting is often high. My work EDCs are all throwers with the KL1 being just acceptable and the Scorp even better.

Re: Quantifying urban throw (the \"washout effect\")

Nice job Andrew /ubbthreads/images/graemlins/thumbsup.gif
I realy like to see these real word tests.

Re: Quantifying urban throw (the \"washout effect\")

Asdalton, congratulations on a fine piece of work.

Your chart pretty much squares with my practical experiences using flashlight (as a civilian) in parking lots and dimly-lit streets -- whose shadows are the proprietary grazing-pastures of the criminal underclass ever-seeking to surprise us with their sudden appearances.

It looks like -- if one determines 50 yards as the minimum Punch/Throw required -- that the SL Scorpion is the minimal (and bargain-priced) light for pocketable carry. I'd guess the Pelican M6 would also be right there, along with the SL Strion and ASP Triad.

The venerable MAG-Charger should be near the top, but it's a bear to carry, of course. I once used one to thwart a neighborhood car-theft-in-progress by "reaching" across a dimly/ambient-lit street and into an equally-dim apartment bldg parking garage beyond -- to spotlight the thieves. Shoulda seen 'em jump... and the brown streak they left behind.

Your chart places the respected Inova T3 in the 35 yard (arguably marginal) zone. I believe the only LEDs that could make the 50 yd. cut here are the E-Lumens Tri-Star Phazer and the Nuwai Luxeon-III TM-115X.

As LEDs and optics evolve, let's hope we'll see some durable ones with real Ambient-Light-Cutting Throw in the near future. Anyone know what Surefire has in the Throw pipeline?

I hope this provokes a healthy debate. Good work, dude.

Re: Quantifying urban throw (the \"washout effect\")

Oops!

Just realized your "Urban Throw" distances were feet, not yards. (Correct?) If so, that makes my 50 yard "reach" standard impractical, though I still believe the old MAG-Charger could approach it.

Re: Quantifying urban throw (the \"washout effect\")

I did a similar little experiment to answer the question of whether a Surefire E2e would illuminate a bad guy at 25 yards (the questioner's longest back yard sight line). I sent a photo shooting target of a man holding a black handgun to the 25 yard line and shut off the down range lights, but not the line overhead lights (there were other shooters /ubbthreads/images/graemlins/smile.gif). The result was that the E2e with fresh batteries clearly illuminated the target, gun visible, no problems.

I want to guess that anything with at least as much throw as the little E2e should work at 25 yards, which I consider a realistic maximum "encounter" distance, generally.

Bradlee's beamshot of the Inova T-3 and Quickbeam's throw score leaves little doubt in my mind that it would work well, though I can't speak to what effect intervening light would have. Outdoors, moisture in the air and fog may also hamper blue-tinted LEDs more than yellow-tinted incandescents from what people on this forum have said.

Re: Quantifying urban throw (the \"washout effect\")

Great chart, well done.

Re: Quantifying urban throw (the \"washout effect\")

Yes, those numbers are in feet, and they are intended to be a worst-case estimate. For example, a tough challenge would be to illuminate a dark entryway under the shadow of a nearby streetlight.

The useful throw in the presence of ambient light should change inversely with the square root of the ambient light intensity. I used the following equation:

I = I_ref*(d_ref/d)^2

I = intensity (lux) of the beam spot
I_ref = reference intensity of beam at a distance d_ref (these are Quickbeam's lux values at 1 meter)
d_ref = reference distance at which the reference intensity was measured (1 meter in this case)
d = distance at which beam spot is observed

So if rearrange this to find the distance, then

d = d_ref*sqrt(I_ref/I)

Quickbeam used d_ref = 1 meter and I = 1 lux, so his throw values (in meters) were just the square root of the lux values that he measured. I adjusted the numbers for a minimum intensity of 20 lux, which was the equivalent of dividing Quickbeam's throw distances by sqrt(20), or about 4.47.

If the ambient illumination is only 5 lux, then the throw distances would double. ( 1/sqrt(5) = 2*1/sqrt(20) )

Re: Quantifying urban throw (the \"washout effect\")

Andrew,
Good job and interesting read! The other issue that has come up is the contribution of full spectrum light (incan) VS the LED light which is weak in the higher wave lengths (reds).

White wall hunting is a poor substitute for real world experience and effects as most of us will attest to, I don't doubt that there are elements of physics at play that we have not fully accounted for in some of our estimations and assumptions.

What we often forget is that we are using the return of reflected light off of the targets and landscape to provide the visual information. The problem is that many materials and items are very poor reflective surfaces!! Most of the light we send out is either absorbed or reflected at angles which do not correspond with a return to source. The best visual of this is to take a weak or flood type light and shine it down the street. Little is illuminated and yet reflective street signs and license plates may light up a couple blocks away.

A measly 5 mm LED light will throw across the universe! Our main concern is the return to us of the photons we send out! I have no doubt that a cocktail of a full spectrum of photons is the best transmission of light in practice since we rarely know in advance which wave length(s) of light have the best chance of being reflected back, in sufficient volume, to provide visual information. Now it has been experienced or so stated by some that the LED is not as effective as incandescent in ambient levels of exterior lighting. Is it just a question of lux as your charting presumes or is there more to it? /ubbthreads/images/graemlins/icon3.gif

I would like to suggest that "throw" is really about sending light out and should be measured at the target. In otherwords, send someone down a straight and level road and tell them to stop when they can no longer see the light you are shining at them. This will give you a measure of the "throw" of the light in question. Now in terms of illumination for the user, what we are really interested in is bounce back of the light. Unless we are signaling for help or sending light out so the BG knows our location, what we really want is lights with good return bounce!

Measuring lux at one meter from the source gives us some idea of how we are sending the light out but is it indicative of how well the light will bounce back home in the real world?!? /ubbthreads/images/graemlins/thinking.gif

Depending on the level of ambient, I believe our eyes are more attuned to specific wave lengths of light and it is likely a question of our rod and cone receptors. To effectively see, we send out light and process that which returns to us. How well we process this return of information dictates how well we "see". Measuring transmission may not be as informative of a return or bounce for reception as one might presume. /ubbthreads/images/graemlins/icon3.gif

One final comment about the short coming of using a lux measurement to quantify or predict the ability of a light to illuminate is that a single lux measurement does not provide any information regarding beam divergence. A tight colimated beam of a lower lux measure can do a better job of illuminating a distant target than a more divergent beam scoring a higher lux reading; unless the reading is taken at the target. We need spotting scopes that measure the return lux from target! /ubbthreads/images/graemlins/nana.gif

In parting, with so many factors and so much physics involved, trust your eyes more than the numbers or reports! /ubbthreads/images/graemlins/grinser2.gif (I believe these guys who report that LED's can't compete with incans but I would like to get to the underlying reasons for this being the case)

Re: Quantifying urban throw (the \"washout effect\")

Just thinking out loud, but it seems this might be a useful experiment.

Find two lights, one incan and one LED that have similarly sized spots at say 15M. Keep looking until you find two that meet that criterion and also average similar lux at say 5M. Then do subjective test of the difference in how they light a target at various distances.

My thinking is that bounce-back from a given target is simply a ratio of the light reaching the target, so controlling for spot size and lux, you could get a more realistic view of the differences of the two designs - LED vs. incan.

Re: Quantifying urban throw (the \"washout effect\")

I'm sure that wavelengths matter to some extent. All else being equal, my bluish LEDs seem a bit weak compared to my nice white ones. But we shouldn't begin to worry about the effects of tint until we are comparing two lights that are roughly equal in both lumens and projected hotspot intensity as measured by a light meter. For example, we don't need to speculate about color to explain why the Surefire 6P stomps over the L4 in throw.

As for beam divergence, this is a bit simpler than you might expect. When light emerges from the business end of a flashlight, is is composed of a complex mixture of rays having different angles. Some of these rays point inward and crisscross for a while, but after a few feet the crisscrossing is over and all of the rays are diverging. This is the point where the beam is spreading with distance but not changing shape.

This is also the point where the 1/r^2 law applies. A flashlight beam traces out a cone in space, and the divergence of the beam changes only how narrow the cone is. All cones have a width that increases linearly with the length traveled, and the cross-sectional area increases with the square of the length traveled. Unless there is smoke or some other obstruction in the air, the same quantity of light (lumens) continues on through space. The same amount of light is being spread over a larger and larger area, so the intensity of light along any ray within the beam cone is decreasing with distance.

The beam area increases with the square of the distance, and so the intensity along any ray must decrease with the square of the distance. The most important rays for measuring throw are the brightest rays, which are almost always found in the center of the beam.

This scaling law means that we can take a single lux measurement at a single known distance and we will know everything about how the beam behaves over long distances.

There are two catches:

1) We must make sure that our single lux measurement is taken in the 1/r^2 regime, which means not too close to the flashlight bezel. The beam must not be changing shape at this distance. A distance of 1 centimeter would almost certainly be a poor choice; something like 1 meter (what Quickbeam used) is quite reasonable.

2) We cannot extrapolate the 1/r^2 law back to short distances in front of the flashlight. This would predict an infinite intensity at zero distance anyway, which is absurd.

But assuming that the lux reading was taken at a good distance from the bezel, we can apply the following relation for long distances:

Intensity * Distance^2 = constant.

Re: Quantifying urban throw (the \"washout effect\")

Andrew,
I agree that there is a cone of light and that once you are in the "1/r^2" region, the lux measurement will be most indicative of the beam's intensity. However I do not think one meter is far enough away for some of the lights being tested. They are also tested with a light meter that has an averaging sensor that does have physical size to it and is measuring light that could either be in column or still in the process of divergence; giving both conditions equal weight, if I am not mistaken. If the optics and source of light are similar, this is no issue. It becomes more of an issue, IMHO if the measurements are of significantly dissimilar light sources; an 8" parabolic reflector compared to a 1/2" spherical optic would be an exaggerated example. A truly collumiated beam has a hole in the center which might be called the light sources shadow, for want of a better word. This hole could be averaged into the lux measurment and underestimate the beams effect at distance.

As I understand it, the more effective your collumation of light is, the less the "1/r^2" rule will apply. Isn't this rule based on lambertian distribution of light? /ubbthreads/images/graemlins/thinking.gif

Re: Quantifying urban throw (the \"washout effect\")

You raise a legitimate question about whether Quickbeam's choice of distance is large enough.

The 1/r^2 law is always valid for each point that emits light. It becomes a good approximation for the entire flashlight when the distance at which the intensity is measured becomes much larger than all the dimensions of the emitter, reflector, lens, etc. For very large reflectors, 1 meter might not be enough. My Vector spotlight has a beam spot that doesn't settle into its final shape until about 2 meters away. In constrast, most of my handheld LED beams do this in well under 1 foot.

You can always find some distance past which the 1/r^2 law takes over. With the exception of lasers or searchlights, that distance isn't going to be enormous.

An improved method would be to make the extrapolation more rigorous by taking the lux measurement at a larger distance, such as 5 meters. But most flashlights have small bezel dimensions, and I doubt that improved measurements would shake up Quickbeam's ranking system very much.

Re: Quantifying urban throw (the \"washout effect\")

Andrew,
I think I am following, for the most part! /ubbthreads/images/graemlins/blush.gif At some distant point from the source of light, when you are in the section of the diverging cone of light, your inverse square rule should hold up. I can see where readings taken at various distances beyond might however imply a starting point or source of light that is not truly the case in possibly location as well as intensity.

"1/r^2 law is always valid for each point that emits light" <-- isn't this based on the presumption of a lambertian distribution?!?! If the point source is a photon sniper rifle instead of a shotgun, wouldn't this law be effected? No matter...........

As far as the spectrum of light output, I do think this may have bearing but to what extent, I do not know. If we consider a good, full spectrum light source that all of us are familiar with and see what it does, we may be able to infer some effects on our artificial and portable light sources.

Consider the sun. Although it is depicted as yellow, the light coming from it is pretty dang white I believe; maybe even on the blue-white side. On a real clear day at high noon, what color is the sky at the horizon? Blue, right? well if it is blue then this means we are seeing blue light from somewhere, which of course is from the sun. That blue that we are seeing is being reflected and bounced in the atmosphere so it obviously has not traveled in a straight line from the sun and it has not made it to target. The blue light has not been absorbed by the atmosphere but dispersed and scattered all about. So it would seem that blue is not a good light to be sending out in an atmosphere with hopes of it staying on course and bouncing off reflective objects in a distance. If you look at a relative spectral power distribution from Lumileds, you can see that a bunch of a luxeon's output is in the blue spectrum. The further you send this light in any atmosphere, the more likely some of it will be dispersed and not useful in "bounce back". To some extent, I believe this is cause for some statements about incans having better results at distance than LED lights. Perhaps space police would have better luck with LED's than those earth bound. /ubbthreads/images/graemlins/smile.gif

OK, it isn't a question of whether I am in this over my head. More important, can I handle the pressure of being below the surface so deep! /ubbthreads/images/graemlins/icon15.gif /ubbthreads/images/graemlins/wink2.gif

Re: Quantifying urban throw (the \"washout effect\")

My head hurts /ubbthreads/images/graemlins/thinking.gif

Re: Quantifying urban throw (the \"washout effect\")

McGizmo,

According to this page, a Lambertian source is a point that radiates energy following a cos(theta) law, where theta is the angle away from the perpendicular. A Lambertian surface radiates the same amount of energy in all directions independent of angle.

So for a Lambertian surface, the 1/r^2 law would apply in a particularly simple way: the intensity of light would fall off as 1/r^2 at long distances and be independent of viewing angle. This is how the HD Luxeon emitters work. In the case of flashlight beams, I was making a more limited claim: that the intensity can vary in any way by angle, but it will fall off as 1/r^2 along any particular ray. The constant of proportionality can vary with angle. The ray that we are most interested in is the in the center of the beam. The 1/r^2 law along a ray is based on geometrical considerations and is independent of any assumptions about the distribution of light emission from the source.

I found a lot of stuff about light on this page.

For light scattering in air, there definitely is an effect called Rayleigh scattering that increases for shorter wavelengths. However, the attenuation length is 30 kilometers even for violet light. The attenuation of blue light over 100 feet or so would not be measurable. This makes sense, since sunlight travels through about 100 km of atmosphere and still a lot of blue light gets through.

This may come as a surprise: "The nature of the multiple scattering also shows that no color penetrates fog better than any other." So much for those goofy yellow headlights. /ubbthreads/images/graemlins/smile.gif

For the differences between incandescents and LEDs, I'm still putting my bets on simple matters of lumens and focusing. We notice the issue more with LEDs because LED flashlights are capable of having both high lumen output and poor collimation, two things that almost never go together with incandescent flashlights.

So far, we have very high output LED emitters (the Luxeon V) and small focusable emitters (Luxeon I and Luxeon III). But we don't yet have both. If the Streamlight ProPolymer Luxeon had its output doubled from around 30 lumens to 60 lumens, its throw would increase by a factor of sqrt(2) to 78 meters on Quickbeam's original scale. That would match the Mag 3D.

Existing Lux V flashlights could have their emitter dimensions halved, while maintaining the same output. This would cut the beam angle in half, quadrupling the intensity at a given distance and doubling the range of throw. This change would make the Surefire L4 throw better than the existing L5, and the L5 would increase its throw to 74.

It's only a matter of time. /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/thumbsup.gif

Re: Quantifying urban throw (the \"washout effect\")

Well, this is all very interesting. But, at the risk of annoying some folks, and with all due respect to those who have put forth this effort, I have to wonder if it isn't yet another valiant attempt to validate feelings that LEDs are simply better???

I THINK I understand the theories being discussed here; and I think the real question is NOT about "throw", or reach, or whatever you want to call how far a given light is a viable illumination tool.

Forgive me if I wasn't clear, but in the scenarios (real deal in a cluttered back yard, etc.; not a parking lot) I was in, the LED light was simply swallowed up and never got to the shadow area; regardless of how close or far away I was. When I said the light was useless outside the pool of ambient light, I was serious.

I think Don is closer to the real answer; its all about the wavelengths and how they interact. But then, I'm just guessing too... /ubbthreads/images/graemlins/grin.gif

LED lights are cool; I have more of them than I do incans. But they are NOT the jack-of-all-trades some folks would have them be.

Re: Quantifying urban throw (the \"washout effect\")

Asdalton,

First of all, nice analysis. This is not an easy situation and tackling it is neither straightforward nor intuitive.

You said above "But when there are streetlights, your eyes adapt to the background illumination and not to the flashlight. So the absolute brightness of the projected beam becomes important. A diffuse beam in these circumstances will not be useful at long distances even if it puts out a lot of lumens." But these two statements comingle two completely different things. The absolute brightness of the beam could be characterized as the lux reading (flux/area). But this has nothing to do with either the diffuseness of the beam (which is really the beam profile) or the total lumen output (flux). If a "tight" beam from a 115 lumen Lux3 in a Mag head puts 1,000 lux on a target, the beam from a 3,200 lumen Osram bulb in the same Mag head will be much more "diffuse", but will almost certainly give a much higher lux reading and result in better illumination of the target. Granted, this is an extreme example but a valid one I use simply to illustrate the point.

Having said that, I think what is really at issue is the question of contrast in detection. The situation of a dimly lit hallway aside a brightly lit storefront is an illumination setting where the light and low level contrasts in the hallway are seemingly overwhelmed by the contrast against the brightness of the surrounding scene. But boosting those levels of contrasts in the hallway by the same amount by shining a light on them will be detectable whether it is in the country or in the city. So, doubling of the light you put on the target is perhaps not the key consideration, but rather increasing the contrast sufficiently to allow target detection.

I think this is the case based on parallels with machine vision. Image detection is not based so much on the absolute brightness of the scene or field of view, but rather on edge detection, motion detection or contrast detection. Robots that navigate by machine vision don't do lux measurements or lumen measurements. They capture the scene and extract features in a crude analog to what the human visual system does.

What do you think? Am I off base?

Wilkey

Re: Quantifying urban throw (the \"washout effect\")

Andrew,
Thanks! /ubbthreads/images/graemlins/blush.gif I reckon if we have lights that are hitting targets 30 klicks out and we notice that the blue and violets are starting to get lost, we may have other issues of even more significance. /ubbthreads/images/graemlins/icon15.gif

It is reassuring that you too feel that the real difference between the LED and incans is more basic and not due to spectral differences.

With more experimenting and messing around, I am seeing more and more evidence that the LED just kicks the incan's butts at the other end of the power game! I can get good useable light from a Luxeon at 1% of its designed power rating. Do that with an incan! /ubbthreads/images/graemlins/tongue.gif

BTW, peeked at the links and now my head hurts too! /ubbthreads/images/graemlins/smile.gif