Scattering of light

[red02]

Re: Why are warm LEDs preferred by most on this forum?

surprise surprise, the warm penetrates better while the cool is confounded by all the crap in the air between the user and target :whistle:
and thankfully he switches back and forth between both lights a number of times, all with the same result!!

this sounds like a circular argument, your simply restating your premise as a conclusion.

the snow density is continuously changing. The amount of particles in the path of the light is not constant even if you take it second by second and only consider one light. Since the CW light has to travel longer to get to the ground, naturally there would be more snow in it's path. If you look at the hotspots they are nearly the same. Not to mention the efficiency difference between 5A and WC that obscure any results. my point is that there are no meaningful conclusions as the testing method is not scientifically sound.

 

[saabluster]

Re: Why are warm LEDs preferred by most on this forum?

I think both sides are missing something here.

That video was outstanding and for anyone to say they can't see a clear improvement in visibility is beyond me. However there is not any more light on the target. The warmer beam is just not reflecting off the snow as much as the cool white one. It is being absorbed.

I used to do sign work and part of that was installing vinyl that had holes in it so you could still see through the glass. That said if the color of the vinyl was white it made it next to impossible to see through if there was much light at all on your side. If the vinyl was black even with the same lighting conditions you would be able to see the other side far better. The white was just reflecting back so much that it drowned out the rest of the visual information. I believe that is all that is happening in that video.

 

[red02]

Re: Why are warm LEDs preferred by most on this forum?

I think both sides are missing something here.

That video was outstanding and for anyone to say they can't see a clear improvement in visibility is beyond me. However there is not any more light on the target. The warmer beam is just not reflecting off the snow as much as the cool white one. It is being absorbed.

I used to do sign work and part of that was installing vinyl that had holes in it so you could still see through the glass. That said if the color of the vinyl was white it made it next to impossible to see through if there was much light at all on your side. If the vinyl was black even with the same lighting conditions you would be able to see the other side far better. The white was just reflecting back so much that it drowned out the rest of the visual information. I believe that is all that is happening in that video.

The snow fall and wind gusts are random, the output of the two lights are uneven, the angles at which the two lights are positioned are uneven, the distances to target are uneven to add to this the video is grainy. The video auto-corrects tint, which is what we are looking at. It baffles me how anyone can come to -any- conclusion using that video.

 

[saabluster]

Re: Why are warm LEDs preferred by most on this forum?

The snow fall and wind gusts are random, the output of the two lights are uneven, the angles at which the two lights are positioned are uneven, the distances to target are uneven to add to this the video is grainy. The video auto-corrects tint, which is what we are looking at. It baffles me how anyone can come to -any- conclusion using that video.

All we have to do is use that super computer between our ears. I am also going on experience.

Random snow fall? Really? So your telling me that every time the cool white one was on it was blowing hard and then as soon as the neutral white is turned on it magically slowed down? Back and forth? Over and over? Yes it is auto-correcting tint but that is inconsequential. It is not erasing the light by changing the white balance.:rolleyes: And the video is not all that grainy. Besides that aspect is exactly the same for both subjects. Yes the positioning of the flashlights can be an issue but I can tell you from experience that any of that in play here is negligible at best.

 

[was.lost.but.now.found]

Re: Why are warm LEDs preferred by most on this forum?

The snow fall and wind gusts are random, the output of the two lights are uneven, the angles at which the two lights are positioned are uneven, the distances to target are uneven to add to this the video is grainy. The video auto-corrects tint, which is what we are looking at. It baffles me how anyone can come to -any- conclusion using that video.

+1, at least someone sees what I see. But then that is my point as well, Everyone is always going to interpret what they see in their own way. I'm only trying to say that the video is not a smoking gun because the two lights are not given equal footing. At a minimum, one needs their own results seen with their own eyes to draw fair unbiased positions. Three cheers to Locoboy who has done that and has his own opinion from his own experience.

With 6 posts in 3.5 years, you clearly only write when you have a point to make.

For this case, this point, I couldn't agree more.

Hah, CPF's Silent Bob. I love it.

All we have to do is use that super computer between our ears. I am also going on experience.

Random snow fall? Really? So your telling me that every time the cool white one was on it was blowing hard and then as soon as the neutral white is turned on it magically slowed down? Back and forth? Over and over? Yes it is auto-correcting tint but that is inconsequential. It is not erasing the light by changing the white balance.:rolleyes: And the video is not all that grainy. Besides that aspect is exactly the same for both subjects. Yes the positioning of the flashlights can be an issue but I can tell you from experience that any of that in play here is negligible at best.

Yes, I watched the video at least 6 times last night and there are moments when you can see the blowing snow pickup while the WW light is on, and its beam becomes just an obscured and full of lit up snow as the other CW led. You don't see it as much because of timing, happenstance, and the already poor positioning of the lights relative to the blowing snow and the camera angle.

 

[red02]

Re: Why are warm LEDs preferred by most on this forum?

All we have to do is use that super computer between our ears. I am also going on experience.

Random snow fall? Really? So your telling me that every time the cool white one was on it was blowing hard and then as soon as the neutral white is turned on it magically slowed down? Back and forth? Over and over? Yes it is auto-correcting tint but that is inconsequential. It is not erasing the light by changing the white balance.:rolleyes: And the video is not all that grainy. Besides that aspect is exactly the same for both subjects. Yes the positioning of the flashlights can be an issue but I can tell you from experience that any of that in play here is negligible at best.

The amount of particles in the beam's path changes throughout the video. I do not know if it is more snow, more wind or both. I do not know when the wind picks up, when it doesn't. Thats the point, I just don't know.

Therefore I must discard any time the lights are one individually.

If you look at 1:25 when both beams are shown, the CW appears to have more snow. However it eclipses the 5A beam which is not as bright. This means that the comparison is not accurate to begin with.

The video lacks the resolution to show any details. We do not know which light the ccd or whatever that camera uses is more sensitive to. Since it auto adjusts to color, does also auto adjust to amount of light? This would skew the results tremendously. What if the light is more sensitive to CW? Is this a good CCD? Is it a good camera, or a camera phone? This is the bane of our discussion, we cannot trust the video with rendering the amount of snow, since it color corrects we cannot trust it with accurate tint. How can anyone draw a conclusion based on that video if can't trust anything thats on the screen?

Kudos to those who have formed their opinion based on personal experience. Its not a problem to be biased as long as you have personal experience to back it up.

 

[Ragiska]

Re: Why are warm LEDs preferred by most on this forum?

saabluster,
i think some here honestly attribute the tk20's clearer view for lack of gusts, not realizing the snow gusts are still there, but the tk20 simply penetrates it far better. they think every time the tk20 switches on it magically stops blowing, since they don't see it.

 

[saabluster]

Re: Why are warm LEDs preferred by most on this forum?

saabluster,
i think some here honestly attribute the tk20's clearer view for lack of gusts, not realizing the snow gusts are still there, but the tk20 simply penetrates it far better. they think every time the tk20 switches on it magically stops blowing, since they don't see it.

Yes I agree but to use the word "penetrates" is the wrong word. It is not that it penetrates any better it is just that it doesn't throw up a "smoke screen". So it is not what it does but rather what it doesn't do that increases visibility.

 

[PayBack]

Re: Why are warm LEDs preferred by most on this forum?

and if MORE light is getting back to your eye (instead of on target), what does that mean for total light on target?

oh yeah :ohgeez:

You do realise that the light from the target actually has to get back to your eyes for you to see it?

Seriously you're making even less sense now. The extra light getting to your eyes with white light is bouncing off the target.

saabluster,
i think some here honestly attribute the tk20's clearer view for lack of gusts, not realizing the snow gusts are still there, but the tk20 simply penetrates it far better. they think every time the tk20 switches on it magically stops blowing, since they don't see it.

You're still not conceding the fact the warmer light does NOT pentrate better. More light reaches the target when it's white. More light reflects back into your eyes FROM THE TARGET. So less light reaches the target from warmer LEDs so it's not true they penetrate better. It's just having less blue light reflect back makes it look like it does due to reduced glare.

Why can't you understand that? You know it hasn't stoppeed snowing just because it looks like it, so why can't you understand it's not penetrating better just because it looks like it?

 

[FloggedSynapse]

Re: Why are warm LEDs preferred by most on this forum?

Something to do with the wavelength, thats as far as my knowledge goes I'm afraid.

he was referring to neutral/warm tints cutting through haze, dust, and fog better, which is true

Indeed. LEDs scatter much more than incandescent lights. This effect is very dramatic on a night with even a small amount of haze or fog. Try it sometime - compare an incan torch next to any LED torch. In bad fog LED flashlights are nearly worthless.

There are two reasons for this. First, most 'white' LED lights consist of a blue LED combined with a phosphor. Spectrum looks something like:
http://en.wikipedia.org/wiki/File:White_LED.png

You'll note there's a huge spike around 425-450 nm, from the LED. Second, because of Rayleigh scattering this blue light gets scattered - much more so than red/green light. see:

The light does not have to pass through 'hundreds of cubic miles' of atmosphere for this phenomenon to manifest. It's one of the reasons the beam of a blu-ray laser is very visible at night, despite the fact the eye is not very sensitive to 405nm radiation.

However, since both warm and cool LEDs have this blue spike in the spectrum, they both 'penetrate' poorly compared other light sources.

Another thing to keep in mind is blue light is not very useful for seeing things (contrast). This can be measured and has to do with the majority (> 90 %) of the color sensitive cells in the retina being red/green sensitive. So yes most people will agree too much blue light (while it may look keen) amounts to glare.

Another observation about perceived brightness - since the eye is nonlinear in its response to brightness a +/- 50% difference in lumens does not appear dramatic. A coarse rule of thumb is it takes 4 times the power to appear twice as bright.

Personally I prefer the 'neutral' white LEDs. Cool white.. a bit too much blue, and warm is too red. I'd like to see more makers offer neutrals.

Here's another graph comparing the spectrum of various light sources:

 

[kingofwylietx]

Re: Why are warm LEDs preferred by most on this forum?

In the discussion about wavelengths effect in fog and dust, there is a light bar mfg that states their amber leds (claimed to be as bright as the whites) produce less glare. I'll provide the link below. The problem I see with the link, is that they turned off all their other lights and only used a couple of amber light bars. The simple act of reducing how much light they threw forward would reduce glare.

Is it a gimmick? http://www.rigidindustries.com/category_s/93.htm

 

[PayBack]

Re: Why are warm LEDs preferred by most on this forum?

Why do people keep bringing up Rayleighs law?? Fog particles are much to big for this law to be relevant. Flogged, you're correct about blue light but that those space hogging graphs you've included are not relevant...

"This explanation is flawed for more than one reason. Fog droplets are, on
average, smaller than cloud droplets, but they still are huge compared with
the wavelengths of visible light. Thus scattering of such light by fog is
essentially wavelength independent. Unfortunately, many people learn
(without caveats) Rayleigh's scattering law and then assume that it applies
to everything. They did not learn that this law is limited to scatterers
small compared with the wavelength and at wavelengths far from strong
absorption."

@kingofwylietx

Not a gimmick if they're going to be installed by someone with no experience... less blue light = less glare. Well for the majority of human eyes anyway. However well positioned/adjusted white lights can be as effective where the extra light compensates for the minumal increase in glare. Which is why most modern cars come with white fog lights. you just have to be more careful with the beam shape etc.

 

[FloggedSynapse]

Re: Why are warm LEDs preferred by most on this forum?

I think Rayleigh's law still applies. With fog droplets (for example) more rays get scattered off axis (via reflection/refraction/total internal reflection) and those scattered photons will be more much more visible if they are blue/violet. Red light will be redirected just as much by the fog droplets, just most of it will not be visible. Effectively blue/violet light does not travel nearly as far as red light before scattering to a haze. Fog droplets will accentuate this effect dramatically.

OK.. so I could have scaled the graph down more, but it simply points out how much more blue and violet light is scattered by the atmosphere.

Even on a dry and clear night the cone of light from an LED flashlight is much more visible than from an incandescent flashlight. On a foggy night so much light is scattered from an LED torch it's nearly worthless for illuminating things.

I've wondered if perhaps a different phosphor along with a blue filter in the torch lens could help in this regard, though the LED would not be as efficient.

 

[mudman cj]

Re: Why are warm LEDs preferred by most on this forum?

Again, Rayleigh's Law does not apply! If the size of the particles is near the wavelength of the light, then you would use Mie Theory (aka Lorenz-Mie-Debye Theory). The application of Mie Theory is not at all straightforward. I know because I have applied it to a similar situation which I cannot discuss until the patent application is filed. ;)

 

[Ragiska]

Re: Why are warm LEDs preferred by most on this forum?

payback,
i think you are ignoring the tyndall effect, which is very observable over even very small scales. smoke from tobacco and 2-stroke engines (for example) appear blue because of tyndall scattering. blue eyes are blue from tyndall scattering.

fog lamps are used on cars because of the tyndall effect in fog/mist (not rayleigh as you assumed before)

 

[FloggedSynapse]

Re: Why are warm LEDs preferred by most on this forum?

Again, Rayleigh's Law does not apply! If the size of the particles is near the wavelength of the light, then you would use Mie Theory (aka Lorenz-Mie-Debye Theory). The application of Mie Theory is not at all straightforward. I know because I have applied it to a similar situation which I cannot discuss until the patent application is filed. ;)

I tried to explain why I think otherwise in my above post.

Water droplets will bounce more light rays via reflection and (total internal) refraction. Imagine how the light is bounced back and forth between and among the water droplets (think ray tracing). This will accelerate the (rayleigh) scattering of the high frequency blue/violet photons within a volume of space.

Whatever the reason, you can easily observe for yourself that LED flashlights don't 'penetrate' nearly as well as incandescent lights on foggy nights.

 

[mudman cj]

Re: Why are warm LEDs preferred by most on this forum?

I tried to explain why I think otherwise in my above post.

Water droplets will bounce more light rays via reflection and (total internal) refraction. Imagine how the light is bounced back and forth between and among the water droplets (think ray tracing). This will accelerate the (rayleigh) scattering of the high frequency blue/violet photons within a volume of space.

Whatever the reason, you can easily observe for yourself that LED flashlights don't 'penetrate' nearly as well as incandescent lights on foggy nights.

I never argued that there was no difference in fog between light sources with more or less blue content. I only said that you can't predict the effect using Rayleigh's Law.

 

[FloggedSynapse]

Re: Why are warm LEDs preferred by most on this forum?

I disagree, Rayleigh scattering explains the phenomenon. A ray of light passing through a volume of space full of water droplets will spend much more time there bouncing back and forth between water droplets. In effect its length will be greatly increased leading to more scattering within a small volume.

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

See this image of multiple reflections:
http://en.wikipedia.org/wiki/File:Ray-traced_steel_balls.jpg

Perhaps you have an alternative explanation?

 

[mudman cj]

Re: Why are warm LEDs preferred by most on this forum?

I disagree, Rayleigh scattering explains the phenomenon. A ray of light passing through a volume of space full of water droplets will spend much more time there bouncing back and forth between water droplets. In effect its length will be greatly increased leading to more scattering within a small volume.

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

See this image of multiple reflections:
http://en.wikipedia.org/wiki/File:Ray-traced_steel_balls.jpg

Perhaps you have an alternative explanation?

You are still overlooking my point. Rayleigh scattering is only applicable when the particles scattering light are <10 times the wavelength. For particle sizes larger than a wavelength, Mie scattering predominates. There is no way the water droplets in fog are smaller than 50 nm. Droplet sizes in fog tend to be in the range from 1 to 30 microns.

 

[Ragiska]

Re: Why are warm LEDs preferred by most on this forum?

You are still overlooking my point. Rayleigh scattering is only applicable when the particles scattering light are <10 times the wavelength. For particle sizes larger than a wavelength, Mie scattering predominates. There is no way the water droplets in fog are smaller than 50 nm. Droplet sizes in fog tend to be in the range from 1 to 30 microns.

mie theory is a degenerate solution (spheroid particles only) to tyndall scattering, which is EXACTLY what happens to light in fog.