What I meant is, normally lens for flashlight only manufactured up to 15mm focal length and no more. (you want a pencil beam > I want ) The diameter provide is to typical value needed to cover 120degree angle only, thus for particular focal length but larger diameter, it catches more light.
It is stated the viewing angle of XML2 is 125 degree in spec sheet, however that is true for zone(angle area)intensity higher than 50%. (there is still light even more than 160degree outwards)
And of course, 60-75% applies to single lens thats coveres the 125 degree ONLY, multiple optics (reflector/lens/etc) will make it better, but more often is complicated, hard to setup.
It could, but what would happen then is the light rays would cross and diverge again. Like an x in front of the lens but rotated around the lens center. The best you can do and have the die stay in focus at any distance down range is to have the light rays come out parallel to each other. Think of when you were a kid using a magnifying lens to light leaves and such, it only came to a tight point at one distance.
The science behind lens flashlights? For who has had the subject back then: look up your secondary school ray physics (it is actually 17th century science, not 19th, Galilei and Huygens could have helped you design the best optical design for your flashlight, heck, they could have made quite descent lenses for it!), it will tell you most of what is happening with aspheric flashlight optics (reflector optics is more complicated btw).
There are a few complications because we use such short focal lengths. *The lens must be aspheric instead of spheric to produce a better image, *because the cone of light that the lens catches is so wide there is light loss at the edge because of direct reflection, AR-coating will help a bit.
There's good fortune as well for use in flashlights: the lens does not have to be melles-griott quality, a crap chinese lens does the job almost as well because we do not need a perfect die image, a bit blurry is actually more desired, as long as the lens is not so bad that the light gets all over the place.
In short, theory predicts, and I found this in practice as well, using a fixed emitter size and brightness, hotspot brightness (throw) is lineair with aspheric lens surface area. Given that surface area, a shorter focal length lens (thicker) gives a larger hotspot (more light), a longer focal length (thinner) lens gives a smaller (but not brighter) hotspot. So unfortunately to get more throw optically, the only way is using bigger lenses. What does not help is that to get twice the throw, the lens must be twice the surface area, but will be 4 times as heavy, not counting the increased size and weight of the flashlight housing. You can avoid that heavy lens a bit by using a thinner longer focal length lens, that gives the same throw but with a smaller hotspot with more light loss. (I was pleasantly surprised by the UF-1405 that they did not go for the thinner long focus lens but put a massive chunk of glass in there :-) )
I realise that some (a lot) of this post has already been said but I was called away before I could post it, so it is what it is
The degree’s that are mentioned in the data sheet are for 50% emittence, that is the point (in degree’s) where the output is 50% of the output at 90 degree’s.
Look at the image we have all seen before, if you measure the output through a small hole at 90 degree’s then measured it at the red line the result will be 50% of the output.
So from one red line to the other (125 degree’s) you get most, but not all, of the output.
For an Aspheric lens the lower the degree’s the better, the led is throwing more out front.
Have a play here,
With B = the led (62.5), C = center of lens (90), A = ignore
b = half of lens diameter, a = focal length, c = ignore
In the linked calculator enter 62.5 into “Angle A or B” and half of the diameter of the lens into “Side b” then “calculate”.
The top figure is the focal length for the 125 degree angle, less is better, more not so good.
I used a bunch of Optolife aspheric a couple of years back. I was having trouble getting decent 50mm from Kaiodomain, Dino Direct and the usual suspects. Although not MG quality they were a significant improvement over most China glass at a reasonable price. The AR coating was a good option.
So, an aspheric lens cannot be used to produce a very tight spot at, let's say 10 miles, with a led and a reflector cannot be made that does the same, so in reality leds are no good for real long distance throw (several miles), and lasers are all that remains for that purpose?
oh yes they can throw that far, as long as you make the aspheric lens/reflector big enough. Put a dedomed XM-L in one of those WWII air defense search lights and you have a pencil beam that goes 20 miles.
So, like I said, it's not really something we can do in a flashlight, so a laser would be about the only way to go, but a laser beam is too small and not white?
I’ve always wondered about the Bat signal and whether or not it would work as depicted. Wouldn’t a Bat shaped die be the best way to project a Bat shaped image? I guess one is casting a shadow and the other projecting an image. Shadows just seem too easily blurred by distance.
I can not remember the details of Ervin's design, but adding a reflector to an aspheric light does not improve the throw. It will add to the flood around the hotspot, usually in a ringy way.
This has been discussed a lot, so I am saying more than immediately necessary here.
The simple optics approach (expanded):
The ray that goes through the center of the lens from anywhere on the LED goes approximately straight through the lens. So the angle of that ray is not changed. So the angle of the beam is the same as the angle of the LED viewed from the lens. The spot size is the LED size times the ratio of the distance from the lens to the target to the distance of the LED to the lens. For a fixed lens diameter, moving it closer to the LED and making it thicker so it focuses catches more light but makes the spot bigger. Moving it farther from the LED and still in focus makes the spot smaller but loses more light. If the lens is small relative to the angular spread of the LED these two effects compensate and the throw does not change. For typical LEDs and lenses, the outer part of the lens sees a bit less light than the center, so a longer focal length does increase the throw but not as much as it decreases the total output.
More general optics point of view:
Another way to look at it, which I really like, is Dr. Jon’s luminance. Simply put, you can’t make the front of the light look any brighter than the LED. To get a brighter spot, you have to get a wider flashlight.
Relation to other physics problems:
From Liouville’s theorem (Hamiltonian), the phase space of a classical statistical system, such as a beam of light is a constant of the motion. It can get tangled up, such as with a diffuser, so it looks bigger, but it can’t even look smaller.
This is roughly the angular spread times the position spread. So if the beam comes out through an aperture of a certain size, it must have a minimum angular spread. The angular spread of the LED is reduced by the optics, but only by the ratio of the optics size to the LED size. To make the beam narrower than that, you have to remove some of the rays from it.
I know the first point of view is obscure. To me it is interesting that this is a special case of a much more general principle. I know it may not happen, but it would be nice if this helped someone else be aware of the unity of physics.
The other two approaches are simple enough to be of general use to others on the forum. The idea that a ray passing through the center of a lens goes approximately straight is very simple and useful. The idea that the luminance can be understood as the brightness of a surface and especially the idea that this quantity cannot be increased by passive optics have been discussed repeatedly in relation to dedoming (which does increase luminance).
These, and especially the impossibility of increasing luminance, are needed here to give a clear answer and keep the discussion here and elsewhere from bouncing back and forth without a solid conclusion, as it had been.
I am changing the order, so people won’t have to read what they can’t understand to get to what they can.
Well, most of the time, the people who know, can't understand it in a manner that the dummies who know nothing, can understand. My problem is that I walked through high school, in a small town, in the 60's and didn't have to take mandatory classes in 11th and 12th grades, because I had enough "points", by 10th grade, that I could pick for myself. I never went farther than basic math, no algebra, or anything farther than multiply, divide, etc. I never went through any science classes because only 1 year of biology served as the necessary mandatory class. I did literally nothing the last year, taking the mandatory English, speech and athletics classes, (yes mandatory, shows what they were about) and the rest were study halls. I sat through 5 study halls a day. I had absolutely no interest in any of it at all and since they let me do it, I skated through it.
So, when you start quoting stuff like in some of the posts here, it is totally lost on me. What I need is, "you are right, you need the biggest, thinnest aspheric lens you can get and no matter what, you will not produce a small beam at 1 mile or more. It's not possible" That's the kind of thing I need. Hopefully, the smart people here will gain something from the answers, but I won't. Now you know why I will never, ever take an electronics class and try to learn circuits. I would have to take years worth of other classes, to even begin to understand the basics.
The American school system has a poor record. I have been taking care of kids. They are enormously motivated to learn. Somehow the schools beat that out of us. Most successful people in the US either went to school in some other country or learned somewhere else.
Examples: I have a really excellent scientific education and my career ground to a halt early, while my brother who never finished any school, as far as I know, is doing well paying, very interesting work on car engines.
I think he situated the lens within the reflector so that without blocking light that hits the reflector the unfocused light that normally misses the reflector is focused by the aspheric. The aspheric is smaller in diameter than the reflector and sits below the rim.
) The diameter provide is to typical value needed to cover 120degree angle only, thus for particular focal length but larger diameter, it catches more light.
