Aspheric lenses, what do we know about them? Which are the best for which leds?

Plano Convex, Convex, Aspheric - The domed lenses that you see on most any zoomie and the custom ones for projectors or car lights, that we tend to use in flashlights.

My questions are:

  1. Why does a larger diameter lens make a smaller beam when focused?
  2. How does the ratio of diameter to depth of the lens affect the size of the beam? If a 75mm diameter by 25mm thick lens and a 75mm diameter by 15mm thick lens were to be used on the same led, which one would give the smallest focused beam and why?
  3. What is the relationship of led die size to lens diameter? In other words, is bigger always better? LOL, I know I'm asking for trouble there, but keep it in reference to these lenses.Wink
  4. Is there an optimum size for the different leds, as far as diameter of the lens?
  5. Is there an optimum diameter vs thickness for different leds?
  6. Is there a way to determine this ahead of time. A scientific way of determining?

Have at it. I want to learn something before I go and buy some big aspheric lens and end up getting the wrong one.

I don’t know how to answer any of your questions (tho I’m definitely interested in reading other’s answers) but the people at edmunds lenses are more than willing to answer your questions if you don’t get satisfactory answers here.

I can tell you this.... They STINK as a headlight lens!!! I thought they would be great... WRONG! They put out a "Line" of light across the road, if you raise them up to where the line is gone... they are just not bright!!

I have some literature on the Aspheric. The larger ones make the smaller beam because the deeper the lens, the more dense the output. The radius of the dome tightens the beam & acts like a projector lens. I'll read up on the rest of the questions and see fi I can help.

Dan.

Can’t help… Have similar questions. Sub’d.

Good to see you here. Been a while.

  • Why does a larger diameter lens make a smaller beam when focused?

  • The same shape lens and larger has a longer focal length. The size of the spot at a given distance is about the LED size times the ratio of that distance to the focal length.

  • How does the ratio of diameter to depth of the lens affect the size of the beam? If a 75mm diameter by 25mm thick lens and a 75mm diameter by 15mm thick lens were to be used on the same led, which one would give the smallest focused beam and why?

  • The thicker and more curved the lens the shorter the focal length. It also depends on the refractive index of the material. So the thicker 75 mm. lens would have a larger spot.

  • What is the relationship of led die size to lens diameter? In other words, is bigger always better? LOL, I know I’m asking for trouble there, but keep it in reference to these lenses.Wink

  • If the whole light scales up together, LED and lens included, the angular beam spread does not change.

  • Is there an optimum size for the different leds, as far as diameter of the lens?

  • A larger lens of the same focal length will catch more light. It doesn’t depend much on the LED.

  • Is there an optimum diameter vs thickness for different leds?

  • To get the same spot size with a larger LED one needs a larger or thinner lens.

  • Is there a way to determine this ahead of time. A scientific way of determining?

  • Most everything about optics is understood scientifically, but of course it takes more data and more work to get more detail.

    The focusing specification of a lens depends on its focal length, which lesser than 15mm is quite rare. (eg.: lens placed 15mm away from LED, to get fully focused)

    Thus with higher focal length, they need bigger diameter lens to cover the effective light ray from LED (XM-L2 is 125 degree i think)

    Example: (using 120 degree, easier calculation)

    15mm focal length
    ±52mm diameter lens

    20mm focal length
    ±69.5mm diameter lens

    Even so, the light passing through lens according to calculation above is around 60-75% of the light produced only.
    There are still light more than 125 degree angle. Lens is always the larger diameter, the shorter focal length, the better.

    However, smaller the lens (in fully focused senario), the thinner the beam it is. (pretty much useless, unless for flashy that is)
    Ideally for thrower (especially crazy type), its better to have crazy wide lens.

    For exact figure or reference, I couldn’t provide you with anything, since I’m not light field specialist nor a thrower builder (not really need one, maybe I will build a pencil beam for fun) All these are base on optic science I learned, not really precise, but enough I guess…

    Thank you for helping. I have some comments, but I am not sure whether they are about optics or about English.

    To say focal length less than 15 mm., you must be thinking of a particular size lens.

    I don’t know for what optics an XM-L2 will project a 125 degree beam.

    I have a crazy wide lens that is a crazy thrower. That is right.

    60-75% sounds right to me for the best simple lens lights. One can do better with multiple zones. Reflector outside and lens inside.

    Subscribed

    About the science of it, this is old 19th century science. Flashlight optics have not yet progressed as far as 19th century lighthouse optics.

    A few years ago… I was trying out various lenses on lights to see what the results were.
    I live close to Surplus Shed; and if you want to grab lenses off the shelves and try them on lights while you’re there, the owner doesn’t care.
    Of course if you buy a few after your experiments, they appreciate that.
    If you buy some, give good feedback on forums that might entice others to purchase a few lenses… they like that even more.

    I mentioned this on another flashlight forum; and posted the results of some of the lens/light combos. I included the stock numbers of the lenses if there was anything anyone liked and wanted to try themselves.
    I got lectured that you can’t just grab lenses and stick them in lights, you have to do all the math first.
    One member was (politely) being outright insulting.

    Anyway… here’s part of the “lecture thread” I had saved as a Word file (as it has some useful information):

    Re: Aspheric lenses - 52mm/37mm fl - 52mm/33mm fl - 50mm/35mm fl - 50mm/38.5mm fl ~ B

    Melles Griot Asperic Lenses.

    52mm with 37mm focal length is here.

    This is why I wanted to know the distance from the emitter to the back of the aspheric lens:

    See, Fb is 21.9mm - so the pest possible placement of the LED from the back of the aspheric lens would be 21.9mm. MrMimzu found the distance to be 27mm, so moving the emitter slightly closer to the back of the lens should provide more throw.

    The 52mm w/ 33mm focal length, the Fb is only 16mm - which would be bad.

    Conversely, the 50mm w/38.5mm focal length, the Fb is 24.3mm - so if the emitter placement is indeed 27mm, then the 50mm w/38.5mm focal length should throw better.

    The moral here is for maximum throw, the LED should be Fb distance from the back of the asperic lens for best throw. Having the emitter at the exact focal point of the lens should focus the most rays straight ahead - of course the focal point is measured from the middle of the lens, so we have to look at the Fb number.

    Optimal distances from back of asperic lens to emitter:
    52mm w/33mm fl = ~16mm
    52mm w/37mm fl = ~22mm
    50mm w/35mm fl = ~23mm
    50mm w/38.5mm fl = ~24mm

    So, considering the Fb for the 52/37 and the 50/35 are only 1mm apart - I would be surprised if they threw that much different.

    You can get a magnesium fluoride AR coating on the 50mm lenses for $20 and on the 52mm lenses for $40.

    I think he’s saying the output of an XML is 125 degrees so it’s hard to have a lens that is both wide enough to capture that arc at a given distance from the led and has a short enough focal length to focus the beam at that distance. For just about any lens you choose light will be lost to the side.

    As a noob, is that what the Wavien Collar is trying to recoup?

    Yes, hopefully it will continue and we will all be much better for it. It has helped so far.

    I guess one of the things I don't get still, is why isn't there a way to concentrate the light (bend it), to make a smaller spot? Ok, the aspheric does that, but look at the size of a square die pattern of an XR-E and a SST-90. At 100', with an aspheric, the XR-E is very small and the SST-90 is very large. Why can't the SST-90 be focused to a relatively small spot? and if it can be, what will it take to do that? As an example only, say the XR-E square die spot is ten feet at 100 yards and the SST-90 is 30 feet at 100 yards. Why can't the SST-90 spot also be 10' at 100 yards and what will it take to accomplish that?

    you mean by the time of frensel lens ? :nerd_face:

    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 :wink: ) 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 :stuck_out_tongue:

    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 hope that lot is understandable :nerd_face:

    Cheers David