Using MULTIPLE lenses - taking the mystery out of it...

Flashlight Modding and DIY Parts
1

I have mentioned several times, here and there, usually in the context of my IR illuminators i built when i talk about my scratch-built night vision? That i have built several “multiple lens” illuminators. One is a dual lens setup, one is a 3 lens, another is a 4 lens.

It was a several month long journey. I am fairly sure what applied to LED IR emitters, also applies to visible LED emitters…the big difference would be i was working with IR and therefore one main centroid wavelength (mainly 850nm) and if memory serves me, our visible emitters have 2 or more wavelengths. Other than that, and the refraction differences, they should be very similar.

There will be “some math”, but really, its nothing THAT major. If you took “college prep” in high school? i am sure you have more than enough math under you to follow the plot line. Algebra I + II, some geometry, a little trig here and there, and youre golden.

The PURPOSE of this, is to form a sort of “white paper” that will allow you to easily begin using 2 lenses, and i will end it with a description of how if you can use 2 lenses fine, then you can use 3 or even 4 lenses. I should note? while following the math is HELPFUL, its not NECESSARY. I found, the math got me a quick start, and after i had run the math enough times? i began to be able to simply pick up lenses, focus them on a CFL twisty to draw up an image on white notebook paper to judge the 2 lenses FLs… and can simply use them and “play” to design something.

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heres where it started… i had the idea, a strong hunch… that using more than one lens would be “better”. Cameras/telescopes/monoculars/binoculars/microscopes… all use more than one lens, my GUT told me i should be using 2 or more lenses to focus light up at distance… but HOW?

i got 2 thin biconvex plastic lenses, out of the “eyepiece” of an old cheap BBGUN scope (4x fixed) and fooled around and got them to make a cheap keychain laser to grow bigger and smaller on the wall… so, i knew it COULD work, but… i was like a kitten playing in the dark with a flashlight beam (pun intended). I had to find some MATH to understand it, or this would take forever.

Also? i had to first find SOMEwhere, that it was the proper idea to use 2 or more lenses… i first had to QUALIFY the idea, then i could try to QUANTIFY it.

so… the first thing i found that told me i WAS on the right track? a white paper from edmunds scientific, heres the link…

http://www.edmundoptics.com/TechSupport/resource_center/downloads/published_articles/prototyping-illumination-systems-photonik.pdf

Equation 1 ? basically sets up a “NA” or “Numerical Aperture”, which to ME seems about like an F-number in photography. It relates the diameter of the lens to the focal length (D to FL), and provides you a “figure of merit” to compare some things, among them the width of the LED emitter beam and lenses you might want to try.

the first several paragraphs on page 1 of this white paper? Really doesnt do much for us (yet)… but it DOES cover what we all already know here… that to try to illuminate something brightly as possible really far away? using a single lens is the first idea, and its a good idea… also, we all know already that a higher FL makes for a smaller beam that appears brighter and goes further, but, only up to a POINT.

also, that increasing diameter of the lens is good. After equation 3, we see the devils dilema we all know about already… that increasing the diameter of the lens is good up to a point (where it begins to get ridiculously wide diameter) and that you quickly want a longer FL, but, as the FL starts to get “really useful”, you are too far away from the LED, and you lose a lot of light that doesnt “cut” the lens.

its all a DANCE, and we can only “do-zee-do” a certain amount, before we start running out of usefulness… so, what comes next? By the third column on page 1, we see they quickly introduce “step 2”, which is of course, using a second lens! Yippee! (I was happy to find my idea QUALIFIED, as in, some “pro” somewhere thought this was a great idea, too)

The rest of the paper goes on to introduce arcane stuff, such as light pipes and weird arrays and stuff, not terribly useful to me.

Using a scientific calculator, and they are available online too… all the equations on page 1, are easy to simply “plug in” values and get results. ONE of them i think gives the size of the focused emitter (in radians, which converts to degrees easily enough)

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TECH TIP: from experience? i can tell you that figuring numerical apertures of lenses and emitter half angles? is not TERRIBLY useful at your bench… i find better quick results from setting up the lens, and blowing cigarette smoke into the visible beam approaching the LENS focused up… you can easily see the beam then… and a piece of white paper after the lens will quickly tell you if you are “losing” a lot of light that is not cutting the lens…

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before we really get started? we REALLY need a quick, easy way… to discover the ACTUAL focal length (FL) of any lens we pick up, buy, or salvage…

THIS link will tell you how, and its pretty easy. You cant just focus up on the white wall, and measure from the back of the lens to the LED, thats only the actual distance from the back of the lens AT THAT DISTANCE. We mostly all know that even a single lens has a RANGE of focus, and that it changes. Its shorter at long distance focus, and much longer up really close… this link will tell you how to find the exact “real” focal length…

http://ronja.twibright.com/faq/focal_length.php

you will want to know this for the next part… getting started with 2 lenses…

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okay… lets get to the MEAT of the matter. At this point, i was looking around for information concerning “what” happens when 2 lenses are used instead of ONE lens…

there are TWO main ways, that people will use two lenses together…

1) to make whats called a “simple (2 lens) Galilean Telescope”

2) to simply use two lenses IN PLACE OF a SINGLE lens.

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if you were to take two lenses… one lens, say… 100mm focal length
another lens, say… 25mm focal length

arrange them so that the 100mm FL lens was facing towards the MOON… and the 25mm FL lens was by your EYE? and you had them far apart, and slowly brought them together? you would eventually hit a “sweet spot” where you had actually made a simple telescope.

you would see the MOON, except… it would be 4 times bigger/closer than normal with the naked eye. It would BE four times bigger/closer because of the ratio of 100 to 25 is… 4. (and the moon would likely appear to be upside down)

if you wanted a 10x simple galilean telescope? you would use the same 100mm FL “objective” main lens out front, but instead select a 10mm FL “eyepiece”… the ratio would be 100 to 10, and would then be a 10 power telescope.

this is actually anyone’s “first project” who wants to get into building their own telescopes, by the way.

this is ALSO what the laser guys will call a “beam expander”. if you were to fire a already-collimated laser thru it? it would come out the other end a “fat beam” and be 10 times bigger around. Obviously THIS IS NOT WHAT WE WANT TO DO.

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so… what DO we want to do? if instead of making a telescope, we could take the SAME TWO LENSES, and instead of starting far apart and bringing them together looking for the telescope “sweet spot”?? if we were to put the two lenses TOGETHER, we would simply have made whats called a “compound lens”. (This is what we WANT to do…lol)

so… lets take a simple example… lets take 2 lenses of equal focal length… both lenses are 100mm Focal Length? if we simply place both 100mm FL lenses TOUCHING, we have created a compound lens. The focal length of this compound lens is… 50mm focal length.

we cam take a 50mm focal length lens OUT of a flashlight, and replace it with these 2 lenses touching? and the distance to the LED will be about the same, and, the SIZE of the projected focused emitter? will be the same… lenses have two properties, focal length and DIAMETER…

first rule? “any SINGLE lens can be replaced by properly chosen and arranged TWO LENSES, and it will have the same focal length”

second rule? “any TWO LENSES acting as a compound lens? can be replaced by a SINGLE well chosen lens, and it will have the same focal length”

now, replacing ONE lens, with 2 lenses touching to achieve the same focal length? isnt really going to gain us anything. The MAGIC happens, when you introduce an “air gap” between those two lenses.

example…two identical 100mm FL lenses, touching, makes a single compound lens of 50mm FL…. BUT??? (and its a BIG but, lmao) BUT as you separate those 2 lenses with more and more of an AIR GAP, the focal length of the compound lens slowly approaches almost 100mm focal length.

whats the big deal, you say? “i could have simply selected a 100mm single lens in the first place and got THAT result !!” and you are dead wrong… what you GAIN is that the REAR lens? is now getting closer and closer to the LED emitter !!

the distance of the rear lens, to the emitter? is called “back focal length”, and as you introduce a decent size air gap, with well chosen two lenses? you can place the back lens as close as ONE mm away from the emitter !!! (or, even closer. or, you can put it where you WANT it, thats a trick for one of the other reserved posts, in build notes)

its a “free lunch”… you get the HIGHER focal length you want, for a small bright focused emitter beam? and your rear lens is sitting RIGHT on top of the emitter !! you cant GET as better deal than THAT.

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all right, theres one thing left? rules of thumb are NICE and all, but… we want some MATH. (i personally? dont fully TRUST anything without an equation, lol, call me crazy)

You see, i was looking for the equation to let me predict what two lenses of UN-equal focal length would “combine” to be as a compound lens? and i wanted a way to predict what, for example? a 100mm FL lens, and a 35mm FL lens, separated by a 30mm air gap would create as a final compound lens??

THAT equation was nowhere to be found. Where was it hidden? It wasnt on the telescope site i joined, it wasnt on the laser pointer forum, it wasnt on my night vision site, it wasnt on this or the other flashlight site… where WAS that sucker? Well hidden…

well, heres a link the WIKIPEDIA page…

http://en.wikipedia.org/wiki/Lens\_(optics)#Compound_lenses

the answer? its cunningly hidden in plain sight, right out in the open on a wikipedia page, LMAO… i had READ that compound lens section many times, not realizing i had the answer staring me right in the face the whole TIME. BOY did i feel stupid.

I had forgotten a basic principle of math classes (physics, calc, trig, etc etc) when you have an equation that has a PLACE (variable) for everything you already KNOW, and ONE variable that you WANT to know, you have your answer! The equation just might not be in the FORM that YOU require for YOUR answer. The solution was simple… i just had to re-solve the equation for what I needed.

the second equation after the “heading” of “compound lenses”?? has everything we need… the problem is, it is “stated” as a reciprocal, not terribly useful IN THAT FORM.

TECH TIP: i myself thought this, and i noticed fritzTcat fell for this too… everyone thinks that using more than one lens “adds up” to be a “big” lens… actually? it DOESNT… it gets smaller in focal length, not bigger, when you put them together. The fact that the wikipedia page says that two lenses are “additive” when put together? is very misleading! Actually, the RECIPROCALS are “additive”…

in the second equation after the “compound” heading?

f = resultant focal length of the compound lens
f1 and f2 = the focal lengths of the two lenses you have selected
d = distance of the air gap between f1 and f2
(alll measurements are in millimeters, “mm”… you CAN do everything in INCHES, but, whatever unit you select, you have to be consistent. SOME lenses are sold by “inches” of focal length? just convert to mm’s… 1 inch = 25.4mm)

now, you CAN simply put the focal lengths in, and put the airgap distance in, and work out the equation as a algebra problem? its TEDIOUS that way… its a lot easier to just solve the equation in terms of ” f ” instead of ” 1 / f ”

which yields this equation…

New Focal Length = (F1 x F2) / (F1 + F2 - Distance)

the BFL (back focal length) equation is fine the way it is…

( F2 x (D - F1) ) / ( D - (F1 + F2) )

NOTE: in both of these equations? they are “plug in” equations now (easy kind, lol)
F1 and F2 are the Focal Lengths (FL) of your two lenses, D = airgap distance…. both answers are in MM, by the way.

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so…… lets say we want to know what a pair of 100mm focal length lenses will “make” when touching (touching is airgap of zero mm’s)

new focal length = (100 x 100) /( (100 + 100) - D)
new focal length = (10000) / ((200) - 0 )
new focal length = 10000/200
new focal length = 50mm FL

NOTE: when the lenses are touching? the BFL calculation is trivial, its the same answer

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so? lets see what happens when we use the same 2 lenses (both 100mm FL) and introduce an airgap of 50mm, shall we?

new focal length = (100 x 100)/((100+100)–50 )
new focal length = (10000)/((200)–50 )
new focal length = 10000/150
new focal length = approximately 66.7mm

BFL = (100 x (50 - 100))/(50 - (100 + 100))
BFL = (100 x (50)) / (50 200)
BFL = –5000 / –150
BFL = 33.3 (approximately)

this means? i could have used a SINGLE lens of 66mm FL… but then it would have been 66mm away from the LED emitter… i would be LOSING a good portion of the LED light, because of the wide cone of light the LED makes…

instead i can take two 100mm FL lenses, and put them 50mm apart from one another? and when focused, the REAR lens will only be 33mm away from the LED !!

I get the SAME smaller/brighter emitter for longer distance? but… i lose less light by having the rear lens HALF the distance away from the emitter!

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When you start to use DIFFERENT focal lengths, for the 2 lenses ? you start to get unexpected results you would not have guessed, without doing the math…

lets for the last example? take a 100mm FL lens, and a 40mm FL lens… and we will try them TOUCHING (zero airgap) and also run them again with a 30mm airgap…

ZERO AIRGAP (touching) = (100 x 40) / ((100 + 40) - 0 ) = 4000/140 = 28.6mm new focal length
30mm airgap = (100 x 40 ) / ((100 + 40) - 30 ) - 4000/110 = 37mm (roughly rounding)

its NOT what you expected, was it?? i know it wasnt what I first expected…

in fact? “adding” two lenses together, with or without an airgap? will always produce a new compound lens… that is LESS of a focal length. In fact, after working the results out all night, playing “what if”… you will quickly see that you can take a 600mm FL telescope main objective, and if your rear lens is a short focal length lens?? your final compound lens will be a LOT closer to the focal length of the smaller FL rear lens…

by the way? if you pick the 2 lens focal lengths “appropriately” by playing “what if”, and keep running the BFL ? you will eventually find a situation where the REAR lens is only a couple mm off of the emitter… (wink!)

so? lets recap… what do we already have?

1) we can pick up ANY lens that we beg, borrow, steal, or salvage… and we can figure out the “true” focal length
2) we can take ANY two lenses, and make a new compound lens out of both of them, and we can figure out the focal length of that new lens we made, AND when we use an airgap? we can predict how close to the emitter the back lens is

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we can pinch this post off… we’ve covered a LOT of ground !

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so? lets start to get nuts with it ! Lets have FUN… i think it was n10sivern wanted to know what happened when a 25mm FL lens was used with a 100mm FL lens ??? lets see…

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Okay, these have been some LONG posts… i apologize for that.

to make up for it? heres a SHORT post…lol

not a long explanation how to add a third lens into the build… you already have 2 lenses with an airgap, which work together too make one compound lens??

you treat those 2 lenses as a SINGLE lens (because thats what a compound lens is, really one lens) and you simply ADD a third lens…

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example?

i already have a 125mm FL lens, and a 100mm FL lens… with a 50mm airgap? working as one lens? fine… i might place a 40mm FL small diameter lens close to the emitter, and simply move my compound lens back and forth to focus these “two” lenses up… remember, the compound lens is now ONE LENS for all practical purposes.

you MIGHT even have ONE compound lens that is made from 2 lenses with an airgap…
and make ANOTHER compound lens that is made from 2 lenses with an airgap…

and? separate both compound lenses with an airgap…

thereby, creating a 4 lens system…

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you can run the SAME equations to find out the new focal length of “both” compound lenses, working now together with an airgap…

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it should be obvious? that when the lenses are well chosen…you end up with 4 lenses, with small spaces between each of the 4 lenses… and the back lens of the “back lens” is very close to touching the emitter…

…and you have a great situation! you very FIRST lens is almost touching the emitter, losing almost NO LIGHT… and each lens is a small jump to the next…

THIS is where you begin to create a great efficiency, and lose less light. It IS TRUE that each and every lens you go thru? will take a toll on the light, and you lose a little… but, it CAN be less than what you lose if you were to simply grab ONE long focal length lens…

actually? in real life? MOST optical devices are made of MANY lenses, its actually kind of RARE to have an optical device that only has one monolithic single lens…

a CAMERA lens, for instance? will easily have 17 individual lenses in it? yet… they are all working together as ONE compound lens, and you treat it as such.

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for my IR illuminator builds? I use… 1,2,3 and once even 4 lenses. (i forget if the 3 or the 4 was my “best”, my memory tickles me the 4 lens system barely squeezed out the 3 lens one)

since i am a gun dude? i have a lot of old scopes laying around… from tiny bbgun scopes, to 40mm and 50mm 3-9 hunting scopes… i took many of them apart. Also? old binoculars, old spotting scopes, old monoculars, anything i get for free or doesnt work or i can buy at a yard sale that has interesting looking lenses? gets taken apart to get lenses to play with!

also? telescope main objectives?? come apart with threads, most of them… and contain a super high FL biconvex lens that is simply truly ridiculously high FL… and broken camera lenses?? arent worth a dollar of they no longer work, LMAO… and contain MANY many lenses inside…

also, surplus shed is a treasure trove of lenses to pick from… and whats best is you can PICK your diameter and FL…

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some practical experiences?

its typical to use the higher FL lens on the FRONT end of the flashlight, and to use the shorter FL lens close to the emitter.

i found my best setups came? from having the smaller diameter lenses closer to the emitter, and the wider diameter lenses at the front end… and when using more than 2? to arrange them getting wider and wider to the end of the flashlight…

i generally set up the “compound lens” of 2 lenses first? THEN i add the smaller lens close to the emitter… just works for me easier that way.

for making prototypes? your going to hit the hardware store, lol… one word here… “PVC”, lol

pvc pipe, and pvc couplers, in many sizes? is your friend, trust me. (hey, guys make big expensive telescopes out of PVC pipe and couplers… they arent ashamed, why should we be ashamed to use PVC ?)

i also made good use of cardboard tubes before i go PVC, to rough out a plan… cardboard, tape, PVC pipe and couplers, hot glue, JB weld… i know it looks like HELL, but… when it works, it works.

i was working with IR LED’s that only used a 1 amp driver (actually 1.04amp) so, i wasnt making GOBS of heat… YOU guys using 9 amp drivers?? you MIGHT wanna “play” with a low power driver to make a compound lens, and get it working well… THEN at the end when you are in love with your lux meter or whatever?? you can break out the impressive aluminum fabricating to make the final lens carrier up and affix it to a flashlight.

i can see it now? giant BAZOOKA lights, lantern style handle over flashlight setups… have fun.

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one of my favorite “little” IR illuminators? is very tiny… but it performs above its station in life, simply by using 2 thin lenses about 1” diameter each, separated by an airgap… it performs similar to a larger diameter aspheric, as far as what it does for how tiny the setup is.

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almost all of my work? has been with IR LED emitters… so, my “beamshots” are taken off of a monitor screen… the important thing is? i was using the same IR sensor as everyone else was using… but… i was “seeing” my focused emitter much farther away.

i was seeing my “scopeless” digital night vision setup? make the emitter play infrared light on trees 800 and even 1000 yards away… when others were reporting 400, 500, 600 yards as more typical results.

what was the differenc ein my setup? THEY were using “one big aspheric”… I was using…

a 50mm diameter gun scope main objective (150mm FL), coupled to a 40mm diameter gun scope main objective (125mm FL)… and a third lens behind those two…. and a tiny lens down inside my p60 reflector close to my IR LED emitter…

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dont get the idea, you just calculate it all out, and BANG it works wonderfully… there is still MUCH trial and error, and even when you know you are close? there is “tuning”… i spent hours, moving the 2 big lenses closer and closer by a hair at a time… and deciding exactly where the “middle” lens went… and finally added in the tiny lens down in the p60 reflector, before i had my first huge success…

when i was all done? i could use it all like a regular single lens… IE, my emitter was inside a copper coupler? and i moved the copper coupler in and out of a tube, to focus in, or to flood out… instead of moving the lens, i moved the light…

but, you could definitely use 2 lenses that you can move closer and farther apart (increase and decrease airgap)

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i already posted my “beamshots” and “build pictures” in another thread, but, later on i will edit this last post to show some of them… just remember? this is all done with IR and a low power LED… but, the principles all apply to visible light too

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oh yes… what IS an aspheric? an aspheric is a lens like any other… it has a diameter, and it has a FL… the only difference is? the shape of the lens is such that it reduces image distortion… IE, it keeps your square emitter looking SQUARE… instead of “rounded out” (barreling distortion)

theres nothing wrong with using other types of lenses. I use a lot of convex lenses, and a few plano-convex lenses… and i use a lot of “achromat” lenses (the main objectives in gun scopes i take apart? are all achromats)

they all “work”.

some convex lenses will make some blue or other color around the emitter, or, the square emitter will come out a little bit rounded… i never found it objectionable. my tiniest IR illuminator? is just 2 small thin regular convex lenses… and it doesnt make much rounding of the square emitter at all, sometimes you get lucky.

since you are going to be using more than one lens? theres no reason to get HUNG UP ON “aspherics only!”… they are very big and bulky in glass… you MIGHT wanna try regular convex or plano-convex lens(es) inside, and just use youre aspheric for the final lens…

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okay! its TIME!

time for what? time for… someone is going to go get dr jones, or some other expert, to see if i am making this all up, or, if i actually know what i am talking about. (i already know it works)

the only thing i HAVENT done? is play with a “light meter” and see the meter reading go up, from using a second lens… i dont OWN one (yet) and they dont really make one for infrared emitters, lol…

hope this is clearer than mud, and have fun.

you will definitely want to start with “2 lens” and fool around till you get the meter reading up, i suppose.

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oh yes, one more thing? once you get “used” to how lenses ACTUALLY act when you start using compound lenses? you dont have to run “math” as much, and you get a strong hunch “playing” at the bench surrounded by salvaged lenses…

also? you can simply PLACE a small lens of short FL directly ON or CLOSE TO your emitter to start with? then focus up your lens(es) to THAT… when you do THAT? you are “setting” your BFL (back focal length) then going and focusing by finding the airgap that corresponds to the BFL you have set as a fixed design feature. ALL of which? requires no math…

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subscribed…

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(reserved)….

for comment after beamshots are added…

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Reserved for comment after Sirius9’s comment is posted. :stuck_out_tongue:

8

I’ve been thinking about doing this for a while using sk68 lenses, then you posted the 8mm ones in another thread, and got me thinking about it again.

Looking forward to reading the rest of what you write up.

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So, my question is, if you reduce the beam at the LED or shortly after to say 10 degrees, then run that through the aspheric lens, what do you get, other than a damn laser beam? I had looked at something like this Ledil. It is 16.1mm across so it could be modified to press fit into the pill on top of the LED/PCB. Plus it’s only 9.7mm high. My concern is the 93% light transmittance. The loss through this then through the aspheric could amount to 20% decrease in transmittance.

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I think the losses in the lens are mostly recovered in the light that ends up hitting the front lens instead of the inside of the head.

In theory the final beam ANGLE is dependent on the final lens, and the projected image is actually larger, not smaller than with a single lens.

11

One question I have, does the smaller lens become the focal point of the main lens, or is the focal point still the LED?

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I don’t see much advantage in using multiple lenses, over a very thick aspheric or an aspheric-like Fresnel lens. I think you get about the same converging power per thickness with one thick lens or a stack of thin ones. Can you show us a successful design to show what you mean? DX has some aspherics, of useful quality. Illumination supply has the very good Ahorton lens. Good multi-element refracting Fresnel lenses are harder to find. Edmond has some and there are some used for stage lighting.
As far as I know, the only ways to get a large fraction of an LED’s output into a narrow beam are Fresnel lenses, and possibly very deep reflectors. Flat many element refracting Fresnel lenses, such as used for stage lighting, can give a very high diameter to focal length ratio in a small apace, but if that ratio is too big one loses throw. The other type of Fresnel lens has one or more refracting elements and one or more reflecting elements. That is what Fresnel himself used in lighthouses. They are used by LED Lenser, Coast and POP Lite. See the Wikipedia article on Fresnel lenses. The light emitted forward is focused by a lens, while that emitted to the sides is reflected. In most cases, total internal reflection is used to avoid loss in a silvered coating.

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Price and availability are two reasons I think of to choose the 2 lens setup. I’ve looked for short focal length lenses and they are often hard to find, expensive, or both.

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to fritz? uhm, you havent even TRIED using a second lens at all, and you already dismissed it out of hand? With no experimentation, no math, nothing… it just “doesnt seem” like it “should work”?

well… what can i say?

my PREDICTION?? is that the guys with “the meters” are going to get interested? and if my IR illuminators went “farther”, then visible light should do the same. Oh, the lenses will be a little closer or farther apart, and slightly different FL’s will be “best”… but it works. (infrared is simply another “color” just outside of the visible wavelengths (colors), thats all. it refracts a little different angle, but its still light, and it still does what light does)

how else could my IR illuminator go a couple hundred yards farther than everyone else’s, when the only main difference was this setup? (but, its my prediction that someones meter will show an increase, but we’ll see, i could be wrong)

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PS - someone was worried about where the focal point was? obviously, the focal point is still the LED. Its no different in a camera? you can switch big lenses, and the focal point is still the film, or the digital sensor… as a matter of FACT? you can take ANY LENS, and i mean a single lens, and hold it over the digital sensor, and draw up an image… the old fashioned cameras used to only have ONE lens… this is just making a COMPOUND lens to replace your SINGLE lens you have been using up until now. (i have actually made my own lenses, for my digital night vision sensor… some of them work very well, and produce quite a crisp clean image… they just areny sturdy enough to be useful mounted on a gun and to take into the field… on my future NV spotter unit? i will use some of my modified camera lenses, and even a scratch build)

in a camera? youre doing this all in reverse… the image is out where you see your focused emitter at distance… and your LED is where the film or digital sensor is.

almost all of the information OUT there for lenses? is around cameras and telescopes… i just reversed it and applied it to a LED flashlight.

i’ll just sit back, and wait for “reports” to come in… once a few advanced builders on here see their meters jump, that will be all she wrote, my prediction…

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almost forgot !

i got tired of doing the math, and wrote a small visual basic application, that did the math for me “on the fly” and i just entered the focal lengths of the 2 lenses, and it “spit out” the new focal length and the back focal lengths automatically… it made playing “what if” every night for a week a lot more fun…

crackheads burgularized my house and stole that laptop? but, i still have “screen shots” saved on line… i will have to re-write the little program, its an easy short program, and i found it useful.

takes all the drufge work out of seeing what 2 lenses will “do”, and you can just click-click-click to “play”.

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some coatings will work better than others. I mean AR coatings?

i never had much luck with MOST “ar” coatings… that being the little bit playing around i did with visible light, and certainly “tended” to get much better results with uncoated lenses.

especially the “ruby” and “emerald” coatings? the ones you always see on cheap binoculars and such? they really suck, lol…

some coatings i “polished” off of the lens and they got better… some “older” (70s ???) old school coatings worked fine for infrared, but not for visible, or vice versa.

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if i MAY make a suggestion? for your first trial and error project?

go to walmart, and buy a 8 dollar BBGUN SCOPE… you know, the LITTLE one… 4x15 fixed power?

take the EYEPIECE off, it just unscrews… inside you will find threaded washers holding 2 small thin lenses apart with a plastic spacer (thats the airgap)

take one of them, and put then right over your emitter holding ring, and tape the edge of it down over the emitter plastic holder (like on the 1504 from the group buy?)

you should find you can hold the aspheric that came with the light in and out to find the “focus”, as you have already “set” the BFL for this combo… you are searching for the “airgap” that corresponds to this setup.

you might have to use both little lenses touching each other flat to do it with the light put together normally…

17

naturally? I could be making big mistakes, or, just one big mistake, ruining the whole “plot line”? but i dont think i am off base, or too far off base.

most of what i claim here? i can document it…

1) the whole idea of using a second lens? comes from the edmunds scientific paper i posted the link to in the first post here… right after you dont get “enough oomph” from using a single aspheric (our basic plan A, with no other plan B, lens wise) ?? Edmunds scientific directly recommends using a second lens as “plan B” to get further.

2) the math for using 2 lenses, to see what FL result you get for the new compound lens? isnt a terribly advanced equation… anytime you have an equation, its standard practice to re-solve the equation so that its more useful and handier for you to use. Plus i “proofed” it by going BACK and putting values back into the original equation, and into my solved equation both… to verify everything still worked out. It seems sound, mathematically.

3) I have other “anecdotal” evidence… on my night vision site? I mentioned they were basically trying lens after lens… and as they picked better and better single lens? they slowly got more distance (with IR). Right about the time i was starting to “squawk” about trying multiple lenses? someone took a IR camera lens off of a certain night vision device? and discovered that it was at the time “better” than anything else they were currently using… wonder of all wonders? THAT infrared camera lens, had… multiple lenses inside it.

4) when everyone else was using 50, 60 and even almost 70mm lenses in their single lens builds? I was using a 50mm, a 40mm, a 30mm, and a 25mm lens in my 4-lens build that got better distance… plus my dual lens “tiny” one? only had a pair of thin 25mm diameter lenses in it, and it performed better than a lot of bigger diameter single lens setups. I figured, it was a sort of “handicap” to only be using smaller diameter lenses, and i still made a good showing for myself.

==

before i got into this multiple lens stuff? there was some “doctor” or something on the candlepower site?? he once showed off a multiple lens setup, and we all “oohed” and “aahed” over it… (that guy was a real “optical engineer” or something like that) so, i know this might not be everything i am doing, but, its definitely a step in the right direction…

shrugs nothing to lose, just trying to make myself useful… i just figure i am either going to be a “hero” or a “zero” in pretty short order, LMAO… i’m actually a little nervous laying this all out like this. I’m kinda “new” here… and i dont want to act like i am showing off… but, if i am right?? i dont think its “fair” not to share it and get the big builders here going downstream on this… (I dont have the fabrication skills i see on here, lol)

fingers crossed

18

Cameras use multiple lenses or lenses made by bonding materials with different optical properties to correct aberrations. That is, a single spherical lens works well to first order in a power series expansion around the central ray and the central wavelength, but if you actually calculate rays away from the center, a single spherical lens doesn’t focus them all to the same point. Spherical aberration can be corrected by using a properly shaped aspheric lens. The better flashlight lenses are shaped well enough to give good images of the LEDs. That leaves mostly chromatic aberration. Practical materials usually bend short wavelengths more than they do longer wavelengths. So zoomy flashlights, especially those with large lenses, have colored fringes around the spot. Some materials such as polycarbonate have more wavelength dependent refractive index than others. I avoid polycarbonate eye glasses for that reason.
So an advantage, and I think the main advantage, that one can get from using multiple lenses is to correct the chromatic aberration by using lenses of different materials in combinations. That is done in cameras. For instance one can use a slightly concave polycarbonate lens and a more convex lens of a material with less dispersion, to build up an achromatic lens. I think that would improve a flashlight, though the relatively large size of the LED makes a flashlight much less sensitive to details of the optics than a camera or telescope is.

19

I’ve just finished putting together a 1504 with a BLF17DD driver, XP-G2 S3 3C on a Noctigon board, dedomed, and a 22mm OD lens within a mm or so of the LED. The spot size is nearly identical to a dedomed XP-L. Flood is not quite as compromised as it is with a collar. I’ll post measurements in a bit.

Edit: Initial measurement with the 22mm lens very close to the LED yielded only 220kcd. I added another o-ring under the lens and am letting it set up with super glue. I’ll post numbers with the new position in a bit.

20

Thanks sedstar for posting this, i haven’t had time to read it fully through yet but i will very soon :slight_smile: