How to make custom reflectors?

(Disclaimer: my real knowledge here is limited to "angle of reflection = angle of incidence". )

I just made a conical reflector for my Spyder about 1 foot long, from a diameter of 1 inch to 4 inch exit out of aluminum foil. It made a noticeable difference cleaning up stray spill without dimming, but was horribly crude. The goal was to create the best throw as possible, so I just extended by eye what it seemed to be doing already about 1 foot (a complete guess). The rate of spread wasn't much better, though, which was I was hoping to improve. I figured a long black tube would help, but blech.

I was thinking that the shape for the best throw might be some impossible combination of a regular parabola for major focusing, and a long thin cone/parabola for refinement of the resulting beam, but that's only based on lazy imagination. I know a know a nice parabola is best for focusing to a spot, but I have no idea what is the best for focusing from a point source to a beam of more or less parallel rays. Even with a good parabola, most of the light going out the front, not focused by the reflector, is diverging badly, which I'm guessing a shaped lens is required for (also under the catagory of "how the heck?").

It made me wonder how good it could be if done properly. Has there been any discussion on this? Is a conical reflector nearly as good as a parabolic, or not (it's a lot simpler, and I have no idea how to fashion a parabolic by hand).

Is the shape of the reflectors delivered with most lights anywhere near optimal? I just saw a thread where a D reflector was put on a mini, and it worked a lot better. Should I go buy a 12" telescope and rip out the reflector for my pocket light? Raid the nearest observatory? Or is bigger != better past some point?

I'm not too knowledgeable about such things, but I'm led to believe that a collimating lens will help focus the light output and offer increased focus at the expense of spill. If you haven't already, see an Inova X1's beam, with miniscule pseudo-reflector and collimator. Target is starting to carry them. The beam throws farther with less spill than it has any right to for a reflector and lens of that size. The X1 also demonstrates how this can be a bad thing by having a beam too small for close-up work and too weak for long-range work outside of niche applications.

Not an answer to your question but I hope you don't mind going off on a tangent.

Try a fresnel lens. Get a page magnifier like those they sell at Office Depot. I saw some at Big Lots last month too. They are flat with concentric ringed grooves.

Warning - you have now entered the most expensive part of playing with flashlights. I should know - been working on this for a long time.

OK, so the Fresnel works, as will much cheaper aspherical lenses which can be purchased on line and in junk dealers. They are well engineered and available at a reasonable cost.

Reflectors are also available in a wide range of sizes and shapes - it is MUCH cheaper to try to find an existing one than to have one custom made. If you must make one, a P type using simple finite element approximation will work fine.

Combining the two elements requires them to be "optically coupled", and not necessarily the way you might expect. Not only must they be coupled, but the precision of alignment is far higher than either of these optical elements is usually manufactured to - which is why this is not done very often. The few people that have done this usually have access to a PhD in optical engineering and fairly powerful optical simulation software - myself excepted, I just spent lots of money.

HarryN said:

Warning - you have now entered the most expensive part of playing with flashlights. I should know - been working on this for a long time.

Click to expand...

I figured as much. Optical stuff eats $$$, which is why I wondering about alternative approaches.

OK, so the Fresnel works, as will much cheaper aspherical lenses which can be purchased on line and in junk dealers. They are well engineered and available at a reasonable cost.

Click to expand...

Is the OfficeDepot [etc.] plastic page magnifier Fresnel lens at $7 worthwhile? If the Fresnel lenses you're talking about are more expensive than aspherical lenses, then I probably don't understand what you're saying.

Also, do you have any clues on how to guess what kind of lens spec I want (or should I be looking for some focusable thing?), and where are the reasonable cost lenses? My searches so far all yield specialty or camera lenses, which range from $50 to $500 or more (I found *1* aspherical lens on EBay for $10, but I don't know if camera lenses work well for this). I'll have to look locally for "junk dealers", though I'm not sure what that would be: not auto wrecking [junk-]yards, clearly; pawnshops maybe.

Reflectors are also available in a wide range of sizes and shapes - it is MUCH cheaper to try to find an existing one than to have one custom made. If you must make one, a P type using simple finite element approximation will work fine.

Click to expand...

I'll have to look for these also in above "junk" shops.

Actually, I was curious whether there was a cheap and dirty method for making your own, and experimenting with shapes and sizes. Unfortunately, my knowledge of optics is limited (what's a P type?) so I think I need to know

1) what shapes do what

2) how to fabricate prototype level things (a cone is easy, but parabolas?), i.e. with some mylar, styrofoam, utility knife, ruler, and string (I was thinking of the crop circle approach

3) whether anything I can do that cheaply will have any useful effect (not looking for perfection, but some noticeable improvement over stock reflectors).

4) whether I'm just in over my head, i.e. searching on "finite element approximation", I come up with $300 textbooks and equations I'm not sure I'd understand even if my calculus wasn't rotting in my mind's bone yard.

Combining the two elements requires them to be "optically coupled", and not necessarily the way you might expect. Not only must they be coupled, but the precision of alignment is far higher than either of these optical elements is usually manufactured to - which is why this is not done very often. The few people that have done this usually have access to a PhD in optical engineering and fairly powerful optical simulation software - myself excepted, I just spent lots of money.

Click to expand...

Hmm. Then I don't understand adding an aspherical lens is going to help if it isn't "coupled" with the reflector. Is it possible to get a focusable lens to deal with the coupling problem?

the problem you will immediately discover using an optic (lens including fresnel) to focus the light from an emitter is that you will 'project' the die of the emitter out to the world.. 4 obvious squares of light.. it's not a pretty beam... if you use a reflector... it has to be one of two types to be effective at all.. parabolic or elliptical... the 'problem' with an elliptical reflector is that it focuses all the light from a point source to another point source typically a few feet away... you 'defocus' somewhat and get a relatively cylindrical 'beam' of light with a hole of dark down the middle (ala maglite set partially to flood)..

Parabolic reflectors work well with luxeons because the relatively large (not pinpoint) spot of light ends up being a fairly large 'hot spot'.. enough subtle 'bumpyness' to the reflector and it really smoothes things out nicely.

Learn how to use the forumulas for parabolas and ellipses.. and make a spreadsheet in excel and you can figure out some things you might even make a reasonable reflector like you are describing.. 'crop circle simplicity'..

example.. you make an ellipse (drawing) by putting two nails in a board.. tie a string that is longer than the distance between them and drag a pencil trapped by the string around in the 'oval-like' shape.. it will be a perfect ellipse.. you could actually draw a guide to make a real frame to hold your crop-circle reflector this way..

the nails are the foci and the curve is where the reflector would have to be... all light leaving one 'nail' will bounce off the curve and hit exactly at the other nail.. from all directions ... of course you won't have a whole 'egg' shape reflector you will cut the back off and keep just that.. so light going forward will still spill out as 'flood'. You can have an emitter pointing into the reflector and 100% of the light will be captured.. you will have a shadow in the middle of course of the emitter and heat sinking is tricky.. but it's a neat idea.. and if you hand-made a reflector that would be fun you could make one with like a 2' diameter out of foam, cardboard, alum. foil, etc.. it would be an interesting experiment.

there is a very simple way to draw a basic parabola.. on graph paper.. start at the origin and move 2 squares over from zero and 1 square up.. put a dot.. than from origin go 4 squares over and 2 squares up.. put a dot.. next.. 6 over and 4 up.. (the 'x' goes linearly.. the 'y' is the 'square'.. this method can be modified by multiplying either the 'x' or the 'y' by a constant to flatten or narrow the shape but where the 'focus' is is a bit tricky if making it this way.. no 'crop circle' string mechanism i know of to make a parabola but the formula isn't terrible.. if you do a search for andrewwynn 'thread starter' on cpf you should find a thread i mad showing a link to my xcel spreadsheet with the formulas including a luxeon.

-awr

kennyj said:

I'm not too knowledgeable about such things, but I'm led to believe that a collimating lens will help focus the light output and offer increased focus at the expense of spill. If you haven't already, see an Inova X1's beam, with miniscule pseudo-reflector and collimator. Target is starting to carry them. The beam throws farther with less spill than it has any right to for a reflector and lens of that size. The X1 also demonstrates how this can be a bad thing by having a beam too small for close-up work and too weak for long-range work outside of niche applications.

Click to expand...

I'm having a hard time finding collimating lenses not designed for lasers, fiber optics, or specially designed, i.e. I found a PDF which listed some collimating lens sources ( http://www.lumileds.com/pdfs/DR02.PDF ). Looking at: www.reflexite.com lead to:
http://www.display-optics.com/products_lighting.htm and http://www.fraen.com/ and others, which have some interesting lenses designed or Luxeon LEDs, too tiny or modular for an add-on to a flashlight.