Light Pipe Principles 2

[bvanhiel]

I'm designing a part that the customer wants to "glow". The area is about 1-2 in^2, but the geometry may be restrictive in where I put the LED's. I'm looking for books or design guides that I can use to understand the principles involved.

Thanks,

b

 

[IRstuff]

Total Internal Reflection is the basic principle

TTFN

 

[bvanhiel]

I understand the physics. I need to understand the engineering principles for use in designing the part. I'm looking for rules of thumb, materials, finishes, angles, effective LED layouts, etc.

-b

 

[IRstuff]

rule of thumb: Hire a professional lighting designer

TTFN

 

[bvanhiel]

rule of thumb: Hire a professional lighting designer

I'll take that to mean that you are unaware of any design guides. Thanks for all your help.

-b

 

[patprimmer]

Please don't get snooty.

The quality of the answer very often reflects the quality of the question.

The guy you just dismissed has a long history of high quality answers. Your question asks for someone to wright a text at elementary level.

Ask a specific question about a topic that is not clearly defined in basic text and you might be swamped with answers.

Rules of thumb

The material must be transparent.

High rate of light transmission is desirable.

Low haze factor is desirable.

You need to know the refractive index.

You need to understand total internal refraction.

You need to keep the light directed so that it never strikes the internal surface at an angle that will not cause total internal refraction.


Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[MikeHalloran]

I assume you mean you want some object to emit light whe the LEDs are on. The total internal reflection principle applies in light pipes and optical fibers. Basically, a material of high index, e.g. acrylic, is surrounded by a material of low index, e.g. air, and light bounces along the inside of the object. The interface between object and surroundings in normally highly polished in order to maximize transmission and minimize losses. A transparent object like a statue illuminated and finished in that way will be unsatisfactory; only the edges will emit light, and not much of that.

It's necessary to 'frost' (e.g. abrade) the surfaces that you want to 'glow', i.e., make them lose some light.

The trick is in not losing all the light to surfaces near the light source. You may have to frost the object selectively, e.g. coarser finish here, finer finish there, or leave some areas unfrosted in order to highlight or diminish certain features.

If you are doing this for a fixed price and there is no objective measurement specified, you will take a bath on it.




Mike Halloran
Pembroke Pines, FL, USA

 

[bvanhiel]

I'm looking for pointers to books/papers/design guides. Amazon.com doesn't seem to have anything, and manufacturers want the business so they're not giving away any secrets. I'm not looking to have light pipe design explained to me in this forum (that'd be great, but as you pointed out it's not a trivial request).

I understand the basics. I want to find a more advanced resource if it exists.

I guess the nature of my request was not clear. This is evidently the case since both patprimmer and IRstuff seem to have misinterpreted it. In turn I misinterpereted IRstuff's comments as being a little less than helpful.

I've found this:

http://www.plexiglas.com/literature/pdf/81.pdf

and a research paper discussing optimal bend geometry, but little of the nuts and bolts.

The problem can be solved by brute force (25+ LED's worked for the prototype), but I thought I'd try to be smarter this time around.

-b

 

[bvanhiel]

I realize that engineers want to solve problems, so I'll start a new thread that is a more thorough discussion of the part.

-b

 

[MikeHalloran]

I think the subject was covered on not more than two pages of one of those quarto 'design of everything' books, and it's the sort of thing that appears in '...Design...' magazines regularly. I'm not aware of any book devoted directly and entirely to the subject.

You can surely find lots of material on how to design a _good_ light pipe. In this case, you want to design a _bad_ light pipe, that is very lossy, so just do the inverse of whatever is recommended.

I don't think you'll find a nice clear step by step tutorial or recipe or set of equations that will lead you to the solution. Better to invest in lots of different kinds of sandpaper.



Mike Halloran
Pembroke Pines, FL, USA

 

[bvanhiel]

Thanks Mike. I hoped that since there seems to be a couple of pieces of specialty software out there that someone might have written a "... for Dummys" book.

See my other post to get a better idea of what I'm trying to do. It's basically half light pipe and half diffuser. I need the light to get around a component, but have a soft, even glow. LCD backlights have a similar problem of getting a light at the edge of the panel to illuminate evenly.

-b

 

[ToolEng1]

The lighting industry is very secretive, you are right! I should know as I work in one. I can't give you any details because of confidentiality reasons. You need to send a file of your proposal to a tool maker such as DBM Reflex in Canada or INCOS Italy. Approch them as if you need a tool quote and ask them for their ideas and any suggestions to improve the design. This is probably the easiest and cheapest way.

 

[richg1]

Hi,

This is an old thread but having recently gone through this problem thought I'd add my own two cents.

Firstly, review GE Plastics Optical Design information. I've got an old photocopy from a few years ago which covers the basic principles of optics and light pipe design. It was a document titled 'Specific Industry Design Considerations'. Bayer plastics technical information may also be worth checking out which is very good for plastic part design in general.

Secondly, once you have an understanding of the optics, pull apart products that do what you are trying to do. You'll learn heaps and have lots of bits with which to test.

Some ideas:

. Side firing LEDs into a light pipe/guide
. Polycarb / Polyester diffuser films
. TiO2 filler to diffuse the light within the light guide
. Polycarb diffuser films
. For short distances (ie. 1-5mm) don't even try to directly fire the led onto the visible surface as it will hotspot...
. Use Flexible light strip / light pipe modules

 

[mwill195]

Hi, I stumbled accross this thread whilst searching google for 'light pipe design' (it was the first result)!

I eventually found a very useful application note:

http://www.ciri.org.nz/downloads/Lightpipe design.pdf

Alternate link:

http://www.avagotech.com/assets/downloadDocument.do?id=70

This has everything I think the original post is looking for. It certainly helped me.

I thought I had better sign up and post the reply in case anyone else out there finds this thread!

Mark

 

[bvanhiel]

mwill195,

That's exactly the kind of thing I was hoping to find. Thanks for posting.

-b

 

[Newberger]

Hi

I am having some difficultues with a particular light pipe design. It consists of four pipes and when one LED lights up some of the light is visible in a other pipe. The samples I am using for testing are made out of STL models and the surfaces not are polished. The geometry keeps me from polishing by hand. I would like to to make sure the geometry is okay before we go to mold. Is there some program that you can import the 3D Geometry into (IGES or STL)to see how the light would behave whith polished surface. Any advise would be helpfull. Thanks

 

[Heckler]

We use this optics program from Science Lab

http://www.optics-lab.com

to do our lens and light pipes.

Heckler
Sr. Mechanical Engineer
SWx 2007 SP 4.0 & Pro/E 2001
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This post contains no political overtones or undertones for that matter and in no way represents the poster's political agenda.