# Side mounted LEDs on frosted acrylic



## crx091081gb (Feb 25, 2013)

Hi, I'm an installation artist and I'm thinking about making a 'light up' ping pong table using frosted acrylic and RGB LEDS mounted round the side. Does this sound feasible? 

Ideally the LEDS would be glued to sides all the way round but I'm a bit worried they might not give an even light (i.e. it might be dimmer in the middle). Is a ping pong table too wide for this kind of effect to work. 

Would I have to add LEDs in the middle? If I just used LEDS under it would I be able to get away with a thinner piece of acrylic?

Anyone have any idea what width frosted acrylic I'd need for this? 

Would these kind of LEDS be suitable, I was going to glue them and then cover with wood panelling or some such.

http://www.maplin.co.uk/rgb-led-strip-515851

Cheers


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## Ken_McE (Feb 25, 2013)

crx091081gb said:


> I'm thinking about making a 'light up' ping pong table using frosted acrylic and RGB LEDS mounted round the side. Does this sound feasible?



Yes. If you're feeling particularly hard core, you could add a black light shining down over the table and a glowing phosphorescent ping-pong ball and paddles.



> Ideally the LEDS would be glued to sides all the way round but I'm a bit worried they might not give an even light (i.e. it might be dimmer in the middle). Is a ping pong table too wide for this kind of effect to work.



Don't know. But if the middle is dimmer just say you wanted it like that. 



> Would I have to add LEDs in the middle?



If you put them in the middle they will have to face up at it and then the light won't be smooth and even.



> If I just used LEDS under it would I be able to get away with a thinner piece of acrylic?



Time to experiment. Make a coffee table or a cutting board or something just to test the concept. If it is too thin then it would warp.



> Anyone have any idea what width frosted acrylic I'd need for this?



Width or thickness? The width and height would be the same as a ping pong table.



> Would these kind of LEDS be suitable, I was going to glue them and then cover with wood panelling or some such.



They look fine. Consider running aluminum channel around the outside if you can size it to slip over the edge of the plastic. The aluminum will shed heat, give a clean finish, stiffen it mechanically, protect the edges from damage.

If you should care to, we'd like to hear about the results, good or bad.


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## crx091081gb (Feb 26, 2013)

> Yes. If you're feeling particularly hard core, you could add a black light shining down over the table and a glowing phosphorescent ping-pong ball and paddles.



I had thought about this...



> Don't know. But if the middle is dimmer just say you wanted it like that.



Haha, you sound like you've done this sort of thing before.



> Time to experiment. Make a coffee table or a cutting board or something just to test the concept. If it is too thin then it would warp.



Yeh I'm thinking that too.



> Width or thickness?



Thickness. 

Width wise I think the table will consist of two pieces of acrylic each 152.5 cm by 137cm (basically just replacing the wooden panels from a portable table). I intend to use the frame from an existing table to support the design.



> They look fine. Consider running aluminum channel around the outside if you can size it to slip over the edge of the plastic. The aluminum will shed heat, give a clean finish, stiffen it mechanically, protect the edges from damage.



That is a really good idea. Thanks for taking the time to reply.

This was a prototype I did for an arts festival last year with a wooden table bouncing the LED light off the floor, it worked well at night and less well in day light. Hence the need for frosted acrylic awesomeness.

http://www.youtube.com/watch?v=t7TbM9LhGwQ


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## Steve K (Feb 26, 2013)

I've looked at the cost of a piece of clear acrylic, with the intent to use it as a backlight for a stained glass piece. For a moderately thick piece, it's fairly expensive. Certainly more costly than the bit of plywood that a ping pong table is made from. Do you have any idea how strong the acrylic is? If people are actually going to use it, they will be leaning on it, sitting on it, etc. I'd be worried about it breaking or just bending excessively.

Again, if it is actually going to be used for ping pong, will the frosted surface provide enough visual cues for people to know where the surface is? There's probably not a good way to know, but it's something to consider. If was going to be bright, I'd also worry about losing sight of the ball, but that's probably not a realistic concern. 

When I was thinking of this arrangement as a stained glass backlight, my main concern was getting the light coupled efficiently into the acrylic. This would that the light be shining at a near-perpendicular angle to the surface of the acrylic, and that the surface of the acrylic be smooth and polished. If you could drill or mold a hemispherical hole in the acrylic, and mount the led emitter in the center, that ought to take care of the first problem. To smooth the surface, I've seen video of a person using a small propane torch to heat and remelt the surface just a bit. This probably requires some practice, though.
To produce a decent amount of light, you'll need a fair amount of power, and therefore need to heatsink the LEDs. Some aluminum U channel might do well to cover up the LEDs and the holes they sit in, as well as to provide heatsinking and a bit of structural support.


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## bshanahan14rulz (Feb 26, 2013)

You might consider taking apart LCD to see how they get even lighting from just edge lighting.
basics: http://solutions.3m.com/wps/portal/...BehindTheScenes/Optics101/Video?slideIndex=25
idea is that the density of textural bumps in the light guide increases the further from the source you go. 

You might also consider an LED array under the table instead of only edge lighting. It's easier to DIY a diffuser than a nice lightguide.

Progressively thinning the acrylic via sanding the frosted side may be a cheap way to achieve this, or it may end up being a very costly experiment. My thought is to have a sheet of acrylic, playing surface flat and frosted, underside sanded concave. (frosted from sanding)

I do want to say that I do not have any experience with a project with these requirements, so I'm just trying to throw ideas for you to play with or think through.


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## RoGuE_StreaK (Feb 26, 2013)

Attach some sensors to the table and get it to change colour whenever the ball bounces. And/or make a grid of LEDs under the table, implement an IR sensor grid, and light up the grid that the ball bounced on!


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## SemiMan (Feb 26, 2013)

crx091081gb said:


> Hi, I'm an installation artist and I'm thinking about making a 'light up' ping pong table using frosted acrylic and RGB LEDS mounted round the side. Does this sound feasible?
> 
> Ideally the LEDS would be glued to sides all the way round but I'm a bit worried they might not give an even light (i.e. it might be dimmer in the middle). Is a ping pong table too wide for this kind of effect to work.
> 
> ...




It may be dimmer in the middle but even a 30% difference in brightness is pretty tolerable.

I would consider a different construction. For one, though you are starting with a clear material, the end result will not be clear. It can't as you need a way to get the light out, and that takes a surface treatment that intercepts the light traveling in the panel and gets it out. That makes it not transparent. Given that case, you could bond a thinner acrylic panel to a cheaper rigid backing material.

What you are looking or may be available in a close size almost off the shelf. Edge-lit flat backlights already exist for advertising. Not sure the exact suppliers but I have seen them in 4'*6' sizes. Two of those would possible be modified and perhaps you can even find one the right size.

Semiman


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## crx091081gb (Feb 27, 2013)

@SteveK Yeh cost is a factor, the whole project will only go ahead if I can convince the arts council to fund it. I'm worried about rigidity too, I've been talking to some plastics people and they think polycarbonate might be better as its shatterproof.

The problem of the ball being seen, well I'm either going to go for UV paint and a black light (which I've seen before with pong projects) or use a non white ball. I think it should work - ish.

Thanks for the insight into the polished hemispherical holes, that sounds useful.

@*bshanahan14rulz*

Those are some really good ideas thanks. I think under lighting might well be easier but I do really like the effect of side lighting. I suspect it will come down to what works best after the structural feasibility of the design has been considered. If I make the underside concave then I may difficulty attaching it to the existing frame.







This is what I'm thinking of using.

@*RoGuE_StreaK*

Yeh the existing wooden design I posted the youtube link to has piezo mics in the corners, it lights a different colour depending on where it got hit. 

@*SemiMan*

Now this is really interesting, I'll have a google, any idea what these things are called in the industry? What should I be searching for, 'lightboxes'?

Actually that would be really nice as in theory I could hide a lot more stuff (i.e. the piezos) in between the frosted outer layer and the inner rigid layer and lay out the LEDS underneath and just backlight rather than side light.

Hmmm real food for thought...


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## Marcturus (Feb 27, 2013)

Don't ask plastics salesmen, ask signmakers. For slightly damaged acrylics "junk" to play around with beam behavior and adjusted levels of frosting, too.


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## SemiMan (Feb 27, 2013)

crx091081gb said:


> *SemiMan*[/URL]
> 
> Now this is really interesting, I'll have a google, any idea what these things are called in the industry? What should I be searching for, 'lightboxes'?
> 
> ...



- Yes, look for backlit advertising sign panel or similar. Not light boxes as you will get deep boxes which is the more traditional versions. There are a ton of offshore suppliers and some higher quality on shore ones too. It will be easy to tell which are which as they are thin. If you cant find any let me know. I quickly found some last night.

- It is not a simple matter of frosting to get the light out. That is actually where the science is, extracting the right and even amount of light.


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## SRT Mike (Feb 28, 2013)

SemiMan said:


> It may be dimmer in the middle but even a 30% difference in brightness is pretty tolerable.
> 
> I would consider a different construction. For one, though you are starting with a clear material, the end result will not be clear. It can't as you need a way to get the light out, and that takes a surface treatment that intercepts the light traveling in the panel and gets it out. That makes it not transparent. Given that case, you could bond a thinner acrylic panel to a cheaper rigid backing material.
> 
> ...



It won't be a 30% difference in brightness across the sheet - it will be much, much more.

The inverse square law holds true... if it is brightness "X" 10cm from the edge, then it will be 1/100,000th of that brightness at 50cm from that point.

There are two ways to do this

1) Have a light guide manufactured for the distance required, edge light it, and it will have a nice even illumination

2) Back light the panel and use an off the shelf diffuser


I've tried many many ways of doing this over the years and nothing works other than the above - because you can't beat the laws of physics. Edge lighting without a light guide will just result in a few centimeters wide bright "halo" around the edge, and nothing at all in the center. 

It's possible that some kind of at-home light guide could be fabricated, like scratching or machining a series of thin lines into the back of the panel, increasing in frequency towards the center....but there is such a wide differential in light between the edge and center that you'd have the lines so far apart near the light that they would not produce any brightness.

The physical size of a ping pong table would be a technical challenge for even a professional light guide manufacturing company. 


The way to do this is backlighting.


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## SemiMan (Mar 2, 2013)

SRT Mike said:


> It won't be a 30% difference in brightness across the sheet - it will be much, much more.
> 
> The inverse square law holds true... if it is brightness "X" 10cm from the edge, then it will be 1/100,000th of that brightness at 50cm from that point.
> 
> ...





I don't even know where to start with describing how wrong your post is. I am almost wondering if you are not trying to inject light into a a clouded acrylic sheet as opposed to a clear acrylic sheet.

- The inverse square law actually does not apply. The inverse square law assumes that when you increase the distance, the surface area being lit increases by the square. However, that does not apply in this case.

- The acrylic (or polycarbonate) sheet actually is a light guide. As long as the light rays hit the surfaces at less than the critical angle (about 42 degrees), then they will be completely reflected internally and spread throughout the sheet. To that end, the injection source should restrict the angle of the light to +/- 45 degrees as anything beyond that will just be lost.

- There will be losses as the light passes through the acrylic and surface irregularities and defects will also cause losses from the lack of total internal reflection. 


Where the difficulty comes in is in light extraction. The surface is selectively roughened such that total internal reflection does not occur in a controlled manner. Often that is small squares or circles in a controlled regular pattern. There is some technology in the way the surface is "roughened" w.r.t. the structure that is used to extract light.

Virtually every non-LED backlit LCD panel whether a monitor or television is back lit this way. A lot of LED ones are as well. There is even more technology here than in advertising displays due to the desire to control light characteristics to maximize transmission through the panel.

This is done day in / day out for advertising display back light panels up to and including large panels. Here is one of the better companies that do this .... there are a ton of off shore suppliers:
http://www.elumanation.com/LED_Light_Panel.html

Semiman


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## SRT Mike (Mar 3, 2013)

SemiMan said:


> I don't even know where to start with describing how wrong your post is. I am almost wondering if you are not trying to inject light into a a clouded acrylic sheet as opposed to a clear acrylic sheet.
> 
> - The inverse square law actually does not apply. The inverse square law assumes that when you increase the distance, the surface area being lit increases by the square. However, that does not apply in this case.
> 
> ...




You didn't read what the OP wrote and you didn't understand what I wrote.

See post #1 - the OP said he was going to try using "frosted" acrylic. Read post #7 - you said it could be 30% dimmer in the middle with that approach. That's wrong. 

I pointed out the right answer, which is that the inverse square law will dictate that more of the light will be emitted closer to the light source in such a setup. That is correct. The fact that the light is being bounced around by one surface by TIR is immaterial - it is still distributed across a surface and reduces in intensity with distance. I then pointed out that to make it work, you would need a light guide and an appropriately made patterned feature to get an even illumination. That is also correct. 

You said I was wrong and that the "surface should be selectively roughened.... circles in a controlled pattern". You just repeated the same thing I said (my option #1).

Coincidentally, the link you provided for the eLume panel does precisely what I said would need to be done - has a "light distribution grid" engraved into the back of the acrylic panel.

Side lighting any type of acrylic without a pre-designed light extraction feature (that will have increasing frequency as you get to the center) will not result in a panel that is 30% dimmer in the center, but rather one that is bright around the edge, with no visible illumination in the center at all.


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## SemiMan (Mar 4, 2013)

SRT Mike said:


> You didn't read what the OP wrote and you didn't understand what I wrote.
> 
> See post #1 - the OP said he was going to try using "frosted" acrylic. Read post #7 - you said it could be 30% dimmer in the middle with that approach. That's wrong.
> 
> ...



If you read my post "For one, though you are starting with a clear material, the end result will not be clear. It can't as you need a way to get the light out, and that takes a surface treatment that intercepts the light traveling in the panel and gets it out. That makes it not transparent. Given that case, you could bond a thinner acrylic panel to a cheaper rigid backing material." This is what you replied to.

Your description of a "light guide" is not how this type of thing is manufactured. A clear piece of acrylic is inherently a light guide ... it does not require scratching of lines, etc. The scratching of the lines is not a light guide, it is light extraction from a light guide.

Your post "It's possible that some kind of at-home light guide could be fabricated, like scratching or machining a series of thin lines into the back of the panel, increasing in frequency towards the center....but there is such a wide differential in light between the edge and center that you'd have the lines so far apart near the light that they would not produce any brightness." ... shows that you are not getting this concept as there is NOT "such a wide" differential between the edge and the center. With proper light injection (not super difficult) and aluminizing of the the edges to reflect back light, you can achieve fairly good consistency in brightness inside the panel from the edges to the center.

You actually do not need grossly different densities from the edge to the center to achieve reasonable consistency. I am holding a 24" panel in my hands right now (remembered I had it in the lab) and the density of the light extraction circles is not that big really and I have measured this unit as about 75-80% uniformity, the target for these types of products. For backlights on LCD, they target much higher uniformity though, >90%.

Even in your case above, with "even" distribution of light extraction features, the inverse square law does not apply directly. You are shining a beam with a given profile into a sheet. It is not the front of the beam that is lighting the surface, but the side of the beam. It is almost like a wall wash, but in this case any light that does not exit keeps reflecting to exit somewhere else. Inverse square law assumes distribution of light in a 3 dimensional space. We are actually constraining one dimension (100% reflective top or bottom surface), so inverse square does not apply.

Semiman

Semiman


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## Esko (Mar 4, 2013)

crx091081gb said:


> Hi, I'm an installation artist and I'm thinking about making a 'light up' ping pong table using frosted acrylic and RGB LEDS mounted round the side. Does this sound feasible?



A nice project. I don't have many advices on how to make it, I just want to point out one downside. This kind of lighting makes it more difficult to play the actual game. A comparatively bright light on table (which our eyes will adjust to) and the unlit ball and surroundings makes it harder to see the ball and estimate the movement. Personally, I like the black light idea better.


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## Ken_McE (Mar 4, 2013)

Some minor suggestions:

Fix up a bit of kit, go down to a glazers, tell them you have a project in mind and would it be OK for you to shine some lights on some pieces of frosted plexiglass? If they say "no", go to the next glazers. If they say "yes" then you bring out a side light and a back light, apply them to various pieces they have around the shop. This approach will let you see what patterns and materials are available, what sizes they come in, what prices are. My local glazer has perhaps half a dozen different surface finishes in stock. You might find one you like better than a plain frosted surface.

The new pieces of plexi/Lexan/Perspex (whatever they call it there) will all have a thin sheet of plastic stuck on either side. This protects it during shipping & handling. It will interfere with your tests but they really do need it to keep the faces pristine. All these plastics are dust magnets. Once the dust is on the face of it any little brush may scratch up the surface, then they have to open up a new panel. 8-(

If you do need to light from underneath, there is something I know as a cavity light that might be of interest. I am familiar with them from the world of emergency lighting. They work like this:

You have a hollow box, it is white and reflective inside. There are lights sitting along the bottom facing up. Their light goes up, bounces around, comes out through the front of the box reasonably well diffused. A variation on this might work for a back light.

Picture a pyramid made of light wood (I would use Luan, not sure what's British equivalent) black on the outside, white or silver inside. The bottom is open, that is where the light comes out. Your LEDs are mounted so the diodes are inside, the heat sinks outside in free air. You would have have however many of these black pyramids hanging off the bottom of your sheet. Pyramids would enable you to cover the space efficiently, and almost all of your woodworking would be simple straight cuts.


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## SemiMan (Mar 5, 2013)

If you are considering making a backlight, one of the easiest ways of doing this is with an array of side-emitters using an optic such as this http://www.carclo-optics.com/opticselect/intranet/optics/details/optics_pdf.php?id_optics=88

You would put these into a box say 75mm deep that is painted with a highly diffuse (yet still reflective) white paint.

The advantages of side emitting optics are:

- Requires less individual emitters in order to achieve a smooth even illumination level
- As emitters are shining sideways, the light mixes and becomes more homogeneous reducing issues with color and brightness variation between the individual emitters
- Simple for prototyping

It tends not to be as efficient as direct illumination though and you have to watch for artifacts as some of the light always goes upwards with those optics. That said, directly illuminating that large a surface would likely require a deep box in order keep the number of emitters reasonable for direct illumination. Not sure that is within the realm of your design goals.

I agree with SRT_MIKE on the difficulty of doing side emitting well. Unless you can buy an off the shelf advertising panel and use that, I would go with back lighting as it is much much easier to get right.

Diamond Lyte has panels that are very large. I have to expect anything custom will be very bright.

There is likely one other issue that comes into play with edge lighting which is the mechanical construction of the sidelight. It may be very difficult to make that such that it does not impact playability of the surface. A table tennis table is usable right out to the edges which would need to be maintained. At a minimum you will have some distance from the edge that is not active, i.e. not lit up.


Semiman

Semiman


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## en2oh (Aug 8, 2015)

SRT Mike said:


> It won't be a 30% difference in brightness across the sheet - it will be much, much more.
> 
> The inverse square law holds true... if it is brightness "X" 10cm from the edge, then it will be 1/100,000th of that brightness at 50cm from that point.
> 
> ...




Hi folks,

First post and something I'm moderately interested in.
As far as the "best material", I would suggest looking at LuciteLUX.
This product was developed by Misubishi Rayon a number of years ago for the specific purpose of guiding light.
The product contains nano sized particles of either TiO or similar material that has significantly different RI's of the host polymer.
This results in internal reflection with dispersion, which is what you want in light guides with side glow). The combination of internal reflection and a highly reflective back surface changes this from a simple "end lit" light guide with end glow light to an end lit, "side glow" light guide. Depending on how effectively you reflect the side glow out, the panel will look more or less bright. These panels are ~92% transmissive to visible light ie the dispersant does impact on clarity, but not significantly in a .125 or .250" thick panel.

As far as uniformity of light across the panel, you can address this a number of ways. digitally printed reflective dots on the back surface is one simple way. Precisely machining groves, or if you have access, an IR laser engraver will do the job as previously described.

Finally, think about the inverse square law. Consider the importance of the light source being a "point source" and you'll see why the inverse law doesn't apply here. If it did, you could never have optical fiber technology that was practical: the course illuminator would have have to be so bright as to melt the fiber if you could hope to have the light detectable at the other end of a 10km fiber 

Nice thread.
Thanks
Doug


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