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March 25, 2010


Multi-Touch Table – The Projector

Finally it’s time to talk about the projector.  Why devote a whole post to this one component of the table?  Because picking your projector is the single most maddening, difficult, and long lasting choice you will ever make about any part of your table build.  The projector is forever, Bitch!

Why is the projector so damn hard to choose?  Because many of the choices here will dictate entire sections of the rest of your build – overall table size, overall table height, bounce mirror(s), and camera placement to name a few.  Why does the projector choice affect all of these items?  Two words – Throw Ratio.

Throw Ratio

Simply put, the “throw” of a projector is a measure of the distance from the projector lens to the projection surface.  It’s how far the image is being “thrown” onto the surface.  The throw ratio of a projector is determined by taking the throw distance and dividing it by the  projected image width.  Projectors with large throw ratios will need to be further away from the screen to get a certain projected image size, projectors with small throw ratios (particularly those less than 1.0) are referred to as “short throw” or even “ultra short throw” and can often produce a 40-60″ diagonal image while being only 12-18″ away from the projection surface.

Why does it matter?

The simplest way to get a projected image onto a tabletop surface is to simply aim the projector straight up at the bottom of the surface.  There’s a catch with that though, what if you want the table surface to sit 25″ from the ground?  Well, let’s say you have a projector that’s 10″ long from front to rear.  If you take that projector and point it straight up at the bottom of the table the lens is probably going to be sitting about 12-13″ from the bottom of the surface (remember you need to leave room for the cables to go into the back so that the actual front to rear length of the projector is going to be closer to 12-13″ rather than the 10″ length of the projector itself) leaving you with 12-13″ of “throw” to the bottom of the tabletop.   Let’s say you want a 45″ 16:10 diagonal image – to get that in this setup you’d need a projector with a throw ratio around 0.31 – 0.32.  This is because a 45″ 16:10 image is about 38″ wide.  12″/38″ (Throw/Width) = 0.3157.  This is a pretty short throw projector!  Well that’s great you say, you know the throw ratio you need, now just go find a projector with that ratio and you’re done!  Well, no, not exactly.  You also need to worry about the offset of the projector.


The offset of a projector is a measure of the distance between a line shot straight out of the center of the lens and the actual bottom (or top if you’re talking ceiling mounted) of the projected image.  This is really hard to show without an image so thankfully the nice people over at InFocus have gone and made one that I will now share with you here.  That doc does a far better job explaining offset in a few pictures than I ever could in words.  Here’s the important takeaway – a projector with a zero offset means the image is evenly split in height around the center of the lens.  This would be great for just pointing it straight up – center it in the table and go.  Only slightly less good than an offset of zero is any offset less than or up to 100.  With an offset of 100 you just center the projector width-wise and place the center of the lens aligned to the bottom (or top) of the projection area and you’re good.  This works because most tables have a lip of some kind typically a few inches wide and most projectors are only 3-4″ tall, so you can easily “hide” the 2″ or so the projector sticks out past the projection inside the table.  Problems arise as the offset increases and in almost every case it will be greater than 100.  This means if you just point the projector straight up the distance it has to be offset from the actual projection area gets larger and larger and soon the entire projector will need to be placed “outside” your table enclosure to get any kind of image within your desired projection area.  Now we have two problems to deal with – throw, and offset – so how do you deal with them?


I’ve already spoken about mirrors in a previous post and I’m just too lazy to link to it here but you can find it if you poke around.  Mirrors can do two things for you – increase your throw distance and/or correct for an offset angle.  So great, just tear the bathroom mirror off the wall and you’re set, right?  No, not at all really.  Most of the mirrors we deal with in daily life are what’s called second surface mirrors.  This means the actual reflective surface is placed behind some kind of clear protective layer – typically glass, but often times acrylic.  The reflective layer is typically very thin and fragile so putting it behind something makes a lot of sense – it allows you to clean the mirror without rubbing off the very thing that makes it a mirror!

A problem arises though if you try to use a second surface mirror as a way to collapse a projector’s throw distance.  You get ghosting.  This is maybe best explained by thinking about the old rear view mirrors in cars (not the fancy electrochromic auto-dimming ones we have today) that had that little lever on the bottom for “night driving mode”.  These mirrors worked by exploiting the fact that the front surface of the transparent layer of the mirror ALSO acts like a mirror it’s just way less good at reflecting (meaning it’s less bright) so you typically can’t see it.  The fact that it’s less bright is a good thing for a rear view mirror but this effect – when sitting a few inches away from a 2500 lumen projector is pretty noticeable.  So what you get is a reflected image from the rear of the mirror, and a second “ghost” reflection from the front of the mirror.  There’s an easy (but not cheap) way to solve this and that’s to use a… wait for it… First surface mirror.

A first surface mirror, as the name would imply, moves the reflective layer up to the front of the mirror substrate.  This way there can be only one (!!!) reflection since the image never passes through to the back of the substrate and is only reflected off the “first” surface.  The problem with first surface mirrors is that they are really expensive and also really fragile in the sense that it’s easy to damage the reflective layer, not that they are more prone to breaking – thought hey do tend to be pretty thin.  First surface mirrors are used a lot for guiding lasers and hence tend to require really strict optical properties which the dumb-ass like me who just wants to bounce a projected image around has to pay for.  There are some cheaper variants around but quality really can vary so be sure of the product before you buy.

All that being said – for just futzing around and general “aiming” of your image a second surface mirror can and will work.  I plan to do some rough planning and design with a cheap IKEA acrylic mirror before dropping the cash on a good first surface mirror.  Yes there will be ghosting, but you can scratch and scrape that cheap mirror without much worry during the early stages of a build and not cry about wrecking it.

As you can guess – if the offset of my projector is too much (and I am confidant it will be) I plan to use a single mirror to adjust and bounce the image properly to my screen.  There may be some challenges to this but hopefully less than the original two-mirror bounce I was planning on the previous table build.  The more mirrors you add the harder it becomes to solve for the right angles at all points.

Are We Ready to Buy a Projector Yet?

Well, no.  So we’re already worried about throw ratios, lens offset, mirrors and angles – what else can there be?  In addition to those items you also need to decide on the projector brightness (which is further impacted by your choice of projection material, the thickness of your acrylic “sandwich”, the expected brightness of the area the table will be used in, etc.) which is measured in lumens, the resolution and aspect ratio of the projector, the actual projection technology (LCD, DLP), the cooling and airflow, the available inputs and outputs, and even the overall size and weight.  And let’s not forget flat out COST too.  Oh and then there’s optical zoom, lens shift, keystone correction, networked/serial/remote control options, and probably about 20 other potential pluses and minuses to consider.  Now you begin to understand why this one piece of technology can be a f$%king nightmare to decide on.

Ok, Are We Ready to Buy One Now??

Yeah, sure.  I’m sure you’re thinking, armed with all this knowledge, that I spent days upon days planning, searching, and optimizing every aspect of my projector choice… nope.  Here’s the reality – you do need to be aware of all the factors involved in your choice but when it comes down to it, you’ll have a budget, and you’ll go to some of the big projector reseller sites on the net and you’ll filter on price, then you’ll filter a bit more, maybe for projectors with a short throw, and then projectors with the resolution you want, and then you’ll have like 5-6 choices.  NONE of those choices will be the projector you want.  The projector you WANT is going to start somewhere around 5x your budget and it will be 3x larger in size than your table design can accommodate.  But you’ll have your 5-6 choices and now you need to start doing your homework.  Pick the “best” of those choices.  Hooray, you’re a winner!

The good news is that you are not the first person to try and build one of these tables and there are places you can go to see what projectors other people have used.  The NUI Group forums are a great place to do this.  However, if you are a dumb-ass like me you’ll avoid that completely and try a brand new, just released, totally untested projector.  Which brings me to…

The Dell S300 Projector

FINALLY!  This is what I bought.

Meet the S300

The Dell S300 is pretty damn new.  It wasn’t even shipping from Dell when I ordered it but the specs all seem pretty good and the price was what made me willing to take the risk of buying it.  The S300 is a short-throw, 1280×800 (16:10) DLP projector.  It supports both digital and analog inputs as well as network and/or serial control.  The light output can reach 2200 lumens and it even has support for 3D.  Typically a short-throw projector with this resolution and feature set would cost over $1000, probably closer to $1500 (and many still do), but the S300 can be had for a mere $899!  It’s almost enough to make you think that it’s LESS than $900!!  Seriously though, $899 is not a bad price so I went for it.

The projector is both light and small which should help with placing it.  The throw ratio is right around 0.51 meaning I can get a 45″ diagonal image (my desired size) with the projector only about 19-20″ from the screen.  The catch here is that the smallest image the projector does is 44.6″ diagonal.  Why is that bad?  Because it’s really damn close to 45″ and if I’m off just a bit the projector won’t be able to focus the image.  The throw distance between a 44.4″ image and a 45″ image is going to be a fraction of an inch but the complete and utter madness of wondering why the image is just never quite focused properly and tweaking it (accidentally moving the projector 1/8″)  and then trying to re-focus and seeing the image too big… it’s going to suck.

How can you not be drawn in by the sumptuous curves of that lens?

The projector sports a pretty significant array of connections which I hope to use to control it from the PC.  This way when you power on the PC it will them power on the projector, and when you shut it down, it’ll power it off.  It’s a small thing but it’s a nice feature to have rather than trying to rig up some physical or purely electrical way to do the same thing.

In my rigorous and extensive testing (playing XBOX360 games through the HDMI port) the S300 looks and works quite well.  I am able to get  something around an 8′ – 10′ diagonal image with the projector around 3-4′ from the screen (wall).  There is no tearing or ghosting that I have noticed (in both Left4Dead and Left4Dead 2) but that might be related to the fact that I’m just thrilled as hell to have an 8′ gaming screen.

This thing has enough inputs and outputs to make a porn starlet blush!

I’m not going to say that my “testing” hasn’t had any effect of delaying the build of the table.  I’m just going to let you figure that out by yourself.  Go ahead, figure it out, I’ll wait.

So why is this a really dumb thing to do?  Because I still don’t know what the real offset of the projector is.  It’s not really addressed in the manual and I have not yet tried to measure it out myself.  Either way, while the throw really is short, it’s not so short that I can just stand it on end and have the image size I want within the table height that I want.  So I will almost certainly have a mirror in play.  But depending on the offset adding that mirror might be really easy, or it might be incredibly frustrating – I simply don’t know yet.

Poorly photographed and smudged with fingerprints

I might be the first person to try a table build with this projector so there’s no one else’s experience to draw from.  There is no reference design for these tables.  That’s part of the fun, but it’s also part of the risk.  I may have dropped $900 on a projector that won’t actually do what I want/need it to do.  I might get it aimed and bounced perfectly but find it’s not bright enough, or too bright.  But this is what hacking things like this table is about – picking the best parts from what you can afford and then finding a solution to the engineering challenges that arise from those choices.  I’m sure I can make this thing work, I just can’t tell you exactly how yet.

Next time I’ll cover some of the other parts I already have that are key to the working of the table.  I’ll also try to make a post that’s less than 2500 words.  Even I’m getting sick of reading this post at this point.  Until next time…

10 Comments Post a comment
  1. Niltz
    Mar 26 2010

    My profession is a movie theater manager. As such I find this topic fascinating. I am not the technical engineer but I have some thoughts.

    Firstly, I think large scale theater projection is actually less fiddly than this. Ha! We use really expensive custom lenses to eliminate most of the guesswork. I guess that isn’t really an option here =)

    Units like this are similar to the small DLP units we use alongside our feature projectors to display ads and such between films. One thing to consider is a lot of the major chains are going to be upgrading to digital units and chucking the old 35mm projectors. In the process theaters like ours may be getting rid of the small ad units in bulk and using the low power mode of the big DLPs to run the ads instead of a separate unit. They may be able to be found cheap through a reseller. Most of these come with electronic zoom lenses so you can adjust the throw while keeping the unit still. Probably still much more expensive than the one you are using, though. I also find it dubious they would work for a throw that short but based on some quick googling of the stats they are in the throw range. Although they are 16×9.

    I am not sure what the cost of replacement lamps are for that one but I know they are ridiculous for our purposes, like 200-300 bucks for a 1000 hour lamp. This may be a hidden cost you need to prepare for.

    Due to space issues, some of our units are mounted at weird angles, including at 90 degree angles from the screen. We use standard second surface mirrors because they need to be cleaned, and it doesn’t seem to give us ghosting issues. At least I think they are second surface. Ha, I have to check now. They might just be really tough first surface.

    Dagnabbit, reading this made me realize how much I DON’T know about my trade. And I had to take a licensing exam to do it! I guess this is why we still have to call technicians for the big jobs. Heh. I’m more like Kirk sitting in the big chair making decisions. Scotty is the guy making the projector/warp core work. I just know enough to keep the place running.

  2. RexRemus
    Mar 26 2010

    I’m givin it all she’s got cap’n!

    The rated lamp life for the S300 is 5000 hours. Do I believe that? No. But at least if I get half that I’m doing pretty well, and yes the lamp is around $200 to replace but hopefully that’s 2-3 years from now.

    16×9 wouldn’t really change things all that much. I just didn’t want a 4×3 ratio. Certainly any 1280×720 projector was on the list, but if I can get 80 extra pixels at a reasonable cost – why the hell not?!

    Oddly enough I found your comments just as interesting as anything else I’ve seen. If you have a line on those mirrors I might be interested in the details. Something more durable would be great. You know where to find me.

  3. Niltz
    Mar 27 2010

    I think our mirrors are mylar, and not actual glass. They are a more durable front reflecting mirror that requires minimal cleaning. I have no idea what the cost is off hand. My company usually has someone in purchasing take care of these things when we need an installation. I did a little bit of web searching and these guys seem to do custom size mylar mirrors.

    They don’t have price quotes on the site though.

    You probably won’t have to worry too much about upkeep. We run our units all day every day, so 1000 hours burns up quick. If you can get 2500 out of 200 bucks then that is fair for these. The lamp cost is why I usually tell people to avoid DLP for home theaters. Don’t be surprised if you get a lot less time if you buy cheap lamps, or refurbished ones. The savings aren’t worth the headache. Also the lamps have a tendency to dim long before they expire, which is why we go through them so fast.

    And for the record about playing games: yes I have oft daydreamed about hooking my PS3 up to the big DLP at work, but the company monitors those things remotely and would know I was messing with it and be quite unhappy with me. They like to spoil all the fun. Things like that is why everyone who works in the business dreams of opening their own instead of working for someone else. Sadly most of us don’t have the MILLIONS of dollars to start one up. Heh.

  4. RexRemus
    Mar 27 2010

    I sent a request to that site to try and get some info on their products. If it looks good this might be just the thing – light, durable, and hopefully affordable. I’ll pass it on to the rest of the NUI group community if it looks good. i do want to make sure those mirrors have a substrate that won’t warp from heat (the stuff you get might be specially treated to resist it if it’s intended for projection) and I asked them for a small product sample to examine it first-hand. That’s a great find. Thank you!

    As for the gaming – don’t you guys rent out your theaters? All the places around here rent them out during the day when things are slow for business meetings and birthday parties – which means 40ft video game action in 7.1 surround! You could then say you were “prepping and testing” the system to insure optimal birthday party satisfaction. If you’re not doing that, maybe you should send it up the chain for consideration.

  5. Niltz
    Mar 27 2010

    Technically we do rent our theaters, but they signed some contract with a 3rd party advertising company a few years back to handle all of our in-theater ads. Part of the deal was that they get to coordinate and run meetings or anything else that isn’t feature film related. Technically they own the little DLPs that run our ads since they are their ads. That’s why I am a little sketchy on the tech details of them. Since I can’t hook a video game system up to a film projector I am stuck using their stuff, and if I am fiddling without permission I can be in violation of our contract. If we were to schedule a video game thing it would have to go through them and they would help us set it up. And we have yet to have any requests for that sort of thing. I have pushed to try to start hosting tourneys of new release games or pushing game time rentals but the people at corporate aren’t quick to adjust business models or listen to a peon way beneath them. Besides, it would be ridiculously expensive for a rental.

    And there are no ports to hook it up to on our big DLP as far as I can tell. (Believe me, I looked. Heh.) Besides, those generate logs too because movie studios are paranoid with piracy and are afraid we would run special shows without their permission.

    …. Ahhhh isn’t it refreshing how byzantine modern corporations make things?

  6. RexRemus
    Mar 28 2010

    Impressive. I realized all of Hollywood treated their customers like criminals but I am surprised to see them treat their “extended” employee family as criminals as well. Learn something new every day.

  7. just checking
    Apr 26 2010

    long post about buying a project while guessing it might work with mirrors. Have you pointed it at a mirror yet? Good luck! Let us know what you discover in another month or so.

    • RexRemus
      Apr 27 2010

      I was able to try with a small mirror but without great success – the lesson learned here is that short throw projectors really do make a LARGE image in not very much space making it very difficult to get a mirror in front of that that can bounce the entire image but also not cause the projector to self-shadow. Either you need a very large mirror ($$$) or better geometry skills than me. Further testing is planned.

  8. jonathan
    Sep 14 2010

    Hi, nice site!

    Have you managed to figure out the offset of the Dell S300 projector yet? I’d like to get it!

    I’m working on a project and bought the Epson EB-450W but the offset is crazy.

    Need a low / zero offset projector quick! :)


    • RexRemus
      Sep 17 2010

      I have not tried to calculate the exact offset but I wouldn’t call it “crazy”. What I have found is that with any short-throw projector it can be really f’in hard to try and fold the throw with mirrors – the image just gets so large over such a short distance. I’ve had a good bit of frustration over this and am actually at a point where I’m just taking a break from it for a while. I’ll come back at it with a new battle plan at some point.


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