How the LED-LP Effect is Similar to LLP

The following is a response of mine to a user on the nuigroup forums regarding LLP safety and the LED-LP setup.

User gfunk2k writes:

"Im having problems with my setup. as you know i was going to try LLP but was put off as im worried about the dangers of the lasers as i have a little one running around so thinking of the LED-LP approach as this is allot more safe. if you remember my pic the LEDs would only be on the left and right of the screen and also the the screen is 19" but the LEDs would be around 60cm away from each other. would this still work and do you have any idea what LEDs i will need to look for?
If I had to have the LEDs top and bottom they would again be 60cms away as this is the size on the glass top im using.
Thanks again"

Here was my response:

"Hmmm...laser safety is a worrisome problem if you have instances (children, pets, etc) that may somehow redirect the plane to hit someone's eyes. LLP setups are typically better and more safer in vertical wall setups, as it is harder to change the beam path in a way that would hit someone's eyes. It is mentioned that at the low power (under 25mW) that the typical LLP laser is at, after going through a line lens (to create the plane of IR light) that the IR light is much too weak to really cause any damage, but you can never be too careful; since as my Mom would say, "you aint going to get another set of eyes so take care of them." I will let you know that I have tested a CD drive laser and accidentally hit my eyes with it a couple times. It caused some slight pain for a day or so, but like most instances of low power lasers, doesnt create any damage for too long and everything was normal the next day.

As for an LED-LP setup, it is much safer though much more difficult to get similar results to an LLP setup. The "plane" of IR light that you have to create via IR LEDs around your touch surface usually has a much "thicker" beam, so you have to adjust the filters in tbeta very tightly to get very low blob appearance, or you'll create touches when you arent even touching the surface.

My recommendation is to surround all sides of the screen with the IR LEDs. The reason is that you need to create a grid-like plane of IR light. An LED puts out light in a conical beam, so the narrower the beam of the LED the better, since the less light that is sent above the touch surface. This is contrary to the suggested LEDs for an FTIR or DI setup, which should have a much wider beam spread so the the FTIR affect can work better or so that the screen is more covered evenly for diffusion. So think of the LEDs as "lasers" that spread apart the further that the light gets. With these "narrow" beams, we need to essentially create a grid of beams that create a plane, which we touch. By having the beams on at least two sides next to each other (i.e. the top and a side, or bottom and a side), you create this grid (though all 4 sides creates a better grid). By only putting the LEDs on the top/bottom or sides, you are allowing there to potentially be gaps in your beam plane, and thus you will get a strobbing affect if you were to draw across the touch surface. You will get the strobbing affect at the very start of your LED rails no matter what, so make sure to locate the tip of the LEDs off from where you want to begin to touch by at least 1cm or so (this will reduce the strobbing but not elimiate it).

LED-LP setups work better for smaller screen sizes because the conical light from the LEDs doesnt have a chance to spread above the touch surface as much. The screen in my LED-LP table is a 17" (4:3 ratio), which works well. Going bigger should be fine up to maybe a 22" or 24". I do not know the size limitation, but I know that there probably is one, as the LEDs will eventually spread too much to be effective.

As for the type of LEDs to buy, as mentioned above, get ones that have a fairly narrow beam spread. The ones that I bought and used have a beam spread (viewing angle) of about 15-30 degrees, as seen here on ebay. Spacing should theoretically be as close as you can make them, with a max separation of 1 inch (about 2.5cm). The ones on my table are spread apart about 2.5cm on the smaller side and 3cm on the larger side, thus all 4 sides are surrounded. Use an led calculator either found here or here to calculate what resistors to put in series with each parallel rail of LEDs. I am using 4 sides of 12 LEDs each (48 total).

The info above is collected based on my own experiences and prototyping."