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Infrared Optics for MilesTag Systems

 

This is not intended as an authoritative text on optics or lens geometry.  It is only intended to provide a basic understanding of how to construct a functional lens setup for MilesTag or other DIY Laser Tag systems.  

 

 

Lenses        

 

The effective range of an un-lensed infrared LED is very limited, even at high drive currents.  To achieve the ranges needed for a decent Laser Tag system (50 meters to over 200 meters), you will likely need to use a lens.  The simplest setup is a double-convex lens (like a standard magnifying glass).  These can be found from online surplus stores, or science supply companies.  Or you may find a suitable hand magnifier at the local hardware store or office supply store.  2" (50mm) diameter is a good starting point, though you may have great results with smaller or larger diameter lenses as well.  You will need to experiment.

 

The lens and LED are mounted at opposite ends of a tube.  The tube is cut to an appropriate length so that the LED is sitting at the lens' Focal Length.  This may require some experimentation too to get the focus just right.

 

How do you find the Focal Length (FL) of your lens?  If you have ever used a magnifier to burn a hole in a piece of paper, then you already know how.  Of course, if you use a bright light instead of the sun you will be able to test the lens' FL without starting a fire.  The light should be at least 15-20 feet from the lens.  This will get you in the ballpark.

 

It is also important to select an LED with an appropriate "angle-of-half-intensity" or "beam angle" for the lens you are using.  You want as much of the IR light from the LED as possible to hit the lens.  If the LED's beam angle is too large or too small, you will lose efficiency and the range of the system will be reduced.  And don't forget that some LED specifications are for "half-angle".  A 10-degree half-angle is +/-10 degrees, so it's actually a 20-degree beam angle.

 

For example: The TSAL6100 LED has a 20-degree beam width (10-degree half angle).  So it would be well suited to a lens with a 30mm diameter x 85mm FL.  It could also use a lens with a 50mm diameter x 150mm FL.  But these numbers don't necessarily have to be exact; this is another area for experimentation.  A 50mm diameter x 100mm FL lens might still give great results.

 

In general, a longer FL and/or larger diameter lens allows longer ranges.  But excellent results can be achieved with 1" diameter or smaller lenses that are high-quality, well-focused, and well matched to the LED.  

 

Note:  Any given lens may actually have a different FL for infrared light than it does for visible light.  

 

 

 

Lens Tube        

 

The tube used to hold the lens and infrared LED is often made from PVC plumbing pipe or generic plastic or aluminum tubing.  PVC pipe usually has various adaptors or fittings available that are well suited to holding the lens and LED in place.  It is also very easy to cut to size.

EXAMPLE LENS TUBE SPECIFICATIONS
Infrared LED Beam Angle   Lens Diameter Lens Focal Length
TSAL6100 20 deg   3/4" (19mm) 2.2" (55mm)
TSAL6100 20 deg   1" (25.4mm) 3" (75mm)
TSAL6100 20 deg   1.5" (38.1mm) 4.3" (110mm)
TSAL6100 20 deg See Below 1.9" (48mm) 4.25" (108mm)
TSAL6200 34 deg   3/4" (19mm) 1.3" (32mm)
TSAL6200 34 deg   1" (25.4mm) 1.7" (42mm)
TSAL6200 34 deg   1.5" (38.1mm) 2.5" (63mm)
TSAL6200 34 deg   2" (50.8mm) 3.2" (82mm)

 

Below is an example of mounting a 48mm lens in PVC "Schedule 40" pipe.  The infrared LED should be mounted in the center of a small cylinder that fits within the barrel (wood or plastic is fine).  The cylinder should be able to slide back and forth to allow focusing of the beam (use a bright red LED for focusing, then replace with the infrared LED).

 

Note:  The actual outside diameter of "1.5-inch" PVC pipe is 1.9 inches. 1.5" x 2" coupler  
 
The 48mm lens diameter is a perfect match for 1.5" PVC pipe. The coupler holds the lens securely in place. Ready to fit LED and mount on gun.