Lens Units


LENSES come in all sorts of types but for tag we want CONVEX lenses. Each lens can be characterised in the following ways.

SIZE - the diameter of the lens

FOCAL LENGTH - the distance at which light is gathered to a point

The longer the focal length of a lens the longer the potential range as the light will stay coherent longer. Of course this is offset by the fact that the further your led is from the lens the bigger it needs to be to catch all the light from the led.

Common lens sizes in use vary between 25mm and 50mm although Ive seen some guns with VERY large lenses. For a 'realistic' size gun a 50mm lens is about the largest you can get away with (I prefer 40mm myself).

LEDS have 2 factors that are important to us.

POWER - this is usually given in mw/SR (milliwatts per steradian, basically the bigger the better)

HALF ANGLE - the angle that the LED puts out its light (so for example an LED with a 16 degree half angle projects its light in a 32 degree cone)

So ideally we are looking for an LED with as much output in as small an half angle as we can get.
The best we have found so far is the OD50L from opto-diode corp. This has a 7 degree half angle and an output of 600 mw/SR. It only has one downside, to get that power it needs to be driven at around 10 amps (which is alot of power).

The next best we have found is the SFH484-2 from Siemans, it has a half angle of 16 degrees and an output power of 80-160 mw/SR, this is at a power of 100 ma, if you pulse the led at about 1 amp it had about a 300 mw/SR output power.
Not bad we get half the output for 1/10th of the input.
The SFH484-2 also has another big advantage, its costs about 0.60 as opposed to 10.

NOTE:-2X the power does not mean 2X the range, it actually takes 4X the power to double the range of a gun so you can see that the smaller led is better overall

CONSTRUCTING A LENS ASSEMBLY

Depending on your skills and equipment available there are lots of ways to do this. I'll stick with a simple way that most should be able to do.

Luckily for home lasertag constructors hardware stores have many many types and sizes of pvc pipe available. What you want ideally is a size of pipe that your lens just fits inside.
If you are using one of the common lens sizes (25mm, 40mm and 50mm) this is easily done.
First you cut a length off that is about 5-10 cm longer than the focal length of your lens (or longer if you like). You also need to cut 2 more lengths one about 2 cm long and the other about 5 cm long.

Take the 5 cm long length and cut it length-ways.



you need to remove a piece sufficient to let it fit snugly inside the long length of pipe like so.....



Glue this section into place. You now have a nice recessed ledge for your lens to sit on. When you fit your lens in place the smaller 2 cm section goes in front to hold it in place.



Thats the lens taken care of. Next comes the emmitter, the easiest way to mount the led is to use a bezel mount.



I find there are 2 easy ways of fixing the bezel in place, one is to cut a disk of plastic or thin wood the same size as the lens and mount it with 2 more sections of tube like the lens.
The second (and easier) way is to take an offcut of your tube and go wander the local supermarket :-)
WHAT !! you say, well you are looking for a plastic cap that fits comfortably inside your tube. Depending on tube size Ive found that all manner of caps from bottle tops to deoderant tops will fit.

Once you have found one that fits just drill an appropriate size hole in the middle to fit the bezel and slide the cap into your barrel tube. The advantage of the cap method is that its easy to slide the led backwards and forwards to focus the lens assembly. When you have it at the correct point just use a small self tapping screw to secure it in place.



FOCUSING THE LENS UNIT

One easy method for focusing in in a lens unit is to use a hyperbight red LED the same as we use for the guns muzzle flash. While it doesn't focus the unit perfectly (due to the slight difference in wavelength between IR and red light) it will work very well. Just light the LED and focus it on a white wall at least 10 metres away (the further the better) and slide the focus back and forth until you see a sharply focused image of the LED. Once you have it fix the cap in place and replace the visible LED with your IR LED.

If you have a video camera sensitive to IR or a set of IR night vision goggles you can be a bit more precise by using the IR LED and checking the focus with the camera instead.