Lenses & Optics

Lenses.

They are a crucial part of the hardware for the emitter/gun circuits… Unless you only want a couple of meters range.

As previously mentioned, again there was very little range about WHY the lens assemblies used works, other than do this and it’ll work.

Which as part of the learning curve I went more or less the opposite direction with it, until all the research came together in a blinding “Well of Course moment!”

Now, the lens assembly is intended, along with a suitable power output to increase the range of the Infra red signal/beam, and it’s the ‘how’ that I had to wrap my head around so I could get a decent combination of parts for the best results.

So, an LED has what’s known as the Half-angle/viewing angle/angle of half-intensity. This is basically the area where the beam is brightest, and the half-angle is about the point where the brightness of an LED effectively drops to half it’s peak output

For a more detailed description have a look here or here (Wikipedia links for led and half angle)

Now in an ideal world, you want the beams from the LED to be as parallel as possible, otherwise as the beam spreads out/diverges it effectively looses.. well.. power/brightness. In short, the more the beam spreads out, the dimmer it seems.

A good example is this video I dug out from Youtube for a variable focus torch.

So, a perfectly parallel beam is what the lens assembly/barrel is aiming for.

Now, I use an Infra-red LED with a 7 degree half angle. I had originally tried led’s with a 15 degree half angle, thinking it would mean that I would need a shorter ‘barrel’ to get the maximum amount of max-brightness infra-red to hit the lens.

The complication comes down to the refraction of the lens. A convex lens can only bend the light towards parallel so far. So, the closer to parallel the infra-red rays are at the start the better.

An LED with a 7 degree half angle has a beam/rays that from the start are much closer to parallel than an LED with a 15 degree half angle. 

So, an LED with a 7 degree half angle is much easier/needs a much less powerful lens than a 15 degree half angle led to focus the rays towards parallel.

So, now following that wall of text, it’s time for the pictures now.

15 degree half angle emitter

This represents an LED with a 15 degree half angle. For illustration I’ve given the lens a 5 degree refraction.

Not so great is it? Certainly not what I would call coherent.

7 degree half angle emitter

Now, Stick in an LED with a 7 degree half angle, and while you do need a slightly longer barrel the resulting beam is considerably more parallel or convergent than the 15 degree half angle emitter.

The other difference is the size of the spot/beam cross section – The large the size of the spot, the easier it is to hit the sensor, but the sorter the range.

So, there is no right or wrong way to build a lens assembly, it all depends on what you want the final design to do – a “Shotgun” type prop I would go for a shorter range with a broad spread, whereas a pistol-type-prop I’d go for a low(ish) range and reasonably small spot.

The PPG’s for the Babylon 5 game final design has a rough range of between 30 and 40 meters (Depending on the vagaries of electronic components), which for a small pistol design, with a tiny lens (16mm diameter lens, but due to the housing it is effectiveley 14mm) and 7 degree half angle led is not bad.