Choosing a Lens

Choosing a lens for a "laser tag" application requires awareness of several considerations. For my first tagger, I used a standard asymmetric double-convex lens sold on the Miles Tag website, with diameter of 48 mm and focal length of 108 mm, and didn't have to consider this.

Shape

The desired effect is to take a small light source (the IR LED) and to collimate the light into a cylindrical beam. Ideally, the beam is just a slight bit conical, with the narrower end at the lens. Because the light source is neither a laser nor a point source, a lens that is shaped for perfect collimation of a point source should give the desired slightly conical pattern.

How wide should this conical pattern be? If it's too narrow, then a shot could fall between sensors without registering on either. This would cause the shot to miss, even though apparently properly aimed. So, at range, the pattern should be more than 30 cm wide, and probably two or three times that wide. If the pattern is too wide, the beam intensity will diminish quickly with range, since the energy of the beam will be spread over a larger and larger area. This will cause shots beyond a certain range to fail to register on the sensors, and the tagger will have a short range. For some taggers, this may be a desired effect, and a lens can be chosen that gives the desired range and beam spread tradeoff.

Plano-convex lens Asymmetric double-convex lens

The simple lens shape that does this with the least spherical aberration is plano-convex, and a double-convex lens does nearly as well; there should be little or no detectable difference between the two in this application. A positive meniscus lens should also be similarly effective, but may be hard to find with an appropriate diameter and focal length. With a plano-convex lens, the flat side should be placed towards the emitter; with an asymmetric double-convex, the flatter side should be placed towards the emitter.

Compound lenses will work well in this application. Since we are concerned with a nearly monochromatic light source, the advantage of a compound lens (reduced chromatic aberration) is not important. Compound lenses are generally more expensive. If you find a cheap surplus compound lens, it may be worthwhile; otherwise, don't bother with the extra expense.

Concave lenses (plano-concave, double concave, negative meniscus) cause the light to diverge into a wider cone, rather than to converge. They are not the correct lens shape for this application. They may conceivably be useful as part of a compound lens assembly.

Spherical lenses do not focus as accurately when the angle of incidence is high, which is most pronounced towards the edges of the lens. Aspheric lenses can be shaped to compensate. The effect is negligible for this application. It may be hard to find an aspheric lens of the appropriate diameter and focal length, and spherical lenses will often be cheaper for the same optical quality.

For more information on lens shape, consult a good reference. A basic explanation is given by the Molecular Expressions website.

Diameter and Focal Length

The diameter and focal length of the "perfect" lens depends on the shape of the light emitted by the IR LED. The standard IR LED recommended is the Vishay TSAL-6100, which has a conical emission pattern that reaches half intensity at an angle of 10° off the axis. Or, to put it a different way, on a plane through the axis of the cone, the portion of the beam that is of at least half the maximum intensity of the beam subtends an angle of 20°. The Vishay TSAL-6100 has a fairly quick drop-off from full to zero intensity, and using the half-intensity contour is a pretty good approximation for the usable portion of the LED output.

Where:
F = focal length
D = lens diameter
θ = half angle of usable portion of LED output

The optimal lens will make use of as much of the IR emission as possible. If we take the half-intensity emission angle as the usable shape of the IR beam, these conditions dictate a relationship between the focal length and lens diameter for a specific IR LED. The minimum lens diameter D is given by D ≥ 2 F tan(θ), where F is the focal length and θ is the half-intensity angle from the axis. For example, for the Vishay TSAL-6100's 10° half intensity angle, the lens diameter should be no less than 0.35 times the focal length, or the focal length should be no more than 2.8 times the diameter.

Long focal length gives smaller parallax error for a given width of light source.

The IR LED is not a point source of light. The lens will be trying to collimate this light, and the parallax error is larger when the light source is farther away from the lens' central axis. A larger parallax error makes the beam "spread", giving a shorter and wider effective volume for the beam. The amount of error depends on the angle from the lens' central axis. Thus, a longer focal length, which reduces the subtended angle of the IR LED, produces a more collimated beam. Therefore, the optimal long range lens will have as long a focal length as possible.

These two considerations give a simple list of maximum usable focal length for a selected diameter:

θ = 10° (e.g., Vishay TSAL-6100)
selected D 15 mm 17.5 mm 20 mm 25 mm 30 mm 40 mm 50 mm 60 mm
optimum F 42 mm 49 mm 56 mm 70 mm 84 mm 112 mm 140 mm 168 mm

In addition, the lens must fit into the tagger. This generally requires the lens is small enough for the available space, and the focal length is short enough for the available space. For a firearm-shaped casing, the diameter is a more serious constraint than the focal length. The mounting method will, most likely, cover some portion of the lens, and should be taken into account when designing the casing.

Surface Finish

The surface of the lens can be polished in various ways. Nearly all lenses sold for optical purposes will be good enough for laser tag. Pitch polished lenses (where the lens is embedded in pitch as a sort of temporary glue, and rubbed against progressively finer abrasives) are most common. Fire polished lenses (where the lens is heated until the surface liquefies briefly, forming a smooth surface) are generally cheaper, but can have a less precise surface. Casting, usually used with plastic lenses, is another process that works well. The surface quality is often expressed in "scratch/dig numbers", where the first number is the allowed size of any scratches in μm, and the second number is the allowed size of any digs in 10 μm units. Optical lenses will usually have scratch/dig numbers under 100/50, while laser quality lenses will be closer to 10/5.

An uncoated lens will transmit most of the infrared light we are using. Some lens coatings increase light transmission, and will be marginally helpful. A few lens coatings block infrared, and will make the lens useless. If your lens is coated and you don't know which kind you have, it's worth checking. Using the same IR-sensitive camera that you'll be using to focus the device, check to see if the IR from your LED is visible through the lens.

Lens Material

Glass is the most common lens material. It's fairly hard to scratch, transmits well in the near infrared, somewhat heavy for its size and power, and brittle. There are a number of different types of glass, most of which work well enough for collimating a monochromatic infrared source. Glass lenses need to be protected in a sturdy mounting.

Plastic is another common lens material. It's easier to scratch (in most forms), transmits well in the near infrared, is light for its size and power (usually), and hard to break. If you're using a plastic lens, you'll want to take some care when cleaning it to avoid scratches, and will want to put it in a protective mounting.

More exotic materials (sapphire, calcium fluoride, etc.) tend to be more expensive, and don't give better results in this application. Perhaps an IR-transmissive visually opaque lens might be interesting for a design.

Vendors

Surplus Shed (huge variety, low cost), Edmund Optics (large variety), Rolyn Optics (large variety), JP Mfg, Inc. (plastic lenses), Knight Optical, Newport, Ealing Catalog, Laser Surplus Sales, Thor Labs, OptoSigma, Anchor Optics, sp3 plus, ESCO products, Lens-Optics, Basic Science Supplies (38 mm dia only), Optotronics, MK Photonics, G-S Plastic Optics, China Optics