Selecting the right lens can be the key to achieving perfect results with your CO2 laser system. The parameters are set, the focus is adjusted, but somehow the engraving is not precise. Has this ever happened to you? This error is most likely a lens issue. The correct lens is just as essential as the parameters and optimal focus adjustment. To select the proper lens, you must consider the output of the laser device, the material type and thickness that you are processing, and the detail level and resolution (dpi) of the graphics.

Selecting the Right Lens for Your Laser Application

By David Stevens, TROTEC LASER INC

Selecting the right lens can be the key to achieving perfect results with your CO2 laser system. The parameters are set, the focus is adjusted, but somehow the engraving is not precise. Has this ever happened to you? This error is most likely a lens issue. The correct lens is just as essential as the parameters and optimal focus adjustment. To select the proper lens, you must consider the output of the laser device, the material type and thickness that you are processing, and the detail level and resolution (dpi) of the graphics.

As a rule of thumb, when marking or engraving more delicate and detailed graphics, a smaller lens should be used. A 1.5-in. lens is ideal for graphics with a resolution of 500 dpi or higher and for rubber stamps with thin letters, in addition to other graphics with small or intricate details. A 2.0-in. lens will work well for most standard engravings or graphics with medium detail or resolution, while a 2.5-in. lens may be used for heatsensitive materials that require a large surface to be engraved away. The extra distance between the laser head and the material will prevent the material from overheating or warping.

For cutting, ideally a 2.0-in. lens will work with thicknesses up to 3/8 to 1/2 of an inch, but anything thicker may require a 2.5- or 4.0-in. lens. Keep in mind that as the focal distance increases, the amount of energy the laser provides will seem to decrease—so laser power will come into play here, as well. You can also use larger lenses for low-detail engravings with higher laser power.

Not all laser systems offer every lens, so make sure you do your research and talk to your local laser expert so they can make a recommendation based on your material and application.

Glossary of Terms

Beam diameter

The diameter of the laser beam, which becomes greater and greater before and behind the focus. The longer the focal distance of the lens, the greater the beam diameter. The energy of the laser is focused by the lens onto a defined focal point. The greater the focal distance of the lens, the greater also the surface onto which the energy of the laser is applied. This may mean that if you use a large lens, the material is heated rather than cut.

Focus tolerance (depth of focus)

The area in which the beam has the smallest diameter.

The larger the lens (= the larger the focal length of the lens), the longer the focal tolerance. This means that the focus tolerance of a 5-in. lens will be twice that of a 2.5-in. lens. This is especially important if you want to cut through thick materials.

Focal length

The distance between the lens and the point of the smallest beam diameter.

Focus

The area where the laser beam is optimally focused and converges to the smallest possible beam diameter.

David Stevens has been in the laser industry for more than 20 years and is renowned in the industry for his knowledge, teaching abilities, and helping customers improve their laser processing techniques. He joined Trotec Laser Inc in 2017 as the Industrial Applications Manager and heads the West Coast Training, Applications, & Inspiration Lab.