Plano Convex Lenses for a CO2 Laser Cutter

In an earlier article we explained about the two main types of Zinc Selenide (ZnSe) lenses used within a laser cutting machine and that the most commonly used of these is a Plano (flat) Convex (curved outwards) lens

Aside from physical dimensions this type of lens has three defining characteristics:

1. Focal Length (F/L): the point from the lower (flat) surface to the point of beam convergence
2. Focal point: the point of beam convergence
3. Depth of focus: a range measured between a point above and below the focal point in which the lens delivers a desirable result.

The focal range is application specific and will vary depending on the material being processed and whether the application is laser cutting or laser engraving.

During manufacture, by variation of the convex surface the point of convergence can be shortened or lengthened and in turn the size of the focal point and depth of focus will vary too

As with photography, selecting a specific lens for certain applications can enhance significantly the output quality; for example, a close-up lens or a telephoto lens. The same is true for lenses within a laser cutting or laser engraving machine. 

Lenses in a laser cutter are interchangeable.

By increasing the focal length of the lens, the truncation of the beam becomes narrower and the focal point becomes larger. This is particularly useful for laser cutting through materials of a greater thickness or for laser engraving surfaces that are not uniformly flat because longer focal length lenses have a greater depth of focus.

Some manufacturers refer to long focal length lens as a 'cutting lens' and a short focal length lens as an 'engraving lens'. Technically, this is not accurate.

Longer focal length lenses are more useful when engraving high debris materials, such as wood for example, where the greater distance between the lens and the material provides for a lower risk of lens contamination.

Using a lens with a short focal length will have a very wide truncation with a very narrow depth of focus, so the uses for such a lens are very limited, however the resulting very small focal point provides for the highest possible resolution and also for maximum energy density of the laser; good for removing more material in a single pass. A good use for this type of lens would be marking photographic detail or engraving rubber stamps

The short focal length lens is commonly misunderstood as being only for laser engraving. Actually, it can also be useful for some laser cutting applications. For example, some materials are very heat sensitive and a lens with a very small focal point is less likely to draw heat away from the cutting point into the material causing adverse effects, such as material warping.

In a typical laser cutting or laser engraving machine the range of Plano Convex ZnSe lenses will be as follows:

1. 38.1mm (1.5”) F/L: high resolution engraving, <1mm material thickness cutting
2. 50.2mm (2”) F/L: good resolution engraving, <5mmmaterial thickness cutting – this is the best 'all-round' or 'general purpose' lens
3. 63.5mm (2.5”) F/L: lower resolution engraving, 3-10mm material cutting
4. 101.6mm (4”) F/L: low resolution engraving, 10-20mm material cutting

Lenses of longer focal length are available but only used for cutting and will not generally be suitable for smaller format machines.

At Lotus Laser Systems we manufacture a wide range laser, marking and engraving solutions with a variety of lens options. Our experts would be happy to advise you which configuration would suit your application best.