Laser Focusing 101

laser focusing

Introduction

In the world today, it is not enough to just own a laser engraver. You must go the extra mile to understand how best it works. As a business owner, learning the fundamentals of a laser engraver will help you maximize profit and deliver quality jobs. Hence, one of the most critical factors to look out for when using a laser engraver is the focal length.

The focal length is embedded in the concept of laser focusing. It explains how the quality of markings on materials come out after engraving. It explains how you need to adjust the engraver's lenses to ensure that the engraver and material distance is suitable for a perfect engraving. You can liken this concept to how you have to adjust a camera's focus to get a clear image. Without further ado, let's get right into it.

What is “Focusing” all about?

Firstly, focal length refers to the distance from the focal lens to the top surface of the material being processed, which is often measured in inches. On the other hand, focus refers to the smallest possible beam of a laser. And this beam has the maximum density of laser packed in it. Focus is an inherent property of focal length; it’s basically the focal length at which the best engraving quality is realized. For example, when an engraver is designated to have a focal length of 1.5 inches, this implies that when you place a material 1.5 inches from the focal lens to the top surface of your material, only at that distance to the material is the engraving sharpest. Therefore, you must pay attention to details when purchasing your laser engraver to note the lens's designated focal length.

How to Select the Right Focus Lens?

Knowing the right focus lens for you is not rocket science, and your final decision should have some factual basis. The truth is these factors are interwoven and intertwined in some ways. In view of this, presented below are some factors to consider when trying to decide:

The Thickness of Material: The rule of thumb guiding laser focusing states that the larger or thicker material, the larger the lens, spot size, and depth of focus. Consequently, it is evident that a thin material will require a small spot size and small lens. Also, the lens will only be able to engrave small and fine details on the material. For fine detail and efficient cutting, the proper focused laser spot will be between .1 - .3 mm.  This analysis is the opposite if you have thick materials.

There is an “unfocused” method of engraving applied to create larger lettering on bulky materials, for example, 3" x 6" letters on a 3' x 4' piece of plywood. You can "unfocus" the laser spot and enlarge it to around 1mm-1.5mm. Always expect some trial and error when trying out a new engraving technique.

The takeaway is that you must consider the thickness of the material you will be working with when purchasing an engraver. Knowing fully well that your laser engraver must not underperform, neither should it overperform to save cost. Failure to do this could limit your profit and capacity as a business entity in the laser engraving space.

Degree of Detail and Resolution: This factor determines how sharp the engraving comes out when it's done. Here is where the thickness of material and resolution intersects: Larger lenses have lower resolutions, and remember that for thick materials, you need a large lens. This implies that you must be able to strike a balance between these two factors. In the end, you want a lens that can work on thick materials and still give good details and a fine resolution. Generally, the 2-inch lens (focus) is considered as the most universal among the classes of lenses available- 1.5, 2,2.5, and 3-inch, respectively.  

Laser Power: The machine’s laser power is best controlled through the focal lens, making your focusing length the most important factor. To maximize the laser tube’s full wattage power, you must have the proper focal lens for your specific project and understand the basic focusing factors. While you may think that the thicker the material, the larger the lens needed, the more likely the resolution and detailing is lower, and the more power is required. But this relationship isn’t always true. The main factor of power depends on the focal length of the lens. You can have an 18mm diameter focal lens with a focusing distance of 3". You can also have a 25mm diameter focal lens with a 1.5" focusing distance. The trick is in manipulating your focal length to achieve your desired laser power relative to the size and thickness of your project materials.

Type of Material: Typically, all materials vary in their composition, and this should inform the right lens choice for you. Different materials have varying absorptivity for laser engraving via the focal length of the lens. For example, the appearance of engraving on glass and wood differs even at the same lens focus. This means that you have to pay keen attention to how each material responds to the engraver's focus.

Various lenses and their characteristics:

1.5-inch lens:

  • Finest detail and best resolution (greater than 500dpi).
  • Suitable for rubber
  • Easy to fall out of focus
  • Most suitable for thin materials and small engravings.

2-inch lens:

  • The overall best lens with the right balance of detail, resolution, and depth of focus
  • Medium resolution between 100 and 500 dpi.
  • Average laser power input requirements

2.5-inch lens:

  • Low graphics detail and resolution less than 500 dpi
  • Appropriate for heat-sensitive material (e.g., laminate).

3-inch lens:

  • Not suitable for engraving, best for laser cutting wood and acrylic.

Final Thoughts

In conclusion, we have done our best to highlight laser focusing and its relation to some factors considered above. More so, these lenses and their focal length always determine the quality of the laser beam available for engraving. In general, ensure you do your homework and define your expectations before purchasing a laser engraver. OMTech has the best CO2 and fiber laser machines with the appropriate accessories for optimal result and marking quality. Our doors are wide open to receive you for demonstration and consultancy at our Southern California showroom.