What is depth of focus or focus tolerance?

The depth of focus refers to the distance along an optical axis required by a laser beam to double its focus area - starting from the focus in the focal plane. The depth of focus and its basic physical makeup are important in laser technology, as they are used to determine the right lens for the respective application.

Effects of focus tolerance on the laser beam

For example the F-theta lens with a wavelength of 1064nm
Laser Beam Diameter: 7mm     Beam Quality: 1.3 M²

Focal length(F-theta lens) Laser Beam focus area Focus tolerance
100 0.025㎛ 0.165mm
163 0.041㎛ 0.438mm
210 0.053㎛ 0.726mm
254 0.063㎛ 1.063mm
330 0.083㎛ 1.794mm
420 0.105㎛ 2.905mm

The meaning of Focus tolerance is: within this certain value range, in theory, the engraved item cannot clearly distinguish the fluctuations of the marking lines with the naked eye. It's useful when engraving items with a slight fluctuation or a certain arc. We can achieve it within a certain range by changing a large lens.

In the same way, a small lens is more sensitive to focal length and the marking lines are more delicate. A large lens has a greater depth of focus, but the marked lines are also thicker.

Of course, focus tolerance is only a reference factor when choosing a suitable F-theta lens.
Generally speaking, the larger focal length of F-theta lens, the larger engraving format, the larger focused spot, and the smaller pulse energy density.

The attenuation of laser energy in the air is very severe. In this way, the larger focal length, the greater the laser energy loss, and the thicker and shallower the lines engraved. When it reaches a certain range, your items may not be able to be marked or there may be no depth. In this case, be sure to choose a laser with higher power to match it.
On the contrary, it fails to meet the engraving requirements.

In addition, the shorter focal length, the smaller focus tolerance, and the higher requirements for flatness of surface of engraved items. Therefore, the surfaces of wavy or cylindrically curved objects are not suitable for use with short focal length F-theta lens.

Therefore, you need to choose the most appropriate F-theta lens according to different processing areas and item shapes or have several F-theta lens with different scanning ranges in reserve.


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