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F-Theta Lenses
Frequently Asked Questions
An F-Theta Lens is a large aperture compound lens built in such a way that the beam deflection distance from the axis on the imaging plane is directly proportional to the focal length (F) and the incidence angle (theta). This is unlike the flat-field scanning lens where that distance would be proportional to the focal length * tan (theta) and unlike the spherical lens that generates curved projection plane.
F-Theta lenses allow steering and focusing the laser beam without material deterioration of the focused beam spot. This effectively eliminates the need of complicated compensating optics, electronics and software that would be required with a spherical or a flat-field scanning lens. As a result it enables high-resolution scanning for various laser marking, laser engraving and laser projection applications.
No, as covered in the previous questions, scanheads utilize f-theta lenses to ensure distortion free projection of the focused laser beam across the entire work field on the focal plane. Even if you find a simple spherical lens of the matching diameter, the laser beam will become materially distorted as the scan angles increase from the coaxial direction.
Chances are if you are in the market for an f-theta lens, you are replacing an existing one. If that’s the case your search parameters will be narrowly defined by your equipment, including wavelength range, focal length, diameter, aperture, acceptable scan angles and damage threshold. The same set of key parameters will also be important considerations if the purchase is not for a replacement.
F-Theta lens is a compound lens that combines several meniscus lenses to create a predetermined barrel distortion. Parallel laser rays travelling through the series of these lenses always intersect on the focal plane independent of the incidence angle. This allows moving (scanning) the focal spot across the focal plane by deflecting the laser beam’s incidence angle.
The damage threshold measures the maximum energy density that the lens can withstand before being damaged. Most of the lenses come with antireflective (AR) coating and this also factors in to the damage threshold. This parameter becomes especially important when working with laser engravers, laser cutters and laser welders that employ high energy laser pulses.
There are a number of reasons why your F-Theta lens keeps getting damaged (burned) by the laser. The first thing to ensure is that the damage threshold specified for that lens is not lower than the maximum energy densities you are exposing the lens to. Assuming you are operating within the range, a few other culprits can be present. Dirt, debris on the lens such as dust, oil, even fingerprints, can cause significant absorption of the laser radiation that causes thermal damage. Another culprit could be a “hot-spot” in your laser beam. If your laser beam profile deteriorates and stops being smooth, it can have high intensity spots (hot spots) across the cross section of the beam. While the overall energy density can be below the damage threshold, the intensity at the hotspot might increase the damage threshold. This is a common cause of lens damage for many laser applications. If in doubt, you should check with your laser service provider to ensure that the beam quality is not an issue. There exists beam profiling equipment that they can use to test that. Another less common cause could be a back reflection to the mirror from the workpiece that somehow focuses on one spot causing a thermal image. Finally you should ensure that if your lens has an AR coating it is designed for the wavelength range and power of your laser.
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