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F-Theta Objective

F-Theta objectives – also called scan objectives or flat field objectives – are lens systems often used in scan applications. Located in the beam path after the scan head, they perform various functions.

For instance, the objective focuses the laser beam at the focal point. When scanning, it also ensures that this focal point is always positioned in the working plane, perpendicular to the objective's optical axis. Additionally, the position in the working plane approximates the F-Theta condition, meaning the scan length (image height) is approximately proportional to the set scan angle. Deviations from this proportionality can be compensated by controlling the scan mirrors.

Key quality indicators of good F-Theta objectives are low field-curvature, a large image field and good, homogenous focus quality throughout the entire image field. Here, a crucial factor is whether the focus is diffraction limited. Additional requirements for F-Theta objectives can apply, depending on the laser's parameters. Examples here include avoidance of back reflections from the objective's lens surfaces, and/or thermal-lens effects.

Focal Diameter (Focal Spot)

The spot diameter is defined as the diameter in the beam waist that contains 86.5% of the total power and corresponds to the 1/e² beam diameter in the working plane for Gaussian beams.

Unit: µm

Calculation of Focal Diameter  (Spot Diameter)

The focal diameter, which the below-mentioned equation approximates, depends on the light source (laser wavelength and quality of the coupled laser beam) as well as the scan system’s aperture and focal length. The approximation only yields useful results for gaussian beams.

d = M2  · k(A, D) · λ · f / D

d = Focal diameter (1/e2)
λ = Wavelength (typically 193 nm – 10.6 μm)
f = Focal length (typically 30 mm – 2,000 mm)
M2 = Beam quality (laser-dependent)
k = Correction factor, depends on A and D (ideally 1.27; more typically between 1.5 and 2.0)
A = Aperture of the scan system
D = Beam diameter prior to focusing (typically 6 – 70 mm)

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see Angle Terms

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