What Is the Thorlabs LC1439-B?
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It’s a plano-concave spherical lens.
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Made of N-BK7 optical glass.
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Diameter: ½ inch (12.7 mm).
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Focal length: –50.0 mm (i.e., it diverges light).
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Has an AR (anti-reflection) coating for 650–1050 nm (“B” coating).
Optical and Mechanical Specifications
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Center thickness: 3.5 mm.
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Edge thickness: ~4.3 mm.
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Radius of curvature (concave side): –25.7 mm.
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Back focal length: –52.3 mm.
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Surface quality: 40-20 scratch-dig.
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Focal length tolerance: ±1%.
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Centration: < 3 arcmin.
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Clear aperture: > 90% of diameter.
Material & Coating Notes
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Substrate: N-BK7. This is standard BK7 glass, a very common optical glass.
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Coating (“B”): Broadband AR for 650–1050 nm. Useful if you are working in red / near-IR wavelengths, since it minimizes reflections in that band.
Uses / Applications
A lens like this is typically used to diverge collimated beams (because of the negative focal length). Some common use cases:
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Beam expansion: can be part of a beam expander to increase divergence on one axis.
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Aberration compensation: negative lenses like this can be used to correct for spherical aberrations introduced by other optics.
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Optical design: for optical systems where you need to do ray shaping or add divergence.



