by BobH » Tue Dec 24, 2019 9:55 am
That is an interference filter, not a spatial filter. It's used with broadband light to narrow the wavelength range that is transmitted, and transmits a range that is centered on a laser wavelength.
A spatial filter uses a lens to focus laser light to a point that is centered on a pinhole made in a steel disk. Light that was scattered by things like dust particles on lenses or mirrors upstream gets focused to different, off-axis places by the lens in the spatial filter assembly. The pinhole only passes the main beam, not the scattered light, and thereby gives the resulting (now expanding) beam a smooth uniform appearance.
That is an interference filter, not a spatial filter. It's used with broadband light to narrow the wavelength range that is transmitted, and transmits a range that is centered on a laser wavelength.
A spatial filter uses a lens to focus laser light to a point that is centered on a pinhole made in a steel disk. Light that was scattered by things like dust particles on lenses or mirrors upstream gets focused to different, off-axis places by the lens in the spatial filter assembly. The pinhole only passes the main beam, not the scattered light, and thereby gives the resulting (now expanding) beam a smooth uniform appearance.