What is a collimator and what does it do?

A collimator is an optical lens or lens system that converts a divergent light beam into a parallel beam. Essential with laser diodes: the bare diode emits a strongly fanned-out beam; the collimator turns it into a bundled, parallel beam. Construction: usually an aspheric glass lens in an M9 × 0.5 or M12 threaded housing. High-quality collimators feature an AR coating (anti-reflective) for minimal light loss at the interfaces. Key parameters: focal length (typ. 3–6 mm), aperture (typ. 4–8 mm) and the AR coating's spectral range (e.g. 420–680 nm for visible light). Glass collimators are more temperature-stable than cheaper acrylic lenses.