Collimated light consists of rays that travel nearly parallel to one another, so the beam diameter changes only gradually with distance. It is used in optical measurement, illumination, imaging, alignment, and sensor systems.<\/p>\n
This article explains the basic principle, practical optical layouts, adjustment methods, and important design trade-offs.<\/p>\n
Perfectly parallel light is an idealization. Real beams always have some divergence because of diffraction, source size, lens aberrations, and alignment errors. In practice, a beam is considered collimated when its divergence is sufficiently small for the intended working distance and accuracy.<\/p>\n
Collimation is therefore a specification rather than a simple on\/off condition. The acceptable angular spread and beam-diameter change should be defined from the application.<\/p>\n
A point source placed at the focal point of a lens produces an approximately collimated output beam. Conversely, parallel incoming rays are focused near the rear focal plane. The focal length determines the relationship between source size and output divergence.<\/p>\n
If the source is displaced along the optical axis, the outgoing beam converges or diverges. Lateral displacement causes beam tilt. Accurate positioning of the source and lens is therefore essential.<\/p>\n
A laser diode emits a strongly divergent and often elliptical beam. A small aspheric lens is commonly positioned close to the emitting facet. Because divergence differs in the fast and slow axes, a circular beam may require cylindrical optics or an anamorphic prism pair.<\/p>\n
Lens numerical aperture must be large enough to collect the emitted light. Working distance, focal length, coating wavelength, and allowable aberration should be selected together.<\/p>\n
An LED has a larger emitting area than a laser diode, so rays from different points cannot all become perfectly parallel with a single lens. The achievable divergence is limited by source size and focal length.<\/p>\n
A larger focal length reduces angular spread but increases system size, while a larger numerical aperture collects more light. The required uniformity, efficiency, and allowable divergence must be balanced.<\/p>\n
A parabolic mirror produces collimated light when the source is placed at its focus. Mirrors introduce no chromatic aberration and are useful for broadband or multiple-wavelength systems.<\/p>\n
Off-axis parabolic mirrors separate the incoming and outgoing paths, but they require careful three-dimensional alignment and can introduce coma when used away from the intended geometry.<\/p>\n
A beam expander combines two optical elements to increase beam diameter and reduce divergence. Galilean designs are compact and have no internal focus, while Keplerian designs can include a spatial filter but create a real intermediate focus.<\/p>\n
The magnification, aperture, wavelength, wavefront quality, and damage threshold should be checked for the actual laser.<\/p>\n
First align the source and optical element mechanically to a common axis. Then observe the beam diameter or wavefront at two or more distances. Adjust the source-to-lens spacing until the measured diameter changes as little as possible.<\/p>\n
A shear plate, wavefront sensor, autocollimator arrangement, or distant target can improve sensitivity. Adjustment should be performed after the source and mount reach thermal stability.<\/p>\n
Simple evaluation compares beam diameters at separated positions and calculates the approximate divergence angle. For Gaussian laser beams, the beam waist and Rayleigh range provide a more accurate description.<\/p>\n
When wavefront quality matters, interferometry or a wavefront sensor can measure residual curvature and aberration. The evaluation method should match the required angular accuracy and aperture.<\/p>\n
Residual convergence or divergence usually indicates incorrect axial spacing. Beam tilt indicates lateral decenter or angular misalignment. Nonuniformity may come from the source distribution, lens aberration, clipping, contamination, or coating mismatch.<\/p>\n
Thermal drift and mechanical play can change collimation over time. Stable mounts, appropriate materials, controlled temperature, clean optics, and repeatable adjustment mechanisms improve long-term performance.<\/p>\n
For lasers, choose a lens that captures the source numerical aperture and provides the required beam diameter and divergence. For LEDs, define acceptable divergence before optimizing collection efficiency. For broadband systems, consider an achromatic lens or parabolic mirror.<\/p>\n
If illumination uniformity is more important than the smallest divergence, a light pipe, diffuser, or lens array may be added after collimation. Every addition introduces loss, size, and alignment trade-offs.<\/p>\n
Collimated light is created by placing a source near the focal point of a lens or mirror and adjusting the spacing until residual convergence and divergence meet the application requirement. Practical performance is limited by diffraction, source size, aberrations, aperture, and alignment.<\/p>\n
Define the required beam diameter, divergence, wavelength range, uniformity, and working distance first, then select the lens, mirror, or beam-expander configuration and verify it under actual operating conditions.<\/p>","protected":false},"excerpt":{"rendered":"
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