Laser Spot Size Calculator
Free calculate laser beam diameter at distance, divergence angle, and focused spot size. Get instant, accurate results with our easy-to-use calculator.
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What is Laser Spot Size?
Laser spot size is the diameter of a laser beam at a given distance from the source or after focusing. It depends on the initial beam diameter, divergence angle, wavelength, and distance.
Laser beams spread out (diverge) as they travel due to diffraction. The spot size increases with distance according to the divergence angle. When focused with a lens, the minimum spot size is limited by diffraction and depends on wavelength and lens properties.
Understanding spot size is crucial in laser cutting, welding, medical applications, optical communications, and any application where beam size matters for precision and power density.
Laser Spot Size Formulas
Spot at Distance
Focused Spot
Divergence
How to Calculate
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1
For spot size at distance
Dₒ = D + 2L×tan(θ/2), where D is initial diameter, L is distance, θ is divergence.
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2
For focused spot
S = 4M²λf/(πD), where M² is beam quality, λ is wavelength, f is focal length, D is beam diameter at lens.
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3
For divergence
θ = M²λ/(πD), converting to mrad or degrees as needed.
Practical Examples
Example 1: Beam Expansion
A 5 mm diameter laser with 1 mrad divergence travels 10 m. What is the spot size?
Solution:
Dₒ = 5 mm + 2 × 10 m × tan(0.001/2)
Dₒ ≈ 5 mm + 10 mm = 15 mm
Example 2: Focused Spot
A 632.8 nm laser with 5 mm beam diameter is focused by a 100 mm lens. M² = 1. Calculate spot size.
Solution:
S = 4 × 1 × 632.8×10⁻⁹ × 0.1 / (π × 0.005)
S = 16.1 μm
Applications
Laser Cutting
Determining cutting precision, kerf width, and power density for material processing applications.
Optical Systems
Designing laser systems, optical communications, and understanding beam propagation in optical setups.
Medical Lasers
Calculating treatment area, power density, and ensuring safe and effective laser therapy parameters.
Research
Understanding beam characteristics, designing experiments, and analyzing laser performance.
Frequently Asked Questions
What is beam divergence?
Beam divergence is the angular spread of a laser beam as it propagates. Lower divergence means a more collimated (parallel) beam that spreads less with distance.
What is M² (beam quality factor)?
M² measures how close a beam is to an ideal Gaussian beam. M² = 1 is perfect, M² > 1 indicates worse beam quality. Higher M² means larger divergence and spot size.
How does wavelength affect spot size?
Shorter wavelengths allow smaller focused spots (better resolution). This is why UV lasers can achieve smaller spots than IR lasers with the same optics.
What is the diffraction limit?
The diffraction limit is the smallest possible spot size, approximately λ/(2×NA) where NA is numerical aperture. It's a fundamental physical limit.
How do I reduce spot size?
Use shorter wavelength, larger beam diameter at lens, shorter focal length, or better beam quality (lower M²). These all reduce focused spot size.