Laser Beam Spot Size Calculator
Calculate focused spot size or beam size at any distance
How to Use This Calculator
Select Calculation Type
Choose whether you want to calculate the focused spot size (when focusing a beam with a lens) or the beam size at a distance from the waist.
Enter Values
For focused spot: Enter wavelength, focal length, and input beam diameter. For beam at distance: Enter wavelength, beam waist, and distance from waist.
Calculate
Click the "Calculate Spot Size" button to get the beam radius in meters and micrometers.
Formulas
Focused Spot Size
w₀ ≈ (4λf)/(πD)
Where:
- w₀ = Focused spot radius (in meters)
- λ = Wavelength (in meters)
- f = Focal length (in meters)
- D = Input beam diameter (in meters)
Beam Size at Distance
w(z) = w₀√(1 + (z/z_R)²)
Where:
- w(z) = Beam radius at distance z (in meters)
- w₀ = Beam waist radius (in meters)
- z = Distance from waist (in meters)
- z_R = Rayleigh range = πw₀²/λ
Example Calculation (Focused Spot):
Focusing a 532 nm laser with 5 mm beam using 10 cm lens:
λ = 0.000000532 m, f = 0.1 m, D = 0.005 m
w₀ = (4 × 0.000000532 × 0.1) / (π × 0.005)
w₀ ≈ 1.35 × 10⁻⁵ m (13.5 μm)
About Laser Beam Spot Size Calculator
Laser beam spot size is crucial for many applications, from laser cutting and material processing to optical trapping and microscopy. This calculator helps you determine either the focused spot size when using a lens to focus a laser beam, or the beam size at any distance from the beam waist. Understanding spot size is essential for calculating power density, determining resolution, and designing optical systems.
When to Use This Calculator
- Laser Processing: Calculate spot size for cutting, welding, or marking applications
- Optical Design: Determine beam sizes in optical systems
- Microscopy: Calculate resolution and illumination spot sizes
- Research: Plan experiments requiring specific spot sizes
- Educational Purposes: Understand Gaussian beam propagation
Why Use Our Calculator?
- ✅ Two Calculations: Calculate focused spot or beam at distance
- ✅ Instant Results: Get accurate spot sizes immediately
- ✅ Multiple Units: Results in meters and micrometers
- ✅ Educational: Includes formula explanations
- ✅ 100% Free: No registration required
Common Applications
Laser Cutting: Spot size determines the cutting width and power density. Smaller spots provide finer cuts but require more precise alignment. This calculator helps optimize cutting parameters.
Optical Trapping: In optical tweezers, spot size determines trap strength and resolution. Smaller spots provide stronger traps and higher resolution for manipulating microscopic objects.
Material Processing: For welding, marking, or surface treatment, spot size affects processing speed and quality. This calculator helps select appropriate optics for desired spot sizes.
Tips for Best Results
- Use consistent units (meters for all lengths)
- For focused spots, smaller input beams and shorter focal lengths produce smaller spots
- Shorter wavelengths enable smaller focused spots
- Beam waist is typically at the 1/e² intensity point
- For distances much less than Rayleigh range, beam size stays approximately constant
- Remember that spot size is the radius; diameter is 2× the radius
Frequently Asked Questions
What is the difference between spot size and beam diameter?
Spot size typically refers to the radius (w), while beam diameter is 2w. The calculator gives the radius at the 1/e² intensity point. For the full beam diameter, multiply by 2.
Can I achieve smaller spots than calculated?
The calculated spot size is the theoretical minimum for a Gaussian beam. Real beams with M² > 1 will have larger spots. You can reduce spot size by using shorter wavelengths, shorter focal lengths, or larger input beams, but fundamental diffraction limits apply.
What is the Rayleigh range?
The Rayleigh range (z_R) is the distance over which the beam size increases by √2. It's approximately πw₀²/λ. Within the Rayleigh range, the beam stays relatively collimated; beyond it, the beam diverges significantly.
How does spot size affect power density?
Power density is power divided by area. Since area is πw², power density is inversely proportional to the square of spot size. Halving the spot size quadruples the power density, which is important for material processing applications.
Why use the 1/e² intensity point?
The 1/e² (about 13.5%) intensity point is the standard definition for Gaussian beam radius because it contains about 86.5% of the beam's power. It's a convenient and physically meaningful definition that's widely used in optics.