Laser Brightness Calculator

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Use this calculator to compare how “bright” two lasers are, either by engineering brightness (radiance), a quick diffraction-limited index (P/lambda^2), or an eye-response estimate in lumens for visible wavelengths.

Pick the meaning of “brightness” you need. Radiance is for how tightly a laser can be focused (engineering). Relative (P/λ^2, where λ is wavelength in nm) is a shortcut that assumes an ideal diffraction-limited beam. Perceived spot (lumens) is an eye-response estimate for visible wavelengths.
Average output power for a continuous beam. Example: 100 mW = 0.1 W.
Laser color in nanometers. Only visible light is about 400 to 700 nm. Wavelength is still used in radiance math even outside visible.
Full beam diameter (not radius) where it exits the laser. Used only in Radiance mode.
Full-angle divergence in milliradians (mrad). Many specs list full-angle. Used only in Radiance mode.
Advanced options
Radiance mode options
If your spec says “half-angle”, the full-angle is 2x the half-angle. Default assumes you entered full-angle.
Radiance formulas are usually stated for Gaussian beams. Flat-top beams can have different peak vs average behavior; this tool reports average radiance using the entered geometry.
Note: This tool outputs an average radiance from your geometry. It does not estimate peak intensity for Gaussian beams.
Perceived spot (lumens) options
Photopic V(λ) is for typical daylight vision and peaks near 555 nm. This is a rough estimate for comparing visible colors, not a safety limit.
The calculator can use a built-in lookup table for V(λ). Coarse tables are fine for comparisons but not lab-grade photometry.
Comparison (optional)
Turn on to enter Laser B and get an “X times” comparison.
Comparison uses the selected mode. If a Laser B field is not used in that mode, it is ignored.
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How to use our Laser Brightness Calculator

  1. Choose a mode in "What do you want to calculate?" (Radiance, Relative (P/lambda^2), or Perceived spot (lumens)).
  2. Enter Laser power (W) for Laser A (example: 100 mW = 0.1 W).
  3. Enter Wavelength (nm) for Laser A (example: 532 nm green).
  4. If you chose Radiance mode, enter Beam diameter at aperture (mm) and Beam divergence, full-angle (mrad) for Laser A.
  5. Open "Advanced options" if you need to change whether divergence is half-angle, or to turn on "Compare two lasers".
  6. If comparing, enter the Laser B inputs that match your chosen mode (power and wavelength always; diameter and divergence only for Radiance mode).
  7. Click Calculate.
  8. Read "Assumptions and warnings" to make sure you did not mix definitions (radiance vs index vs lumens) and to see any N/A results (like lumens outside the visible range).

Definitions

Radiance (laser brightness): Engineering brightness, meaning power per beam area per solid angle. Units are watts per square meter per steradian (W/(m^2 sr)).

Beam diameter at aperture: The full width of the beam as it leaves the laser. This calculator converts diameter to radius before using it.

Divergence: How quickly the beam spreads with distance. Specs may be full-angle or half-angle; this matters because radiance uses the half-angle.

Steradian (sr): A unit for solid angle (a 3D angle). A narrower beam has a smaller solid angle.

Relative brightness index (P/lambda^2): A shortcut index that scales with how bright an ideal diffraction-limited beam could be. It is mainly for comparisons, not a measured radiance.

Perceived brightness (lumens): An estimate of how bright the laser power looks to human day vision using V(lambda) eye sensitivity and the 683 lm/W scaling at peak sensitivity. V(lambda) is tied to standard photometry tables. [1][2]

Methodology

Inputs and unit conversions

lambda_m = wavelength_nm * 1e-9

w0_m = (beam_diameter_mm * 1e-3) / 2

theta_rad = divergence_mrad * 1e-3

if divergence_is_half_angle == "no" then theta_half = theta_rad / 2 else theta_half = theta_rad

Mode 1: Radiance (engineering brightness)

Radiance is computed as power divided by beam area times beam solid angle, using a small-angle approximation for a symmetric beam.

L = power_w / ( (pi * w0_m * w0_m) * (pi * theta_half * theta_half) )

Interpretation: Higher L usually means the beam can be focused to a smaller, more intense spot if other conditions are comparable. This is a geometry-based estimate, not a full beam-quality (M^2) measurement.

Mode 2: Relative brightness shortcut (diffraction-limited index)

This mode reports a comparison index used in many quick rankings. It assumes an ideal diffraction-limited Gaussian beam, so it is best used for comparing lasers rather than reporting a physical radiance.

Index = power_w / (lambda_m * lambda_m)

Mode 3: Perceived spot brightness (lumens estimate for visible lasers)

This mode estimates luminous flux for a monochromatic laser line using the photopic luminous efficiency function V(lambda) (day vision). V(lambda) comes from standard photometry references. [2]

lumens = power_w * 683 * V_lambda

If wavelength is outside about 380 to 780 nm, the calculator shows lumens as N/A because the photopic V(lambda) model is for visible light. [2]

Laser A vs Laser B comparison

If comparison is enabled, the calculator computes the ratio using the selected mode output (radiance, index, or lumens when available).

ratio = value_A / value_B

If value_B is 0 or cannot be computed for the selected mode, the ratio is shown as N/A to avoid divide-by-zero or misleading results.

Validation and edge cases handled

Blocked errors: power must be greater than 0; wavelength must be present and greater than 0; radiance mode also requires beam diameter and divergence greater than 0; comparison mode requires Laser B inputs needed for the active mode.

Non-blocking notes: the tool uses average geometry-based radiance and a simple V(lambda) scaling for perceived brightness. It does not compute eye safety limits, scattering, spot size on a surface, or pulsed-laser peak effects.

Sources

  1. Colour matching functions - AC
  2. Photometry - GSU