Compute a laser's brightness (radiance) from its power and wavelength, run a relative laser brightness comparison between two lasers (dot brightness + Rayleigh beam brightness), and evaluate M² beam quality. Formulas match the Omni laser brightness calculator, with extra modes it doesn't have: waist/divergence radiance and the M² brightness definition B = P/(M²λ)².
This laser radiance calculator works for any laser — HeNe, red diode, green DPSS, argon-ion, Nd:YAG, Yb fiber laser, or CO₂. Switch to the compare tab to use it as a relative laser brightness calculator: enter two lasers' power and wavelength (400–700 nm) and get each laser's dot brightness in lumens plus the beam-brightness ratio.
⚠ Laser safety: never look directly into a laser beam or its specular reflection — not even a 5 mW pointer. Judge brightness with instruments or a spot on a diffuse surface, never with your eyes.
Laser radiance (brightness)
For a Gaussian beam, L = P / (w₀²π²θ²) with θ = λ/πw₀ simplifies to L = P/λ². Enter power and wavelength.
FormulaL = P / λ²
Power in watts
Wavelength in meters
Radiance (unscaled)
Relative laser brightness — compare two lasers
Combines dot brightness (P × photopic conversion, in lumens) with beam brightness from Rayleigh scattering (× λ⁴ ratio). Wavelengths must be in the visible range 400–700 nm.
Laser A dot brightness
Laser B dot brightness
Photopic conversion A / B
Dot brightness ratio (A/B)
Beam brightness ratio (A/B)
Radiance from beam waist & divergence (finds M²)
Uses the unsimplified form L = P/(π²w₀²θ²) and reports the beam quality factor M² = π·w₀·θ/λ. For a perfect Gaussian beam (θ = λ/πw₀) this returns M² = 1 and exactly L = P/λ².
Radiance L = P/(π²w₀²θ²)
Beam quality M² = πw₀θ/λ
Brightness — literature definition B = P/(M²λ)²
The brightness definition used in laser physics literature and datasheets: brightness falls with the square of the beam quality factor. For M² = 1 it equals the radiance L = P/λ².
FormulaB = P / (M²·λ)²
Equivalent radiance L = P/λ²
Common lasers — typical brightness values
Typical published values (not measured). Click a row to load it into the Radiance tab.
Laser
λ (nm)
Typical power
Typical M²
Radiance
Formulas & definitions used
There are several definitions of laser brightness; this tool follows the radiance convention of the Omni laser brightness calculator, and documents exactly which formula each mode uses:
L = P / (w₀² · π² · θ²) → with θ = λ/(π·w₀) → L = P / λ²
Radiance (this tool's brightness). P is laser power in watts and λ is wavelength in meters. It does not depend on distance or solid angle — it characterizes the beam in its entirety.
dot brightness: D = P · ph(λ) [lm]
Dot brightness. Power times the photopic conversion value ph(λ) — the CIE photopic luminous efficiency of the human eye at that wavelength (≈1 at 555 nm). This is what the beam spot looks like.
Beam brightness. Adds Rayleigh scattering: shorter wavelengths scatter ~1/λ⁴ more in air, so blue/green beams look brighter in the room than red ones at equal power.
M² = π · w₀ · θ / λ | B = P / (M² · λ)²
Beam quality. M² = 1 is diffraction-limited; real beams are worse. The literature brightness B makes the penalty explicit: doubling M² quarters brightness.
Photopic conversion: published table values are used for 500–555 nm; standard CIE V(λ) data with linear interpolation covers 380–780 nm. Results are physical estimates for education and comparison, not laser safety ratings.
Scope check — what this tool is (and isn't):
this tool computes the physical brightness / radiance of laser beams (Gaussian-beam radiance, relative dot & beam brightness, M² beam quality).
It is not color-space brightness (see hex to hue/saturation/brightness),
not spectral linewidth (see laser linewidth),
and not projector screen brightness (see projector throw lab).
Frequently asked questions
What is laser brightness?
Laser brightness, also called laser radiance, measures how much power a laser beam carries per unit area per unit solid angle. It tells you how tightly the beam's energy is concentrated, which is why a low-power laser can still outshine a much stronger ordinary light source.
What is the laser brightness formula?
For a Gaussian beam, radiance is L = P / (w0^2 * pi^2 * theta^2), where P is power, w0 the beam waist radius and theta the divergence angle. Using the Gaussian relation theta = lambda / (pi * w0), this simplifies to L = P / lambda^2, with wavelength lambda in meters. This calculator uses that simplified form, matching the convention of the Omni laser brightness calculator.
What units is laser brightness measured in?
With power in watts and wavelength in meters, L = P / lambda^2 gives watts per square meter (W/m^2), commonly labeled W/(m^2.sr) as radiance per unit solid angle. Typical values are huge, so results are also shown in GW/m^2, TW/m^2 or PW/m^2.
How do I calculate laser radiance?
1. Find the laser's power in watts. 2. Write the wavelength in nanometers and divide by 1,000,000,000 to get meters (common values: 473 nm blue, 532 nm green, 650 nm red). 3. Square the wavelength in meters and divide the power by it. The result is the laser's radiance (brightness).
What is relative laser brightness?
Relative laser brightness compares two lasers and tells you which one looks brighter. Two effects are combined: dot brightness, the luminous flux of the beam spot, calculated as power times the photopic conversion value of its wavelength; and beam brightness, the visibility of the beam in air, which adds a Rayleigh scattering factor of (lambda2^4 / lambda1^4).
Why does a green laser look brighter than a red laser of the same power?
Your eyes are far more sensitive to green (~555 nm in daylight) than to red. The photopic luminous efficiency function peaks near 555 nm and falls steeply toward the red end. A 5 mW green laser at 532 nm has about 8 times the dot brightness of a 5 mW red laser at 650 nm, even though both emit the same power.
What is the brightest laser pointer color?
Green, around 532 nm, looks brightest under most conditions. In daylight the eye peaks near 555 nm; at night (scotopic vision) the peak shifts toward 507 nm. At 532 nm you get high sensitivity in both regimes, which is why green laser pointers look dramatically brighter than red ones at equal power.
What is Rayleigh scattering and how does it affect beam brightness?
Rayleigh scattering is the scattering of light by air molecules, and its intensity is inversely proportional to the fourth power of the wavelength (I proportional to 1/lambda^4). Shorter-wavelength beams scatter more, so a blue or green beam is more visible in air than a red beam of the same power. The beam-brightness ratio between two lasers multiplies the dot-brightness ratio by (lambda2/lambda1)^4.
What is M squared (M2) and how does it affect brightness?
M^2 is the beam quality factor: the ratio of a real beam's beam-parameter product to that of an ideal Gaussian beam at the same wavelength. It is computed as M^2 = pi * w0 * theta / lambda. M^2 = 1 is diffraction-limited (the brightest possible); higher M^2 means a fatter, dimmer beam. The literature brightness definition B = P / (M^2 * lambda)^2 makes this explicit: doubling M^2 cuts brightness by a factor of four.
What is the photopic conversion value?
The photopic conversion value ph(lambda) is the relative response of the human eye (CIE photopic luminous efficiency function) at a given wavelength, normalized near 1.0 at 555 nm. Multiplying a laser's power in watts by ph(lambda) gives its dot brightness in lumens. This tool uses published conversion values for 500-555 nm and standard CIE V(lambda) data with linear interpolation elsewhere.
Does laser brightness depend on distance from the source?
No. The radiance definition L = P / lambda^2 characterizes the beam as a whole and does not depend on distance or solid angle. What does change with distance is irradiance (power per unit area on a target), because the beam diverges and spreads its power over a larger spot.
How bright is a 5 mW HeNe laser?
A 5 mW helium-neon laser at 632.8 nm has a radiance of 0.005 / (632.8e-9)^2, about 1.25e10 W/m^2 (12.5 GW/m^2). Its dot brightness is 0.005 W times the photopic value at 632.8 nm (about 0.24), roughly 1.2 millilumens - small in lumens, but enormously concentrated compared to ordinary sources.
Is laser brightness the same as screen or color brightness?
No. This tool computes the physical brightness (radiance) of laser beams. It does not measure display brightness (nits), projector lumens, or color-space brightness like the HSB value in hex-to-hue-saturation-brightness, and it does not measure spectral linewidth like the laser-linewidth tool.
Is it safe to look directly at a laser beam to judge its brightness?
No - never look directly into a laser beam or its specular reflection, even a low-power pointer. Eye damage can occur before any blink reflex. Judge brightness only with instruments, cameras, or by observing the beam spot on a diffuse surface from a safe angle.
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