Laser Brightness Calculator that finds radiance from power and wavelength, compares how bright two lasers look, and gives the irradiance, pulse peak power and a laser safety class.
Diffraction-limited radiance needs only power and wavelength.
Viewing Conditions
Eye Sensitivity Curve
At 532 nm the CIE 2008 daylight curve gives 87.83% of peak eye sensitivity. Filled: CIE 2008 2° V(λ). Dashed: CIE 1924 V(λ), used for SI lumens. 1 nm data.
Common Laser Colors Compared
| Laser | V 2008 | Dot (lm) | Beam glow |
|---|---|---|---|
| 405 nm Violet diode | 0.00497 | 0.00219 | 0.0175× |
| 450 nm Blue diode | 0.0647 | 0.130 | 0.150× |
| 473 nm Blue DPSS | 0.144 | 0.353 | 0.273× |
| 520 nm Green diode | 0.718 | 2.42 | 0.932× |
| 532 nm Green DPSS | 0.878 | 3.02 | 1.04× |
| 589 nm Yellow DPSS | 0.822 | 2.63 | 0.648× |
| 635 nm Red diode | 0.242 | 0.741 | 0.141× |
| 650 nm Red diode | 0.119 | 0.365 | 0.0635× |
Each row uses your power. V column and beam glow use the CIE 2008 daylight curve; lumens use the SI CIE 1924 curve. Glow is side-view brightness against an equal-power 555 nm beam.
Step-by-Step Solution
- For a Gaussian beam, L = P ÷ (π·w₀² × π·θ²) and θ = M²·λ ÷ (π·w₀), so L = P ÷ (M⁴ × λ²)
- L = 0.00500000 W ÷ (1.00000 × (532.000 × 10⁻⁹ m)²)
- L = 17.67 GW/(m²·sr)
How the Laser Brightness Calculator Works
“Brightness” means different things for a laser. A physicist means radiance, the power packed into an area and a cone of directions. Someone buying a pointer means how bright the dot and beam look. The Laser Brightness Calculator covers both, plus the power density used for burns, cutting and safety.
Pick a mode in the Laser Brightness Calculator’s Solve For menu. The wavelength field has presets from a 405 nm violet diode to a 1064 nm Nd:YAG laser, and every mode reports the safety class and how the color suits the human eye. An Eye Adaptation setting switches between daylight and dark-adapted night vision.
The Five Modes
“Radiance from P and λ” gives the brightness of an ideal Gaussian beam from just power, wavelength and M². “Radiance, measured beam” uses a real waist diameter and divergence instead. “Compare two lasers” shows which of two pointers will look brighter, both as a dot and as a beam in the air.
“Irradiance (W/cm²)” spreads the power over the beam area, the number that matters for burning and cutting. “Pulsed peak power” turns pulse energy, pulse length and repetition rate into peak power and fluence for pulsed lasers.
Laser Brightness Formula
Radiance L is power per unit area per unit solid angle, in W/(m²·sr). It is the one brightness measure that lenses cannot raise. A lens can squeeze the spot or narrow the cone, but never both at once.
$$L = \frac{P}{A \cdot \Omega}$$
For a Gaussian beam, the area is the waist, A = πw₀², and the solid angle is Ω = πθ², with θ the far-field half-angle. Since θ = M²λ ÷ (πw₀), the beam size cancels out. Radiance then depends only on power, wavelength and beam quality.
$$L = \frac{P}{\pi^2 w_0^2 \theta^2} = \frac{P}{M^4 \lambda^2}$$
The default 5 mW green laser at 532 nm with M² = 1 gives L = 0.005 ÷ (532 × 10⁻⁹)² = 17.67 GW/(m²·sr). The Laser Brightness Calculator also gives 1.767 MW/(cm²·sr) in the hero line, and the Step-by-Step Solution shows the same working.
Beam quality matters hugely here because it enters as M⁴. A beam with M² = 2 has 16 times less radiance than a perfect beam of the same power. Card 2 shows that loss, and it explains why a cheap multimode laser looks far less “intense” than its wattage suggests.
Brighter Than the Sun
Card 2 also sets the result against the Sun. The Sun’s average radiance is about 2.0 × 10⁷ W/(m²·sr), from the 1,361 W/m² solar constant spread over the Sun’s 0.2666° angular radius. The 5 mW green laser comes out 883 times brighter.
The Blackbody Radiation Calculator gives the same kind of figure for any hot surface. A laser beats the Sun not on power but on how tightly that power is aimed.
Étendue and the Measured Beam
The étendue A·Ω on card 2 is the beam’s size times its spread. For an ideal beam it equals λ², 0.2830 µm²·sr at 532 nm, the smallest any light source can reach. Optics can move étendue around but never shrink it.
In the measured-beam mode, enter a 1 mm waist and 1.2 mrad full divergence for the same 5 mW green laser. Radiance drops to 5.63 GW/(m²·sr), 281 times the Sun, and the tool infers M² ≈ 1.77 from the numbers. The Laser Beam Divergence Calculator helps measure those two inputs.
How Bright a Laser Looks
Radiance says nothing about color. The eye is most sensitive near 555 nm in daylight, so a green laser looks far brighter than a red or violet one of equal power. The Laser Brightness Calculator weighs your power by the eye’s sensitivity curve V(λ) to give lumens.
$$\Phi_v = 683 \cdot V(\lambda) \cdot P$$
The 683 lm/W factor comes from the SI definition of the candela. At 532 nm, V(λ) is 0.885, so 5 mW gives 3.02 lumens. Card 1 also gives the 555 nm power that looks just as bright, 4.42 mW.
Lumens stay on the CIE 1924 curve, the one the lumen is defined with. For “which looks brighter”, the tool switches to the newer CIE 2008 curve, which Omni also draws on. It finds the eye more sensitive to violet and blue than the 1924 curve does.
The table shows both curves, with the power each color needs to match a 5 mW green dot. Each card row in the tool names the curve it uses, so you always know which figure you are reading.
| Laser | V(λ), CIE 1924 | V(λ), CIE 2008 | Power to match 5 mW at 532 nm | Night-vision V′(λ) |
|---|---|---|---|---|
| 405 nm violet | 0.00064 | 0.00497 | 0.88–6.9 W | 0.0185 |
| 450 nm blue | 0.0380 | 0.0647 | 68–116 mW | 0.455 |
| 473 nm blue | 0.103 | 0.144 | 30–43 mW | 0.710 |
| 520 nm green | 0.710 | 0.718 | 6.1–6.2 mW | 0.935 |
| 532 nm green | 0.885 | 0.878 | 5.0 mW | 0.781 |
| 589 nm yellow | 0.769 | 0.822 | 5.3–5.8 mW | 0.0699 |
| 635 nm red | 0.217 | 0.242 | 18–20 mW | 0.00224 |
| 650 nm red | 0.107 | 0.119 | 37–41 mW | 0.00068 |
The curve values come from the CIE tables published in the Colour Science dataset. The power range runs from the CIE 2008 figure to the CIE 1924 figure. A 405 nm pointer also makes many materials glow by fluorescence, so its dot can look brighter than either curve predicts.
Dark-adapted eyes follow the night-vision, or scotopic, curve instead, which peaks near 507 nm. At night, red beams fade almost to nothing while blue and green stand out. The Night option under Viewing Conditions applies this CIE 1951 curve to every mode.
Beam Glow From the Side
A beam seen from the side glows because air scatters a little of its light toward you. Molecular, or Rayleigh, scattering grows as 1 ÷ λ⁴, so a 532 nm beam scatters 1.18 times more than a 555 nm one. Card 4 multiplies that by the CIE 2008 eye response to give the “beam glow” relative to 555 nm, 1.04 times for the default green.
Clean air follows that λ⁻⁴ rule. Dust, smoke and haze scatter much more evenly across colors, so on a hazy night every beam shows up better and the gap between colors shrinks.
Comparing Two Lasers
The default comparison pits a 5 mW green at 532 nm against a 100 mW red at 650 nm. The red has 20 times the power but only 0.119 of the eye’s peak response on the CIE 2008 curve. Its dot still looks 2.72 times brighter in daylight.
The beams tell a different story. Red light scatters (532 ÷ 650)⁴, or 0.45 times as much, so the 100 mW red beam glows only about 1.22 times as strongly as the 5 mW green. Card 3 also gives the red power that would match the green dot exactly, 36.8 mW.
Switch Eye Adaptation to Night and the result flips. Dark-adapted eyes barely see 650 nm, so the 5 mW green dot looks 57.7 times brighter than the 100 mW red. That is the CIE 1951 scotopic curve at work, and it is why green wins for night sky pointing.
Card 4 lists the class of each laser. In the Laser Brightness Calculator the 5 mW green is Class 3R and the 100 mW red is Class 3B, a big step in hazard for a dot that is only modestly brighter.
How to Calculate Laser Intensity
Laser intensity in everyday use means irradiance, power per unit area, often in W/cm². For a round beam with 1/e² diameter D, the average over that circle is P ÷ (πD²/4). A Gaussian beam’s center is twice that average.
$$E = \frac{P}{\pi (D/2)^2} \qquad E_{peak} = 2E$$
The irradiance mode default in the Laser Brightness Calculator is 5 mW in a 1 mm beam. That gives 0.637 W/cm² on average and 1.27 W/cm² at the center. Average full sunlight is about 1,000 W/m², so this small pointer spot is 6.37 times stronger than the Sun on the ground.
The Laser Beam Expander Calculator shows how widening a beam cuts its irradiance by the square of the magnification. Focusing does the reverse, which is how a few watts can cut steel.
Pulsed Laser Peak Power
A pulsed laser delivers its energy in short bursts, so its peak power dwarfs its average. Peak power is pulse energy divided by pulse length, average power is energy times repetition rate, and the duty cycle is pulse length times rate.
$$P_{peak} = \frac{E}{\tau} \qquad P_{avg} = E \cdot f$$
The pulsed mode of the Laser Brightness Calculator defaults to a 2 mJ, 8 ns pulse at 20 Hz. Peak power is 250 kW, yet the average is only 40 mW, with a duty cycle of 1.6 × 10⁻⁷. In a 1 mm beam, each pulse peaks at 0.509 J/cm² fluence and 63.7 MW/cm² irradiance at the center.
Peak power uses a square-pulse estimate, E ÷ τ. Real pulses are rounded, so their true peak differs a little with pulse shape. Laser damage thresholds for optics are usually quoted as fluence in J/cm² for a stated pulse length.
Laser Classes and Eye Safety
Card 3 gives the laser class by power, using the continuous-wave limits from IEC 60825-1. The University of Oregon laser safety manual sets out the same limits for visible lasers.
| Class | Visible CW power | Main hazard |
|---|---|---|
| Class 2 | Up to 1 mW | Safe for a brief glance, blink protects |
| Class 3R | 1 to 5 mW | Risk from staring or viewing through optics |
| Class 3B | 5 to 500 mW | Direct beam injures eyes faster than a blink |
| Class 4 | Over 500 mW | Eye and skin injury, fire, scattered light |
Class 2 relies on the aversion response, which closes the eye within about 0.25 seconds. For invisible wavelengths there is no blink at all, so the tool reports an upper bound and flags the beam as more hazardous than it looks.
To calculate OD for laser safety eyewear, take the base-10 log of the beam’s power density divided by the maximum permissible exposure, MPE, in the same units. A beam 1,000 times over the MPE needs OD 3, and 10,000 times needs OD 4. MPE depends on wavelength and exposure time.
Nits, Lumens and Laser Brightness
Screen brightness in nits is luminance, and one nit is one candela per square meter. It is the photometric twin of radiance, so the same 683 × V(λ) factor converts one to the other. Luminance is radiance × 683 × V(λ).
The default green laser’s radiance of 17.67 GW/(m²·sr) works out to about 1.07 × 10¹³ nits, the Luminance row on card 2. A 900-nit phone screen is roughly 12 billion times dimmer. That gap is why a reflection of even a small laser off a mirror is never safe to look at.
Common Laser Power Questions
These answers use the same physics as the Laser Brightness Calculator, with worked numbers from its irradiance and class results. More optics tools are on the Optics Calculators hub.
How strong is a 100 W laser?
A 100 W laser is Class 4, 200 times past the 500 mW Class 4 threshold. Focused to a 0.2 mm spot, the irradiance mode gives about 318 kW/cm² on average, enough to cut and weld metal. A 20 W or 40 W engraver is in the same class and can burn skin and start fires.
Can a 500 mW laser burn?
500 mW is the top of Class 3B. In a 1 mm beam it delivers about 64 W/cm² on average, more than 600 times full sunlight. It can damage eyes instantly from the direct beam. Above 500 mW, the Class 4 rules add skin and fire hazards.
Is a 1 watt laser illegal?
Rules on buying and owning high-power lasers differ by country. A 1 W visible laser is Class 4 everywhere. In the United States, pointing any laser at an aircraft is a federal crime, with FAA fines of up to $32,646 per violation.
Why is it illegal to point lasers in the sky?
A laser can dazzle and incapacitate a pilot at a critical moment. The FAA logged 10,993 laser strikes reported by pilots in 2025. Even a Class 3R pointer spreads to a wide spot at aircraft distances, and that spot can still flood a cockpit with glare.
Is a laser UV light?
Only some lasers are. Lasers are made at wavelengths from ultraviolet to far infrared. A 405 nm “violet” pointer sits just inside the visible band, while UV lasers work below 400 nm. The tool gives V(λ) = 0 outside the visible range and warns that the beam is invisible.
What material can block a laser?
Laser safety eyewear rated for the exact wavelength and an OD high enough for the beam blocks it. Opaque metal and ceramic stop low-power beams, while flammable materials can catch fire under a Class 4 beam. Mirrors reflect rather than block, and the reflected beam stays hazardous.
Could a laser reach the Moon?
Yes, but it spreads on the way. Even a beam with 1 mrad of divergence grows to a spot about 384 km across at the Moon. The Laser Beam Divergence Calculator works out that spread for any beam and distance.
What is another name for a laser?
Early papers called it an optical maser, after the microwave maser that came first. “Laser” is an acronym for light amplification by stimulated emission of radiation, now written as an ordinary word.