# Laser Brightness Calculator

URL: https://speedycalculator.com/calculators/physics/laser-brightness/
Last updated: 2026-10-30

> The laser brightness calculator estimates the radiance (brightness) of a laser from its power, wavelength and beam quality factor, and compares two lasers.

## Inputs

- Power P (mW, W, kW)
- Wavelength λ (nm, µm, mm, m, pm, Å)
- Beam quality factor M²

## Results

- Radiance L
- Compared with the Sun's surface

## Formula

```
L ≈ P / (λ²·M⁴)  (W/(m²·sr))
```

## Explanation

Laser brightness – radiance – is power per unit area per unit solid angle. Because a laser beam is tiny and nearly parallel, even a milliwatt laser is far brighter than the Sun's surface. Brightness is conserved by passive optics, so it limits how well a beam can be focused.

### How to use the Laser Brightness Calculator

- **Power P**
- **Wavelength λ**
- **Beam quality factor M²**

Results update as you type, every field has its own unit menu, and everything is calculated in your browser.

### How it works

Enter the power, wavelength and M² (1 for an ideal Gaussian beam). The comparison with the Sun uses its surface radiance of about 2 × 10⁷ W/(m²·sr).

### Inputs explained

FieldWhat to enterPower PUnits: mW, W, kWWavelength λUnits: nm, µm, mm, m, pm, ÅBeam quality factor M²A number

### What each result means

- **Radiance L** = [Power P]÷([Wavelength λ]^2×[Beam quality factor M²]^2)
- **Compared with the Sun's surface** = [Radiance L]÷2.0e7

### Example values

How the main result changes when **Power P** changes and everything else stays at the example values above:

Power P (mW)Radiance L (W/(m²·sr))2.57,300,143,106.16533.7510,950,214,659.248514,600,286,212.33076.2518,250,357,765.41337.521,900,429,318.4961029,200,572,424.6613

## Worked example

Inputs — Power P: 5 mW; Wavelength λ: 532 nm; Beam quality factor M²: 1.1.

Results — **Radiance L = 14,600,286,212.33067 W/(m²·sr)**; Compared with the Sun's surface = 730.0143 ×.

## Key terms

- **Radiance**: Power per area per solid angle.

## FAQ

**Why is a laser pointer brighter than the Sun?**
Its power is concentrated into a tiny area and solid angle.

**Can lenses increase brightness?**
No – passive optics cannot increase radiance.

**How does M² affect brightness?**
Radiance falls with M⁴ (M² in each direction).

**Is a brighter laser more dangerous?**
Yes – retinal hazard depends on how tightly the eye can focus it.


## Sources

- [OpenStax – University Physics Volume 3](https://openstax.org/details/books/university-physics-volume-3)
- [HyperPhysics (Georgia State University)](http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html)