# What causes laser speckling?

I just wanted to know, what causes laser speckling? interference with air? uneven projection surface?

Andy Resnick
The speckle is actually on your retina; this can be easily proved if you wear glasses or other corrective lenses- remove them and the speckle remains in focus.

The speckle is caused by spatial coherence within the beam; just as there is temporal coherence related to the frequency spread $$\Deltat\leq\frac{2\pi}{\Delta\omega}$$ , the spatial coherence is related to the size of the source $$\DeltaA=\lambda^{2}/\Delta\Omega$$, where $$\Delta\Omega$$ is the solid angle subtended by the source. Each 'grain' is a region where the light is coherent, adjacent 'grains' are partially coherent with each other.

You can create speckle from any source- its possible to observe the speckle from sunlight, for example.

Claude Bile