Understanding Laser Speckle: Causes and Measurement Techniques

[Airfoil-ed]

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]

You can get objective and subjective speckle.

Subjective speckle as Andy said is due to random phase perturbations introduced at the retina and is readily observable in laser light due to its coherence.

Objective speckle is caused when light hits some other surface that introduces random phase variations, such as a frosted glass pane. You can actually measure the speckle size to measure things like height and refractive index variations of whatever is causing the random phase variations using a technique called correlation interferometry.

Claude.