Laser Speckle
Definition
The grainy shimmer in laser light — random interference from rough surfaces, the bane of laser projectors.
Technical definition
Coherent light scattered from a surface rough on the scale of λ forms a random interference pattern; objective speckle shifts with observation geometry.
Example
Laser cinema projectors fight speckle with vibrating screens — blurring the grains faster than your eye can follow.
Common confusion
Speckle is not dirt on the lens — it is fundamental interference. Cleaning the optics won't remove it; only reducing coherence or averaging over time helps.