Light from a coherent source reflected from a surface rough on the wavelength scale picks up random phases from different points, and these interfere into a granular pattern whose grain size depends on the viewing aperture. Speckle degrades laser imaging, projection and illumination uniformity. It is reduced by lowering coherence, moving diffusers, vibrating multimode fiber or averaging many patterns, and it is exploited in speckle interferometry and flow imaging.
Speckle contrast on the screen is still too high, spin the diffuser faster or add a second one.
Also heard as
- laser speckle
- speckle pattern
Related terms
Coherence length
The path difference over which a light source keeps a fixed phase relationship and can still form interference fringes.
Linewidth
The spectral width of a single-frequency laser's output, usually given in Hz, kHz or MHz at half maximum.
Top-hat
Describes a beam with flat, uniform intensity across its width and steep edges, instead of a Gaussian peak.