Speckle Pattern - Introduction and Applications

A speckle pattern is a random granular pattern produced by the reflection of a coherent light beam, e.g. laser, at a rough surface, such as a metallic surface, a display screen, white paint, or a piece of paper.

This pattern is caused by the interference of the reflected beams of incident light with the relative optical phases. The shape of the speckle pattern tends to vary even when minor changes occur at the direction of the incident beam or in the illuminated spot.

Sometimes, the complex intensity patterns formed in multimode fibers, due to the interference of several waves of the same frequency, are also referred to as speckle patterns.

Speckle patterns can deteriorate the quality of an image generated by laser displays, or alter the position measurements. For this reason, a light source with a large bandwidth can be used to avoid speckle patterns.

Types of Speckle Patterns

The two basic types of speckle patterns are explained below:

• Subjective speckle pattern – Speckle patterns produced at an image plane of a lens are called subjective speckle pattern. Subjective patterns are caused by the interference of waves from the various scattering regions of a resolution element of the lens. In this region, the response functions of the randomly de-phased waves are added, resulting in the formation of speckle patterns
• Objective speckle pattern - Objective speckle patterns are formed when a diffuse object is illuminated by a coherent wave. The speckle pattern size depends on the interference between the waves from various scattering points. The size increases linearly with the increase in distance between an observation plane and an object. Objective speckle patterns are usually observed in the far-field region, and their properties depend on the dimension and form of the region upon which the laser beam hits. In some cases, speckle patterns can also be observed in the near-field, and the properties of near-field speckle patterns depend on the structure of the scattering object

Applications

Speckle patterns find applications in the following:

• Interferometry
• Stellar speckle astronomy
• Eye-testing using speckle
• Speckle imaging
• Measuring activities of biological samples

Sources and Further Reading

• Speckle Metrology - Marquette University
• Speckle – RP Photonics