What is Laser Speckle?

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- GoPhotonics

Sep 20, 2023

When laser light hits a surface with imperfections, it creates a unique pattern of scattered light intensity. This phenomenon leads to the creation of an interference pattern known as speckles. These speckles comprise a pattern characterized by alternating bright and dark spots of varying shapes, distributed in a random manner throughout space.

Laser speckle was first observed in the 1960s, shortly after the invention of the laser. It arises from the interference of coherent light waves scattered by microscopically small irregularities on a surface. These irregularities can be as tiny as a fraction of the laser's wavelength. When laser light illuminates such a surface, each irregularity scatters the light in various directions. As these scattered waves overlap and interfere with one another, they create a pattern of constructive and destructive interference.

Types of speckles

Speckles can be categorized into two primary groups, depending on whether the interfering waves originate directly from the object or pass through an imaging system. These categories are as follows:

  • Objective speckles
  • Subjective speckles

Objective speckles relate to speckle patterns that are acquired without the need of an image-forming system, such as a photographic camera or the human eye. These patterns result from the unaltered propagation of light in free space. On the other hand, subjective speckles refer to speckle patterns that emerge under conditions where an image is being formed. This typically involves the presence of an imaging system, such as a camera or the human eye, which plays a role in shaping the observed speckle pattern.

Generation of Objective Speckles

In the case of objective speckles, there is no use of an imaging system positioned between the object and the recording medium. The formation of objective speckles is shown in the figure above.

To generate objective speckles, a coherent laser beam is expanded and directed onto a diffusively reflecting object, causing it to scatter light waves in various directions. As shown in the above figure, each point on the detector array or photographic plate (denoted as A and B) receives scattered waves from every point on the illuminated object. These scattered waves each travel different path lengths and arrive at specific locations on the recording plane with a number of varying phases.

At certain points on the recording plane, the waves align in phase, resulting in constructive interference that forms bright spots. Conversely, at other points, where the waves are out of phase, they combine destructively to form dark spots. At some points, a mixture of phase differences leads to the appearance of gray spots. However, this type of speckle pattern is challenging to observe and holds minimal practical use for measurement purposes.

The size of objective speckles can be mathematically determined by the following relationship:

The equation above highlights that the size of objective speckles is dependent upon several factors, namely the wavelength (λ) of the laser source, the diameter of the scattering area (a), and the distance between the object and the image plane.

Objective speckle analysis has applications in scientific research, engineering, and quality control where precise and quantitative measurements of speckle patterns are essential. It is extensively used in fields like optics, materials science, non-destructive testing, and medical imaging. 

The advantages of objective speckle analysis are its high precision, reproducibility, and ability to provide quantitative data. By employing mathematical algorithms and specialized software, objective analysis can yield valuable information about speckle patterns, such as statistical properties, correlation lengths, and intensity distributions. This data is important for making informed decisions, conducting scientific experiments, and ensuring product quality. Objective speckle analysis can also be automated, making it suitable for processing large datasets efficiently, and it eliminates the subjectivity associated with human judgment, resulting in more reliable and objective results compared to subjective analysis.

Generation of Subjective Speckles

Subjective speckle patterns come into existence when an imaging system, such as a lens, is employed to focus on the object at the observation plane as shown in the above figure.

The process begins with coherent light emitted from a source being dispersed and projected onto the object. After that, the scattered waves propagate in various directions. A lens is used to collect a portion of these scattered waves and redirect them onto an image plane, which can be a digital detector array or a photographic plate.

In the formation of subjective speckles, waves originating from a single point on the object are focused onto a corresponding point on the image plane. This means that each point on the image plane is illuminated by a finite area of the object. These converging waves travel different path lengths and reach a particular point on the image plane with an array of phases. Consequently, at the convergence point, all these waves either constructively or destructively interfere with each other, giving rise to a specific level of brightness.

The size of the subjective speckle pattern, in this case, depends on the diffraction limit of the lens. Calculating the size of subjective speckles follows a similar methodology as that of objective speckles. The primary difference is the replacement of the cross-sectional area of the illuminated region with the diameter of the imaging lens aperture.

Approximately, the size of the subjective speckle can be estimated using the following equation:

Here, di represents the distance between the lens and the recording plane, λ stands for the laser source's wavelength, and a′ denotes the diameter of the lens. Also, the size of the subjective speckle pattern can also be described in relation to the focal length to aperture ratio, denoted as F (F-number), and the magnification factor, represented as M:

From the equation, when the aperture of the lens expands, the average speckle size reduces. 

Subjective speckle analysis is mainly used in fields where qualitative assessments of speckle patterns are sufficient for the intended purpose. It is commonly employed in industries like art and aesthetics, where the subjective interpretation of speckle patterns on surfaces, textiles, or paintings is valuable. 

The advantages of subjective speckle analysis lie in its speed and simplicity. It allows for quick visual assessments by human observers, making it a cost-effective and accessible approach. Subjective speckle analysis can provide immediate insights into the perceived texture, aesthetics, or overall quality of a surface or object without the need for complex equipment or extensive data processing. However, it may lack the precision and quantitative data offered by objective speckle analysis, which is more suitable for scientific and engineering applications.

Characteristics of Laser Speckle

  • Dynamic Nature: One of the most distinctive features of laser speckle is its dynamic nature. Even when the surface or the laser source remains stationary, the speckle pattern continuously changes. This is because the phases of the scattered waves change as a result of tiny vibrations or fluctuations in the optical path.
  • High Contrast: Laser speckle patterns exhibit a high degree of contrast, with dark and bright spots sharply defined against one another. The contrast is a result of the interference between the scattered waves, with constructive interference producing bright spots and destructive interference leading to dark spots.
  • Sensitivity to Motion: Laser speckle is highly sensitive to any motion or displacement of the scattering surface. Even the tiniest movements can cause rapid changes in the speckle pattern. This property has made laser speckle an important tool in various applications, such as measuring blood flow in biological tissues.

Applications of Laser Speckle

One of the most significant applications of laser speckle is in the field of medicine. Laser speckle contrast imaging (LSCI) is a non-invasive technique used to visualize blood flow in tissues. By analyzing the changes in the laser speckle pattern, researchers and healthcare professionals can monitor blood flow in real-time, which is crucial for understanding diseases like diabetes and vascular disorders.

In manufacturing and quality control, laser speckle is employed to inspect the surface quality of materials. Defects such as cracks, scratches, or irregularities can be detected with high precision by analyzing the speckle pattern reflected from a surface.

Laser speckle can be used in remote sensing applications, such as terrain analysis and vegetation monitoring. By observing changes in the speckle pattern caused by surface movements or vegetation growth, researchers can gain valuable insights into environmental changes.

It has also found applications in security and authentication. The dynamic and unique nature of speckle patterns can be used as a basis for secure authentication and anti-counterfeiting measures.