A laser looks grainy to you because of the wave nature of light or interference. Light coming into your eye from a laser does not exhibit speckle. This ‘grainy look’ or what’s referred to scientifically as Speckle occurs because the laser light is temporally coherent and is scattered from a random surface.
The random surface causes a large number of wavelets to leave the object and overlap in space, each wavelet having a random phase relationship to all the others.
Speckle patterns are usually seen in reflections of monochromatic light, like laser light. This can happen on materials such as paper, paint, or rough surfaces. It can also occur in media that have a lot of scattering particles, like dust or cloudy liquids.
When your eye is exposed to this field of overlapping ripples, it creates yet more overlapping as everything comes into focus. The speckle pattern in your eye is determined by the diameter of your pupil. The speckles will be smaller than the focus spot of your eye or very close if you are corrected to 20/20 vision.
Even if the photo is out of focus, speckle will develop in the picture.
If you are near-sighted, position your head to the left of a laser beam scattering off a surface like a wall. The speckle pattern will appear to move towards the right.
The position of the speckle pattern will change according to where you focus your eyes if you are farsighted. If you focus your vision, the pattern will appear to be still. This movement of the speckle pattern, created by each observer’s individual optical system, is commonly referred to as the Subjective Speckle Pattern.
The direct speckle pattern is more difficult to see when looking at the wall. However, if you enter a dark room with a bright laser focused on one particular part of the wall and then look around to the darker regions of the room, you will notice a large-spaced speckle pattern present on other walls. The size difference between these directly illuminated spots versus indirect ones follow an inverse relationship.
All observers experience this same pattern.
Further Explanation of a Lasers Grainy Look
The grainy look or speckle effect is created by the interference of many waves with the same frequency but different phases and amplitudes. These waves combine to create a new wave whose amplitude (and intensity) varies randomly.
The length of a wave is determined by the sum of its individual vector lengths, which can be anything from zero to the total of all the vectors’ lengths—a 2-dimensional random walk, sometimes known as a drunkard’s walk.
When looking at an illuminated surface from a laser, according to diffraction theory, each point on the illuminated area generates secondary spherical waves as a result of the light wave being reflected. Waves from every point on the illuminated surface are combined to make up the light at any position in the scattered light field.
If the surface is rough, meaning that there are larger variations in path length for different parts of the surface, this will cause phase changes greater than 2pi. This means that the amplitude and intensity of light outgoing from the surface will appear random.
A speckle pattern will not be visible if light of low coherence (i.e., consisting of many wavelengths) is employed, since speckle patterns created by distinct wavelength are generally larger and will usually combine with one another. In certain circumstances, speckle patterns may be seen in polychromatic light.
Laser Subjective Speckle Patterns
When a surface that is illuminated by a coherent light (for example, a laser beam) is imaged, you can observe a speckle pattern in the image plane; this is called “subjective speckle pattern.”
The term “subjective” is used because the speckle pattern’s precise structure is dependent on the viewing system parameters, such as whether the lens aperture size varies. If viewing equipment changes then so the size of the speckles.
If the position of the imaging system is changed, the speckle pattern will adapt and eventually lose its relation to the original speckle pattern.
By looking at a laser spot on the wall, both directly and through a small hole, you can see how aperture size affects speckle size. The latter will appear larger.
The speckle pattern may also vary depending on the position of the eye and the laser pointer. When moving the eye’s position while keeping the laser pointer constant, the speckle pattern will alter. The speckles will remain visible if the eye’s focus is shifted away from a wall.
Laser Objective Speckle Patterns
When laser light that has been scattered off a rough surface strikes another surface, an “objective speckle pattern” is produced.
When a photographic plate or another 2-D optical sensor is placed within the scattered light field without a lens, speckle patterns are produced depending on the system’s geometry and laser wavelength.
The light intensity at any given point in the speckle pattern is a result of the interference between waves scattered from different points on the scattering surface. The relative phases of these scattered waves vary randomly, so that the resulting phase on each point of the second surface also varies randomly. This results in a pattern which does not change regardless of how it is observed, similar to viewing a painting.
Applications
When lasers were initially developed, the speckle effect was seen as a major drawback in using lasers to light objects, especially in holographic imaging, due to the grainy picture generated.
Later, researchers discovered that speckle patterns might contain information about the surface deformation of an object, and this property has been utilized in holographic interferometry and electronic speckle pattern interferometry. Speckle imaging and eye testing with speckle are also utilizing the speckle effect.
The optically observed change in the speckle pattern over time, caused by changes in the illuminated surface is called dynamic speckle. This can be used to measure activity (e.g., with an optical flow sensor or optical computer mouse), and is known as biospeckles when seen in biological materials.
By studying changes in speckle patterns, we can learn about the light source in a static environment.
Speckle has been utilized in quantum computer simulations with cold atoms to generate a disordered pattern. The brightly lit and darkly lit regions act as an analogy for disorder in solid-state systems and are utilized to research localization phenomena.
