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Technical Course | Be Prepared for Something You Don’t Know about Speckles (Part I)

Speckles are commonplace in laser projection. Some time ago, opinion is divided as to speckles in the projection industry. Although we have proved to our customers what kind of laser projection technology is capable of eliminating speckles, we would like to provide further explanations from a professional point of view for our readers to gain a more comprehensive and correct understanding of speckles in laser projection.

This article contains large amounts of information. Be prepared!

Screenshot of the divided opinion

Are speckles exclusive to lasers? No!

Speckles are optical interference in essence, which is a common physical phenomenon in optics. As is stated in university physics, the phenomenon of optical interference only occurs when two light waves have the same (similar) frequency, constant phase difference and the same vibration direction. The classic Young's double-slit experiment (in general physics) and Fourier optics (Section 5.5.4, Speckle Phenomena in Optics, Goodman) both demonstrate that optical interference exists regardless of the light sources. In other words, any light sources including lasers could result in speckles.

Figure 1 Screenshot of the source text

The source text provides the first argument:

Optical interference exists regardless of the light sources. In other words, any light sources including lasers could result in speckles!

This argument confuses different concepts:

Any light sources could result in speckles.

The sun is also a light source. Does it produce speckles?

Read on for some basic knowledge of optical interference and speckle and their relationship, and you'll understand them better.

About Optical Interference

Characteristics of light: In the 19th century, the discovery of optical interference greatly facilitated the development of the wave theory of light. Scientists found that light propagates forward like waves and sometimes demonstrates the characteristics of particles.

(The first photograph of the wave-particle duality by scientists)

The statement that optical interference exists regardless of the light sources is seemingly correct; however, coherent light is required to produce stable optical interference. In modern physical optics, only two coherent light sources with the same frequency and vibration direction and constant phase difference can produce stable optical interference. How can ordinary light sources become coherent? Simply put, optical devices can be used to divide the source waves emitted by the same light source into several wavelets. These wavelets have the same frequency and vibration direction and constant phase difference to produce optical interference. So keep in mind that it is not that easy for ordinary light sources to produce optical interference. Now let's use the well-known Newton’s rings and Young’s double-slit experiment to illustrate the argument.

Newton’s rings was first observed by Newton in 1675 and its schematic diagram is shown in Figure 2. When viewed with monochromatic light through a planoconvex lens with a large radius of curvature on a glass plate, a series of concentric, alternating bright and dark rings appear. If the light is sunlight or white light, the point of contact between the two surfaces is a dark spot surrounded by alternating bright and dark rings. They are interference fringes resulting from the light reflected on the spherical surface and the flat plane. In the Newton’s rings experiment, coherent light is obtained through the division of amplitude. When one beam of light is projected onto the interface between two kinds of transparent media, part of the light is reflected and the other part is refracted. In other words, the spherical surface and the flat plane generating the Newton’s rings reflect two parts of the same incident light. They are coherent for having constant phase difference resulting from going through different paths.

Figure 2. Schematic Diagram of Newton’s Rings

Young’s double-slit experiment is a well-known optical experiment. Thomas Young put a lit candle, which constituted a point source, in front of a piece of paper with a small hole, and then another piece of paper with two parallel slits behind the paper. The light emitting from the small hole passing through the two slits was cast onto the screen, generating a series of alternating bright and dark stripes. In the experiment, coherent light was obtained through the division of wavefront, which is illustrated in Figure 3. According to the Huygens-Fresnel principle, the light diffracted through the small hole (a) is a spherical wave, and any part of the wavefront can be seen as a new light source with the same frequency and the same phase. Therefore, the light waves diffracted at the double slits (b and c) are coherent light sources that generate alternating bright and dark interference fringes on the screen.

Figure 3. Schematic Diagram of Young’s Double-Slit Experiment

Once again, bear in mind that it is not that easy for ordinary light sources to produce optical interference. The advent of laser has facilitated the study and application of optical interference as laser itself is an ideal coherent light source. For example, today's researchers can run a double-slit experiment to obtain an interference pattern by directly irradiating laser through the double slits, instead of needing to generate coherent light with a small hole.

Conclusion

Not all light sources are coherent. Only two coherent light sources with the same frequency and vibration direction and constant phase difference can produce stable optical interference. While certain methods and measures are required to turn ordinary light sources into coherent ones, laser itself is an ideal source of coherent light!

Please keep in mind what we have covered so far, for coherent light is one of the main reasons why speckles appear, which is detailed as follows.

Good job for sticking with us.

About Speckles

Let’s talk about how speckles appear. Coherent light reflected on or transmitted through a rough surface produces innumerable scattering wavelets which interfere with each other in the process of propagation and form a pattern of granular stripes called speckles. Although speckles have been observed by some scientists since the era where Newton lived, there were neither extensive nor intensive studies on them until the advent of laser in 1960.

Conditions for Formation

Two conditions are required to form speckles: 1. Coherent light. 2. An optically rough surface that can create mean fluctuation greater than one wavelength (which can be easily met).

At present, the most common coherent light source is laser. So unless otherwise specified, speckles generally mean laser-induced speckles.

Therefore, the statement that “optical interference exists regardless of the light sources. In other words, any light sources including lasers could result in speckles!” is invalid.

1. Speckles cannot be formed by “any light sources”, but only when certain conditions are met.

2. Direct RGB laser projection onto a cinema screen meets the conditions for speckle formation. However, speckles are undesirable in cinema projection and should be eliminated with certain techniques.

To explain laser-induced speckles in an intuitive manner, we include the laser projection images with and without speckles as shown in Figure 4. It's clear that speckles in the image significantly affect the viewing experience.

Figure 4. Examples of Laser Projection Images (a) with Speckles and (b) without Speckles [1]

In the film industry, attention is devoted to reducing the impact of laser-induced speckles as they are extremely annoying “noise” for a coherent imaging system. However in other fields, researches have discovered that as speckles carry a lot of information about beams and the objects through which the beam passes, they can be applied in many areas. Examples include measuring the roughness of the surface where beams are reflected through the contrast of speckles, measuring the velocity of the object through the dynamics of speckles, using speckles for processing optical information, and even for optometry.

Nowadays, with the rapid development of laser display technology, how to eliminate laser-induced speckles has become a focus of interest for researchers. There are various technical methods to reduce the impact of speckles. These methods roughly work by the following two mechanisms:

1. Reducing the coherence of the light source based on the root cause for speckle formation, such as using weakly coherent light sources, pulse drive, phase delay and multimode fiber.

2. Superimposing multiple unrelated speckles in space or time domain to reduce the contrast, such as superimposing polarized waves and wavelengths and using multi-angle illumination, diffusers and dithering. It is worth mentioning that it can be challenging to reduce the speckle contrast to a level that the speckles are not visually perceptible by using just one method. This is a task that takes a mix of methods to fulfill. [2]

How does phosphor projection technology eliminate speckles? What else we know about speckles is incorrect? Let’s talk about that in our next lesson. Stay tuned.

Reference

[1] K. V.Chellappan, et al. Laser-based displays: a review. Applied Optics. 2010,49(25): 79-98.

[2]J. W. Goodman. Speckle Phenomena in Optics: Theory and Applications.2006.


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