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Sci-Tech Blogger·Lecture | Laser Phosphor Is Born Speckle-Free, So Why Bother?

Sci-Tech Blogger·Lecture

Since 2014, there has been a wave of laser projection in the film industry, as laser projection technology gives viewers a better visual experience by addressing such pain points as inadequate brightness when xenon lamps are used for projecting 3D pictures. Meanwhile, the speckles caused by the high brightness of laser also affect the image quality. Centering on the speckle issue, this article compares two major laser projection technologies in the market, namely, RGB laser and laser phosphor, in terms of approaches to and methods of eliminating speckles for your reference.

The Generation and Influence of Speckles

When an incident coherent laser beam is projected on the surface of a rough screen, the light reflected from the surface of the screen has a fixed phase difference, which, through the imaging system (human eyes), forms interference patterns on the image plane (retina), and such interference patterns are speckles. Interference patterns are bright and dark stripes, so the final image on the retina of human eyes formed by the superimposition of an infinite number of interference patterns is like putting a fogged transparent glass in front of a clear image. Speckles blur the image and dramatically affect the quality of pictures and viewing experience. Speckle elimination has always been the most thorny problem in the application of laser to the projection field and also an important issue in the laser display field for hundreds of years.

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The Approaches to Eliminating Speckles

There are generally two approaches to eliminating speckles: reducing the coherence of light sources, which is the root cause of speckles, or superimposing multiple irrelevant speckles in spatial and time domains to reduce the contrast.

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How RGB Eliminates Speckles

Unable to change the laser coherence from the root cause, the RGB three-color laser projection solution can only take speckle-eliminating measures in the light path by superimposing different speckles in spatial and time domains. The RGB laser projection solution eliminates speckles with low-gain screens, multi-wavelength, vibrating diffusers, vibrating screens, commercial speckle reduction devices, etc. As none of above measures can achieve satisfactory effects, RGB manufacturers have to combine a variety of methods to reduce speckles. However, speckles can only be controlled to a certain extent but cannot be radically eliminated by such combined methods. Some foreign RGB manufacturers have achieved better results with non-vibrating screens, while other RGB manufacturers use vibrating screens with less reliable motor system and subwoofer system, resulting in high cost of system maintenance. Therefore, the combined speckle-eliminating methods employed by RGB are effective in some way, but meanwhile bring the system low optical efficiency and reliability and high costs.

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How Laser Phosphor Eliminates Speckles

As can be seen from the generation of speckles, the root cause of speckles is laser coherence. The laser phosphor solution (shown in the figure below) uses laser-excited phosphor materials to emit a large amount of incoherent phosphor mixed with a very small amount of laser to form speckle-free white light. Thus, the phosphor laser solution is born without speckles, which can eliminate the laser coherence from the source without any additional measures or any negative impact on the system. In addition, the solution features high compatibility and reliability and low cost, so it is the best commercial solution. Therefore, compared with the limited effects and huge costs of RGB solution which reduces speckles through complex incoherent superposition, the laser phosphor projection solution can radically eliminate speckles - the genetic laser problem - so as to present completely clear images to the audience.

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