LED and laser projectors with solid state light engine Overview

As known, Christiaan Huygens invented the first projection device called “Magic Lantern” in the middle of the 17th century.

Of course, modern projectors bear little resemblance to their prototypes. The rapid scientific and technological progress of the last century gave us a huge number of fundamentally new devices, including modern projectors.

Introduction

The models of the last century used powerful lamps as their light source. As known, the image quality directly depends on the lamp brightness. Therefore, they became the main object for improvements. Ultimately, in 1995 Philips was developed a high-pressure mercury arc lamp called UHP (Ultra-high-performance) projector lamp (projector bulb).

Modern lamp projectors continue to use current modifications of UHP bulb.

The development of the semiconductor industry has given projector developers access to relatively cheap and powerful LEDs. As a result, the market was quickly filled with a huge number of LED projectors. Moreover, the enthusiasm of the consumer market has sparked the development of ultra-portable and mini pocket projectors.

Further development of semiconductors has created a new generation of laser LEDs. Of course, the projectors developers did not miss the new opportunity. As a result, the range on offer has rapidly expanded due to laser projectors. Moreover, phone manufacturers have even developed quite a few smartphones with a built-in laser projector.

In addition, many companies use LEDs and laser LEDs at the same time as sources of different colors. Sometimes some experts debate the validity of positioning LED laser as lasers. Therefore, for technical correctness, experts call such models projectors with solid-state light sources. But, of course, a laser diode is fundamentally different from a conventional LED. At a minimum, it uses a built-in optical resonator to generate coherent radiation.

Usually, innovations quickly displace models of previous generations from the market. But for projectors, the situation is different. Despite significant fundamental differences, lamp, LED and laser projectors continue to compete successfully with each other, dominating a certain niche.

Laser LEDs

Mostly modern portable projectors use LEDs. Their brightness usually ranges from tens to several hundred ANSI lumens. But more expensive models provide 1500 lumens and more. Unfortunately, maximum LED brightness will usually reduce color accuracy and thus image realism.

Laser projectors produce images using lasers. As known, laser light does not require focusing.
In fact, the laser diode is a further development of the conventional LED (light emitting diode).

The term “laser” is an acronym and stands for «Light Amplification by Stimulated Emission of Radiation».

This quantum process is based on the emission of photons during the transition of electrons from high to low energy levels. At the same time, they stimulate energy transitions of other electrons. As a result, the radiation process has an avalanche-like character, providing synchronization for transitions of individual electrons. In turn, synchronization ensures the coincidence of phase and frequency for the emitted light waves. Such radiation is called coherent. Its light propagates in a very narrow non-dispersive beam. Therefore coherent laser light has perfect focusing that does not change over great distances.

Structurally, a laser diode differs from a conventional LED by the presence of a built-in optical resonator, which amplifies the corresponding frequency. The laser diode uses a powerful enough pump current to reach the threshold generation level. Then, it goes into a coherent radiation mode with a spectral width of 1-2 nm. For comparison, the spectrum width of an LED varies in the range of 30-50 nm.

Narrow spectrum solves an important problem. Increasing the brightness of lamp and LED projectors is accompanied by green shift. Therefore, their settings always require a compromise between brightness and color accuracy. In contrast, laser models maintain high color accuracy even at high brightness.

Speckle contrast

Unfortunately, laser radiation is always accompanied by speckling artifacts.

High spatial coherence of superluminescent diodes (SLDs) and lasers leads to coherent artifacts known as speckle. This effect occurs because a screen isn’t flat on the scale of the light wavelength. Physically, it’s based on the formation of a random interference pattern due to mutual interference of coherent waves with random shifts in phase and amplitude. As a result, we see an “speckly” image.

Today, speckle noice has been a major challenge for laser projectors. These visual effects are dependent on the screen, the aperture size, etc and drastically reduce image quality. Experts call it “speckle contrast”.

The rotating diffusers are the most common among modern methods in speckle contrast reduction. The diffuser “destroys” the coherence of the laser light, creating continuous variations in the speckle pattern.

Unfortunately, most modern technologies for reducing speckle contrast are accompanied by defocusing, which reduces image quality.

RGB laser and LEDprojectors

LED light source

LED was the first solid-state light source in projectors. Unfortunately, they do not perfectly reproduce yellow tones and have limited brightness, which limits the image size. Of course, companies offer powerful LED models, but they are quite expensive. Low-powered LED projectors are much cheaper, flicker-free, have high contrast, and no rainbow effect or other artifacts. Today they dominate the mini pocket projector segment.

RGB laser source

RGB laser projector generates light using red, green and blue lasers. This technology provides maximum brightness, wide color gamut (BT2020 standard), excellent contrast and high quality HDR mode. Unfortunately, these projectors are very expensive.

Laser phosphor light source (DLP-chip, 3LCD matrix and LCoS)

Laser DLP projector uses a blue laser diode. Its beam falls on a yellow / green phosphor wheel to create yellow and green colors. Partially, the blue beam passes through the hole in the phosphor wheel and after a few reflections is sent to the color wheel. Further, yellow light generates red on the color wheel. The blue and green light pass through the diffusion window. Then, the red, green and blue colors fall on the DLP matrix, which forms the image. Finally, the lens directs the image onto the projection screen.

DLP technology provides high contrast with deep blacks and wide range of color settings. In addition, DLP projectors are offered in a huge range. Their main cons include low brightness, rainbow effect, and flicker. Of course, these problems are much less evident in expensive DLP models, but in this case, they lose the price advantage.

3LCD models also use a blue laser that creates a white color after passing through a phosphor disc.

Next, the 3LCD optical system generates RGB colors from it. Epson introduced the first 3LCD projectors in 1989. Today 3LCD models provide higher brightness and high image quality, but are usually more expensive compared to DLP models.

Unfortunately, Epson’s dominance limits their range of products in the market. In addition, 3LCDs provide lower contrast and black depth, are sensitive to dust, produce curved artifacts on large screens, and motion blur in games and in 3D.

LCoS technology combines the pros of DLP and 3LCD technologies.

JVC calls its similar technology D-ILA, Sony calls it SXRD.

These projectors are rainbow-free and flicker-free and deliver high image quality. Unfortunately, they are very expensive.

Conclusion

Gradually, LED and laser projectors with solid state light engine are replacing lamp models from the market. Today companies offer a huge selection in all price segments, including budget LED pocket projectors, more powerful and expensive models with laser diodes for use as TV, and very powerful RGB laser installation projectors. Moreover, the market offers smartphones with a built-in laser projector. In fact, any consumer can easily choose the best model for their budget. With this, huge lifetime of solid state light sources significantly reduces the problem of model reliability. Except for mechanical damage to the optics, such projector will perform just fine until it becomes hopelessly obsolete.

The video shows the Xiaomi Mijia 8k laser DLP projector with a screen size of up to 150 inches.