Laser display technologies and their applications

1 Introduction

A laser projection technology was proposed in 1965 [1]. The system requires red, green, and blue (RGB) lasers. It remained too costly to be used in commercially viable consumer products and too poor in performance such as very low-efficiency and large-size externals due to water-cooled Ar ion laser systems. Therefore, efficient and compact RGB lasers have been expected to realize the practical laser display [2–5].

By achievement of air-cooled RGB lasers including a red laser diode (LD), a blue LD, and a green second-harmonic-generation (SHG) laser, an advanced laser projection engine was developed in 2004 [6]. The laser projection display using the engine showed low electric consumption and wide color gamut (over 130% NTSC ratio). Figure 1 shows a color gamut of a typical laser display. Mitsubishi Electric Co. developed a commercial laser projection TV [7] in 2008 and a laser liquid crystal display (LCD) TV [8] in 2012 for home use.

Figure 1

Color gamut of typical laser display.

As high brightness and high power efficiency of lasers contribute to downsizing systems and improving electric efficiency, the laser projection displays are expected to bring us big displays anywhere and anytime. Therefore, microlaser projectors [9, 10] are hot topics now and have already been commercialized [11].

Table 1 shows the comparison of lasers, LEDs and UHP lumps. The laser display is attractive because of their low electric power consumption and the wide range of colors that can be realized. The high efficiency and small etendue of laser sources are advantageous for downsizing display systems [12].

2 Optical configurations

Figure 2 shows two different approaches for optical configurations of laser displays. A scanning-type laser projection engine is shown in Figure 2A. The projection display system includes a laser source emitting a laser beam, and a reflecting mirror system for scanning the laser beam over an image to illuminate the image [9, 10]. The reflecting mirror system can be one or more Micro Electro Mechanical Systems (MEMS) scanning mirrors that rotate to raster scan the laser beam over the image. As the reaction of the human eyes is not quick, the image can be reconstructed on a screen.

Figure 2

Two different approaches for optical configurations of laser displays.

(A) Scanning-type laser projection engine. (B) 2D spatial light modulator-type laser projection engine.

A 2D spatial light modulator (SLM)-type laser projection engine is shown in Figure 2B. Conventional SLMs such as the digital mirror device (DMD), liquid crystal, and liquid crystal on silicon (LCOS) are used. In this configuration, RGB lasers are used as a back light [6].

3 Technologies for laser displays

Laser displays require RGB lasers. Blue and red LDs are commercially available. On the other hand, there are no commercially available green LDs except for products with low output power. Instead, the SHG of lasers can be used to provide the green wavelengths [13–15]. In 2009, green LDs are first demonstrated as 515-nm continuous wave [16] and 531-nm pulse [17] operation by InGaN. In 2011, green LDs were mass produced by Nichia Co. (Anan-shi, Tokushima, Japan) and the improvement of those characteristics is progressing.

Speckle noise is the result of the interference of many waves on the screen due to the coherency of the laser source [18] as shown in Figure 3. Speckle is considered to be a problem in laser projection, and it is usually quantified by the speckle contrast. Speckle contrast reduction is essentially the creation of many independent speckle patterns, so that they average out on the retina. This can be achieved by wavelength diversity, angle diversity, or polarization diversity. Figure 4 shows some approaches of speckle contrast reduction. For example, by varying the incident angle of the ray to the SLM, both the reduction of speckle contrast and uniform intensity distribution are achieved. The optics constructed with a rotating lens array and a rod integrator illuminated a SLM. The rotating lens array set just in front of the rod integrator varied an angle of a laser beam and made a different wavefront in each time. The speckle contrast was suppressed by averaging these configurations. A moving screen technique is another solution. The laser TV by Mitsubishi Electric Co. [7] used such a moving screen to reduce the speckle noise.

Figure 3

Speckle pattern image.

Speckle pattern occurs as the result of the interference of many waves on the screen.

Figure 4

Representative approaches of speckle noise reduction.

(A) By laser spectrum; (B) by optics; (C) by screen.

4 Applications of laser displays

Various applications of laser displays are proposed as shown in Figure 5. Here, the laser display includes laser lighting. Table 2 shows the advantages of various laser displays. The laser display systems and applications of the laser display are introduced below.

Figure 5

Various applications of laser displays. It is classified into projectors, TVs, head mount displays, and laser lightings.

Table 2

Advantages of various laser displays which include laser lighting.

	Low power consumption	Wide color gamut	Ultra compact	High brightness	Focus free
Mobile projector
Data projector	○		○
Home projector	○		○
Pocket computer
HMD	○				○
HUD		○	○	○
Laser TV			○
3D	○	○	○		○
Lighting	○		○	○

4.4 Other applications

Various applications of the laser display technologies are proposed [12]. They include laser lightings and laser illuminations. Laser lighting technologies will extend the usability of lighting devices. A laser headlight and a spot lighting by a laser are examined. A laser plant factory and a fishery are promising applications of the laser lighting in the future.

Figure 6 shows the classification of applications as a function of laser power. Laser display technologies will find wide applications by an increase of the laser power.

Figure 6

Classification of applications as a function of laser power.

Summary

Laser displays such as laser projectors, HMDs, HUDs, and laser TVs were developed with low electric power consumption and wide color gamut by using efficient and compact RGB lasers. Table 3 shows the transition of the former and the future on research target of laser displays. Considering a further reduction of electric power consumption, the efficiency of the display part is important and should be as high as possible. By achieving more efficient RGB lasers, a laser TV with the electric consumption power of about 10 W and a microprojection engine with a size of 1 cm3 will be realized in the near future. The laser display will find wide and innovative applications instead of traditional display and lighting systems.