Thermal challenges deriving from the advances of display technologies
Introduction
Starting from the A. Einstein's prediction to the stimulated emission of radiation in 1917, a laser was invented by A. Schawlow and C. Townes in 1958. Real laser oscillation was obtained in a ruby by Maiman in 1960 as the first solid state laser. One should recognize it took 40 years for a laser to oscillate after the Einstein's first proposal. It took further 10 years before the first semiconductor laser was realized in the Bell Laboratories.
As for light emitting diodes (LED), observation of light emission from a SiC by Weardale in mid-19th century was recorded as its discovery. First practical LED was appeared in 1962. Since then its growth in luminous flux was moderated until it was accelerated after mid-1990s to eventually obtain 25 lm/W in a phosphor converted LED for automotive headlamp, thank to reduction of its thermal resistance.
Now market volume of high brightness LED alone is order of 4 billion USD in the world. 40 years after the birth, their business volumes are growing drastically. Such kind of gradual growth pattern has been typical in heavy industry such as electric power line. It takes so long time in replacement of old infrastructure. Strong motivations to use lasers and LEDs for displays are that they are both characterized in: (1) narrow-band, primary RGB colors; (2) high power and high efficiency; (3) compactness, and (4) to be potentially mass producible and low cost.
Section snippets
Recent advances of display technology
A look back at the history of consumer electronics (CE) industry in Japan reveals all sets of symbolic products in 1950s, 1960s and 2005 have display in common, indicating display has been the driver of CE industry in Japan.
Passive cooling challenges of displays
Fig. 13 plots typical CE products commercialized by Sony in double logarithmic scales of surface area (A) and power consumption. Assuming isothermal surfaces and temperature difference ΔT between the average surface temperature Ts-bar and ambient temperature Ta is 15 K, a line Qlimit defined bydivides the CE product plots into two groups, one subject to “fan cooling” and the other “passive cooling”. Since fan cooling is suffered from acoustic
Thermal challenges on light sources
Near-future high power lasers dissipate power in a range of 2–10 W/mm2, corresponding to 2–10 MW/m2. However, heat removal device for such high heat flux density has not been commercially available yet. Therefore, heat spreading becomes more and more important for maintaining device junction temperature sufficiently low. For the purpose of designing a spreader for time-dependant power input, transient thermal spreading model is introduced. In our model, a square heat source and a square spreader
Conclusions
In summary, solid-state light sources such as high-power LED's and lasers have big potentiality for next generation display sources.
Key performance indicators (KPIs) for such light sources are the internal/external luminous efficiency of LEDs for a backlight in a wide-spreading LCD-TV and low noise such as in a GxL device which is characterized in this paper. The result indicates GxL is very promising for a calibrated light source particularly useful in the dark region down to thermal noise
Acknowledgments
Authors would like to thank the chair and vice chair persons of Thermes II 2007, who encouraged us to publish our key note talk for this conference.
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