Conventionally, white light sources, such as a cold cathode tube and a high pressure mercury lamp, have been utilized as light sources for image display devices. In a direct view image display device such as a liquid crystal monitor, red, green and blue colors of light that is wavelength-selected by a color filter, which is made of a material that absorbs unnecessary wavelength light from white light from the light source, is guided to liquid crystal elements for red, green and blue signals.
Further, in a projection type image display device such as a projector, white light is separated into red, green and blue colors of light with the use of a dichroic mirror or a dichroic filter having a wavelength selection function using a thin-film technique, and modulated with the use of a liquid crystal panel or a mirror device, thereby displaying images.
In the light sources as described above, however, a wavelength band that can be used for each color of light becomes narrow in order to improve the color reproduction characteristics of each color of light, and as a result, a light utilization efficiency declines significantly. Further, in the case of high pressure mercury lamps in particular, they have maintenance-related problems, such as a short life span (several thousand hours) and the possibility of bursting.
For these reasons, liquid crystal televisions, projectors, etc. using light-emitting diodes (hereinafter referred to as LEDs) as solid light sources as individual light sources for red, green and blue have become commercially available in recent years. Although the light output of LEDs may not be sufficient, the level of chromaticity of each color of light is unique, which may not be achieved by conventional products.
On the other hand, the use of LEDs involves the following problems. High-brightness LEDs generate a large amount of heat, so that in order to obtain long life spans, a high-performance cooling unit is essential. Further, among red, green and blue LEDs, a red LED is different from green and blue LEDs in a degree of variation in light conversion efficiency and emission spectrum in response to a variation in the junction temperature.
FIG. 6 is a diagram showing characteristics of red, green and blue LEDs disclosed by Lumileds Inc. The characteristics in the drawing show that the light conversion efficiency of each LED changes in response to a variation in each junction temperature. Further, as can be seen from the characteristics in the drawing, the variation of red is larger than those of green and blue. This means that even if white balance is adjusted once, a color gamut occurs due to a significant change in output of each color of light when the amount of heat generation changes as a result of any factor, such as a change in ambient temperature, thereby making desired color reproduction difficult.
Therefore, as shown in FIG. 7, Patent document 1 discloses that a Peltier device as a thermoelectric element is used for a cooling means for an LED 60. A Peltier device generates or absorbs heat when a current is fed to the junction between two thermoelectric materials (bismuth and tellurium). The Peltier device shown in FIG. 7 has a configuration in which N-type and P-type semiconductors 61 and 62 are joined to each other through copper electrodes 63 and 64, and on the outside of the electrodes, ceramics 65 and 66 as electrical insulators having relatively favorable thermal conductivity are placed respectively. The LED 60 as the object to be cooled is joined to the ceramic 65, and a heat sink 67 for dissipating heat is jointed to the ceramic 66.
By properly using the Peltier device and controlling currents, the LED connected to the Peltier device can be cooled or it can be maintained at a certain temperature. By increasing the number of pairs of the semiconductors or increasing the size of the element itself, it is possible to increase the power with which the Peltier device can be operated.    Patent document 1; JP 2005-121890 A
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