A wavelength division multiplexer and etendue conserved optics are used to combine multiple wavelength LED lights into a combined light. The combined light, with higher intensity and higher power than the light from an individual LED, is used to excite a wavelength conversion material such as phosphors to generate a high brightness and high power light. Light generated by multiple LEDs of the same wavelength may be coupled into a optical fiber bundle before inputting it into the wavelength-division multiplexer, further increasing the brightness and power. The wavelength conversion material may generate light of three different color under excitation by different LED lights, or a white light with higher brightness and higher power. Such a light source can be used in image display devices such as a projector or in illumination systems.