Projection systems require a light source that is both compact and high in brightness. Image projectors use light emitted from a lamp to form an image by means of an image-forming element on the basis of image data from a personal computer and video camera, and projecting and displaying that image on a screen or display. An image projector comprises a lamp unit to project light and an image projection unit to project the images formed by the image forming unit, using the light from the lamp unit. The lamp body consists of a lamp bulb supplied with electric power for emitting light, and a reflector fixed integrally to this lamp bulb for reflecting and collecting the light. The projectors have a light source device having a light source lamp (light-emitting tube), a reflector that reflects a light beam irradiated by the light source lamp and a lamp housing that houses the light source lamp and the reflector. Projectors have a plurality of optical modulators for modulating light beams emitted from a light source according to image information and forming an optical image. The projector modulates a light beam irradiated by a lamp unit in accordance with image information to form an optical image and to enlarge and project the optical image.

Many projection systems require a separate source of bright light, and frequently employ high intensity discharge (HID) lamps, such as a metal halide lamp, a xenon lamp, an extra-high voltage mercury lamp, or devices. HID arc lamps have many favorable attributes, such as high intensity, efficiency, and reliability. HID arc lamps produce a point source of intense polychromatic light. Placing the HID arc lamp adjacent to an ellipsoidal reflector allows focusing of the intense polychromatic light with high precision onto a color wheel. Image light projectors, especially those used for projecting a still or moving image onto or through a large screen, typically use high power arc lamps, that operate at 5000 to 7000 watts and higher. High-pressure mercury vapor discharge lamps are commonly utilized for a light source of such optical devices as a liquid crystal projector, with the lamp being positioned so that the light emitting region (arc) is located at the focal point of a reflector mirror such as a parabolic mirror. A discharging lamp is high in intensity and efficiency, but requires a high-voltage power supply circuit. HID arc lamps typically take some time to warm up, after power on, before achieving their full brightness. During this initial (post power on) period, the brightness of the projected images gradually increases. A solid-state light emitting device including a light emitting diode (LED) may be used as an alternative lighting source. The LED has characteristics of being ultra-compact and ultra-light, and having a long life.

Because the light source, the supply circuit, and the lamp drive circuit are heat sources that generate heat in operation. Projectors are usually equipped with a cooling system that takes in cooling air from the exterior of the casing to cool the components in the casing. High-intensity discharge arc lamps are high-intensity, high-temperature light sources. A typical example of such a light source is the high-pressure mercury vapor lamp, which has an internal pressure of 200 atmospheres and an operating temperature of 1000 degrees Celsius. The cooling systems are generally classified into an optical-system cooling system, a light-source cooling system, a power-supply and lamp-drive-circuit cooling system.