Abstract: An illumination device using an innovative design to provide a uniform, highly concentrated and substantially longitudinal illumination. The device includes, at least one light source unit, a compound light guide with a built-in light-extracting feature and a reflective envelope. The compound light guide comprises two optically coupled sub-guides. A first sub-guide has a constant cross-section area with a profile optimized for an integral light-concentrating optics, and a second sub-guide has a varying cross-section area for controlling local light flux density inside the light guide and providing assembly means. Said light extracting feature having variable light extraction efficiency improves illumination uniformity at an area close to a light input end of the light guide without displacing a light source from the central normal line of a light-extracting feature.