An optical system (200, 200d, 200e, 200f), comprising: a light source configured to emit an illumination beam; a filtering module disposed on a transmission path of the illumination beam and comprising a plurality of filtering units (222, 224, 226) of different colors, the filtering units (222, 224, 226) moving into the transmission path of the illumination path in turn, wherein each of the filtering units (222, 224, 226) is adapted to reflect a part of the illumination beam and to allow another part of the illumination beam to pass through; a reflection sleeve disposed between the light source and the filtering module, wherein the illumination beam is transmitted in the reflection sleeve from the light source to the filtering module; a wavelength conversion unit (260a, 260b, 260c, 260d) disposed on a transmission path of the part of the illumination beam reflected by at least one of the filtering units (222, 224, 226) and configured to convert the part of the illumination beam reflected by the at least one of the filtering units (222, 224, 226) into a converted beam (216a, 216b, 216c), wherein the wavelength conversion unit (260a, 260b, 260c, 260d) is disposed on an inner wall of the reflection sleeve, the reflection sleeve being adapted to reflect the converted beam (216a, 216b, 216c) to the at least one of the filtering units (222, 224, 226), such that at least a part of the converted beam (216a, 216b, 216c) passes through the at least one of the filtering units (222, 224, 226), characterized in that the reflection sleeve and the wavelength conversion unit (260a, 260b, 260c, 260d) are configured to form a hollow sleeve through which said illumination beam is transmitted. a light source configured to emit an illumination beam; a filtering module disposed on a transmission path of the illumination beam and comprising a plurality of filtering units (222, 224, 226) of different colors, the filtering units (222, 224, 226) moving into the transmission path of the illumination path in turn, wherein each of the filtering units (222, 224, 226) is adapted to reflect a part of the illumination beam and to allow another part of the illumination beam to pass through; a reflection sleeve disposed between the light source and the filtering module, wherein the illumination beam is transmitted in the reflection sleeve from the light source to the filtering module; a wavelength conversion unit (260a, 260b, 260c, 260d) disposed on a transmission path of the part of the illumination beam reflected by at least one of the filtering units (222, 224, 226) and configured to convert the part of the illumination beam reflected by the at least one of the filtering units (222, 224, 226) into a converted beam (216a, 216b, 216c), wherein the wavelength conversion unit (260a, 260b, 260c, 260d) is disposed on an inner wall of the reflection sleeve, the reflection sleeve being adapted to reflect the converted beam (216a, 216b, 216c) to the at least one of the filtering units (222, 224, 226), such that at least a part of the converted beam (216a, 216b, 216c) passes through the at least one of the filtering units (222, 224, 226), characterized in that the reflection sleeve and the wavelength conversion unit (260a, 260b, 260c, 260d) are configured to form a hollow sleeve through which said illumination beam is transmitted.