An optical source unit comprising: multiple first optical sources (11-1, 11-2, 11-3) for emitting P-polarized light beams (P1, P2, P3); multiple first detectors (11a-1, 11-2a, 11-3a), each for detecting whether one of said first optical sources (11-1, 11-2, 11-3) is normal or abnormal; a second optical source for emitting a polarized light beam (P0'); and a second detector for detecting whether said second optical source is normal or abnormal; multiple light beam combining and splitting sections (13-1, 13-2, 13-3), provided on an output light path of second optical source, each adapted to receive one of said P-polarized light beams (P1, P2, P3), each of said light beam combining and splitting sections (13-1, 13-2, 13-3) comprising: a variable polarization element (13-1A, 13-2A, 13-3A) for changing a polarization direction of said polarized light beam (P0') or an output light beam of a pre-stage (13-1, 13-2) of said each of said light beam combining and splitting sections (13-2, 13-3); a polarization beam splitter (13-1B, 13-2B, 13-3B) adapted to split an output light beam (L1', L2', L3') of said variable polarization element (13-1A, 13-2A, 13-3A) into a P-polarized component (P1', P2', P3') and an S-polarized component (S1', S2', S3'), said P-polarized component (P1', P2', P3') being transmitted to a post stage (13-2, 13-3) of said each of said light beam combining and splitting sections (13-1, 13-2) or a light shield member, said polarization beam splitter (13-1B, 13-2B, 13-3B) being further adapted to combine said P- polarized light beam (P1, P2, P3) of a corresponding one of said first optical sources (11-1, 11-2, 11-3) and said S-polarized component (S1', S2', S3') of said variable polarization element (13-1A, 13-2A, 13-3A) into an output light beam (T1, T2, T3) of said optical source unit. multiple first optical sources (11-1, 11-2, 11-3) for emitting P-polarized light beams (P1, P2, P3); multiple first detectors (11a-1, 11-2a, 11-3a), each for detecting whether one of said first optical sources (11-1, 11-2, 11-3) is normal or abnormal; a second optical source for emitting a polarized light beam (P0'); and a second detector for detecting whether said second optical source is normal or abnormal; multiple light beam combining and splitting sections (13-1, 13-2, 13-3), provided on an output light path of second optical source, each adapted to receive one of said P-polarized light beams (P1, P2, P3), each of said light beam combining and splitting sections (13-1, 13-2, 13-3) comprising: a variable polarization element (13-1A, 13-2A, 13-3A) for changing a polarization direction of said polarized light beam (P0') or an output light beam of a pre-stage (13-1, 13-2) of said each of said light beam combining and splitting sections (13-2, 13-3); a polarization beam splitter (13-1B, 13-2B, 13-3B) adapted to split an output light beam (L1', L2', L3') of said variable polarization element (13-1A, 13-2A, 13-3A) into a P-polarized component (P1', P2', P3') and an S-polarized component (S1', S2', S3'), said P-polarized component (P1', P2', P3') being transmitted to a post stage (13-2, 13-3) of said each of said light beam combining and splitting sections (13-1, 13-2) or a light shield member, said polarization beam splitter (13-1B, 13-2B, 13-3B) being further adapted to combine said P- polarized light beam (P1, P2, P3) of a corresponding one of said first optical sources (11-1, 11-2, 11-3) and said S-polarized component (S1', S2', S3') of said variable polarization element (13-1A, 13-2A, 13-3A) into an output light beam (T1, T2, T3) of said optical source unit. a variable polarization element (13-1A, 13-2A, 13-3A) for changing a polarization direction of said polarized light beam (P0') or an output light beam of a pre-stage (13-1, 13-2) of said each of said light beam combining and splitting sections (13-2, 13-3); a polarization beam splitter (13-1B, 13-2B, 13-3B) adapted to split an output light beam (L1', L2', L3') of said variable polarization element (13-1A, 13-2A, 13-3A) into a P-polarized component (P1', P2', P3') and an S-polarized component (S1', S2', S3'), said P-polarized component (P1', P2', P3') being transmitted to a post stage (13-2, 13-3) of said each of said light beam combining and splitting sections (13-1, 13-2) or a light shield member, said polarization beam splitter (13-1B, 13-2B, 13-3B) being further adapted to combine said P- polarized light beam (P1, P2, P3) of a corresponding one of said first optical sources (11-1, 11-2, 11-3) and said S-polarized component (S1', S2', S3') of said variable polarization element (13-1A, 13-2A, 13-3A) into an output light beam (T1, T2, T3) of said optical source unit.