Patent ID: 8474981

Claim:
An illumination optical system comprising: first and second lamp units each including a lamp and a concave mirror, the lamp having an anode and a cathode arranged along the optical axis of the concave mirror; a light combining portion that combines a light beam from the first lamp unit and a light beam from the second lamp unit, the light combining portion including a first reflecting portion located on the optical axis of the first lamp unit and off the optical axis of the second lamp unit and a second reflecting portion located off the optical axis of the first lamp unit, the light combining portion combining the light beams by reflecting light emitted from a part of the second lamp unit off the optical axis of the second lamp unit with the first and second reflecting portions; at least one light returning portion disposed between the first lamp unit and the light combining portion, or between the second lamp unit and the light combining portion, or both, the or each of the at least one light returning portion dividing a part of light emitted from the corresponding lamp unit into a light beam to be returned to the lamp and a light beam to be led to the light combining portion; and a condenser lens that condenses light from the light combining portion, wherein when, in a cross-section containing the optical axes of the first and second lamp units, two peaks of light quantity distribution along a line intersecting the optical axis of the first lamp unit in the opening plane of the first lamp unit are referred to as first and second peaks, and two peaks of light quantity distribution along a line intersecting the optical axis of the second lamp unit in the opening plane of the second lamp unit are referred to as third and fourth peak, the light combining portion has in order from one end to the other end: a first transmitting portion transmitting light corresponding to the first peak; the first reflecting portion located on the optical axis of the first lamp unit and reflecting light corresponding to the fourth peak; second transmitting portion located on the optical axis of the second lamp unit and transmitting light corresponding to the second peak; and the second reflecting portion reflecting light corresponding to the third peak, and wherein the at least one light returning portion includes a first light returning portion disposed between the first lamp unit and the second reflecting portion and a second light returning portion disposed between the second lamp unit and the first transmitting portion.