Patent ID: 7588352

Claim:
An optical apparatus that comprises a short-arc type discharge lamp with a pair of electrodes that face each other within a discharge vessel and a concave reflecting mirror placed to surround the discharge lamp with its beam axis matching the arc direction of the discharge lamp, in which the concave reflecting mirror comprises a front elliptical reflecting mirror portion, a middle spherical reflecting mirror portion, and a rear elliptical reflecting mirror portion; the front elliptical reflecting mirror portion and the rear elliptical reflecting mirror portion have at least their first focal points collocated between the electrodes, are positioned front and back in relation to the direction of light radiation of the concave reflecting mirror, the middle spherical reflecting mirror portion being located between the front elliptical reflecting mirror portion and the rear elliptical reflecting mirror portion with the first focal points mentioned above as the center position CP; the relationship between an angle α formed by a virtual tangential line VTL, which runs from the center position CP to the outer surface of the electrode positioned towards the neck of the concave reflecting mirror, and the direction in which the electrode of the discharge lamp extends and an angle β formed by a virtual straight line VSL, which runs from the center position CP to the boundary position between the middle spherical reflecting mirror portion and the rear elliptical reflecting mirror portion, and the direction in which the electrode of the discharge lamp extends is β>α; and the relationship between the volume V (in mm 3 ) of the one of the pair of electrodes that is positioned opposite the direction of light radiation of the concave reflecting mirror and the wattage (P) during steady lighting meets the condition, 0.07×EXP(0.014×P)<V.