A lamp unit (3) for use with digital micromirror devices has a direct current short arc discharge lamp (1) in which at least 0.15 mg/mm.sup.3 of mercury is sealed. A short focal length, elliptical reflector mirror (2) both covers the discharge lamp (1) and positions the axis of radiation roughly in line with the long axis of the discharge lamp, such that the short arc discharge lamp (1) is placed with the cathode (14) in a front opening of the reflector mirror (2)