Patent ID: 6956640

Claim:
An illumination system of apparatus for use in exposing a substrate, the system comprising: a diaphragm having an adjustable aperture located along an optical axis of the system; a photoelectric transformation unit having a plurality of photoelectric transformation devices (PTDs) each of which is capable of sensing the energy level of light incident thereon, and the photoelectric transformation unit being operative to output signals indicative of the locations and energy levels of respective portions of light incident on the photoelectric transformation devices thereof: a drive mechanism connected to and supporting said photoelectric transformation unit and operable to move the photoelectric transformation unit between a first position at which the photoelectric transformation unit is juxtaposed with the aperture of the diaphragm along the optical axis of the illumination system and a second position at which the photoelectric transformation unit is offset from the aperture with respect to the optical axis of the illumination system; and a controller operatively connected to said photoelectric transformation unit and said diaphragm so as to receive the signals output by the photoelectric transformation unit and set the size of the aperture of the diaphragm based on said signals.