Patent ID: 7733461

Claim:
An exposure apparatus comprising: (a) an illumination optical system for illuminating a reticle with exposure light; (b) a projection optical system for projecting a pattern of the illuminated reticle onto a plate via a liquid supplied to a space between said projection optical system and the plate; (c) a supply pipe; (d) a supply unit configured to supply the liquid to the space via the supply pipe; (e) a recovery pipe; (f) a recovery unit configured to recover the liquid from the space via the recovery pipe; and (g) a measuring unit configured to measure a refractive index of the liquid, wherein said measuring unit includes: (i) a light source for generating measurement light having the same wavelength, repetition rate and energy output as those of the exposure light; (ii) a liquid reservoir for storing the liquid, the liquid reservoir being disposed apart from the space via the supply pipe or the recovery pipe, and having a transmitting surface for transmitting the measurement light and a reflecting surface for reflecting the measurement light transmitted by the transmitting surface and the liquid in the reservoir; and (iii) a detector for detecting the measurement light reflected on the reflecting surface and transmitted by the liquid in said liquid reservoir and the transmitting surface.