| 摘要 |
PURPOSE:To detect variation in optical characteristics at any time by making polarized luminous flux for monitoring incident on an image formation optical system and photodetecting it, and detecting the variation in optical characteristics of the image formation optical system. CONSTITUTION:Polarized luminous flux MB passed through a polarizing plate 16 and a mirror 18 is also incident on the image formation optical system L of a projection optical device and then photodetected by a photosensor 24 which performs photoelectric conversion through a mirror 20 and a polarizing plate 22. The output of this sensor 24 is supplied to an arithmetic device 26 to detect variation in optical characteristics of the optical system L due to temperature variation, etc., at any time. Feedback control based upon the detection is performed to correct the variation in optical characteristics.
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