A second detection beam is received by a reference illuminance monitor (second sensor) without passing the second detection beam through a liquid-repellent film, and the output (Pref) of the reference illuminance monitor corresponding to the amount of light of the received beam is acquired (step 322). A first detection beam is received by a first sensor after passing the beam through a liquid-repellent film, and the output (Pi) of the first sensor corresponding to the amount of light of the received beam is acquired (step 306). From the output of the first sensor and the output of the reference illuminance monitor, correction information δ (or Ϝ) used to correct the output of the first sensor is acquired (steps 324 to 332). Correct energy information not influenced by the variation of the beam transmittance of the liquid-repellent film is collected.
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