| 摘要 |
PROBLEM TO BE SOLVED: To obtain a filter which can attenuate the incident light at a high attenuation rate, which can decrease the production of ghost light, and which can be easily assembled in a conventional rotary attenuator by disposing a reflection-type ND filter on the back face of an absorption type ND filter and disposing the absorption type ND filter facing the incident light side. SOLUTION: A reflection type ND filter 24b is disposed on the back face of an absorption type ND filter 24a, and the absorption type ND filter 24a is disposed facing the incident light side. The transmittance Td of the absorption type ND filter 24a is controlled to 1/10 to 1/102. By this method, ghost light can be reduced to 1/102 to 1/104 of the incident light and most of the ghost light can be eliminated. The incident light from the objective face for the measurement is successively attenuated by the transmittance Td of the absorption type filter 24a and by the transmittance Tr of the reflection type ND filter 24b, the light can be attenuated as a whole at a attenuation rate as high as Td×Tr. Further, since the thickness of the filter can be decreased, the obtd. filter can be easily assembled in a conventional rotary attenuator.
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