Abstract: A method for modifying optical properties of a synthetic resin increases a transmittance by modifying, removing, or reducing factors which absorb wavelengths in an ultraviolet range, such that it is possible to maintain stable and excellent reliability even after a long-term use. In a method for modifying optical properties of a synthetic resin, a modification treatment which is an energy irradiation treatment is performed on a cured product made of a synthetic resin to increase a transmittance in an ultraviolet range. For example, the modification treatment which is at least one of a photo-oxidation treatment and a thermal oxidation treatment is performed on the cured product made of the synthetic resin to increase the transmittance.

Abstract: An ultraviolet LED optical member has high transmittance in an ultraviolet range, is stable even by long-term use and has excellent reliability. The ultraviolet LED optical member exhibits optical characteristics in which the average transmittance at wavelengths from 200 to 300 nm is 50% or more by having an optical member made of an organic resin. The optical member for this ultraviolet LED optical member preferably exhibits optical characteristics in which the transmittance of at least any wavelength from 200 to 250 nm is at least 60%.