An optical filter used for modifying the spectral power distribution of high color temperature lamp to simulate CIE standard daylight illuminant. The optical filter comprising a heat-absorbing glass plate, a heat insulation washer, a longpass glass plate, and one or more spectral modification glass plate in series. The longpass glass plate and the heat absorbing glass plate determine the wavelength range, and the spectral modification glass plate(s) modify the spectral power distribution of the high color temperature halogen lamp. The CIE daylight simulators with the optical filter have very high colorimetric indexes.