Objective：We propose an ex-situ method to determine the optical constants of aluminum thin films precisely. Comparing to the custom in-situ method

our method doesn’t need the complex optical measurement apparatus attached to vacuum chamber and can be performed simply and fast. Method：Using multi-wavelength fitting method

we fit the ellipsometric and photometric spectra with a combined optimal algorism of genetic and least square. The oxide layer formed on the aluminum in air is included in the fitting process. Result：The optical constants in the UV to visible range are obtained by this method within an error of 0.5%. Conclusion：With these data known

we design and deposit an UV induced transmission filter of one cavity. The tested transmission spectra are well consistent with those designed with a peak transmittance error of 1% at 265nm.