A significant correlation between the track density measured by CR-39 and the affluence has been found in the current study. This correlation can be used to calculate the expected track density in solid state nuclear track detector irradiated by any alpha source of known activity without using the detectors. Nine pieces of CR-39 were irradiated with alpha particles from Am-241 (10 μCi) point source for 0 s, 10 s, 20 s, 30 s, 40 s, 50 s, 60 s, 70 s and 80 s. The absorbance in UV-visible region (before and after etching of the detector) and in the energy band gap showed an acceptable correlation at 650 nm wavelength. This result leads to two semi-empirical equations: first equation between the track density and the absorbance; and second equation between the absorbance and the affluence. These findings can give the track density or the affluence by calculating the absorbance of the detector after etching without microscopic viewing and calibration.