Isochronal annealing experiment was conducted on poly allay diglycol carbonate (PADC) commercially known as a CR-39 detector. The detector films were subjected thermal treatment at 130, 140, 150 and 160 °C for annealing time of 8 h. The qualitative impacts of these treatments have been analyzed by means of ultraviolet/visible (UV/Vis) spectrophotometry in the range 190–800 nm, considering optical properties such as absorbance, band-gap energy, and Urbach energy. As control, CR-39 detector film non-subjected thermal treatment (pristine film) was also examined. This paper shows how the well-controlled thermal treatment of the PADC detector can alter its optical properties in the near-surface layer of thickness beyond a couple of hundreds of nanometers. This finding is more pronounced for cases of higher annealing temperatures and larger annealing time intervals. The absorbance, direct and in-direct band gaps, Urbach energy and carbon atoms per conjugated length proved to be highly affected by increasing the thermal annealing temperature ranging from 130 to 160 °C for 8 h annealing period of time.