<p>A study was carried out on the transmission and reflection of 40-μm-thick polyimide films implanted with 40-keV iron ions at a high fluence of F₁ = 5∙10¹⁶ and F₂ = 1.25·10¹⁷ cm^–2 in the range 200–3000 nm. The integrated transmittance of these thin films decreases steadily with increasing fluence by more than three times, while the reflectivity traverses a maximum, increasing at low fluence and then decreasing to the initial value at high fluence. In addition, a broad reflection band with maximum at λ ≈ 1000 nm is observed in the implanted films associated with the formation of nanoparticles of iron and/or iron compounds. The narrow reflection bands characteristic for the original polyimide sample in the ultraviolet region are suppressed when reflected by the implanted side and shifted toward shorter wavelength when recorded from the nonimplanted surface of the film. These results were interpreted in the framework of a two-layer model specifying a carbonized surface layer with implanted metal and a polymer film layer unmodified by implantation. The determined Tauc optical gap of the carbonized layer is negative due to the predominance of the iron-containing phase and the refractive indices of the modified layer are n₁ = 2.5 and n₂ = 1.3 at low and high fluence, respectively.</p>