Under UV irradiation, poly(vinyl cinnamate) (PVCi) films undergo cross-linking due to [2 + 2] photocycloaddition between cinnamoyl groups. Consequently, electronic configuration of lateral polymer groups changes and also leads to the drop of the refractive index in the visible range. This mechanism is studied with regards to the level of the electronic absorption of cinnamate centered at 278 nm. It acts on refractive index by shifting the absorption UV band. Most of the time, this influence vanishes in the near infrared region (NIR). It is due to antagonist phenomena acting at the expense of the index lowering. The causes analysis leads us to optimize the irradiation spectrum, providing higher index contrasts between irradiated and nonirradiated area. We show that strong index changes can be achieved in PVCi film, up to 2.5 × 10-2 even in the NIR, enabling us, for example, to perform optical photoprinting waveguides for telecommunication wavelengths (1300−1550 nm).