Rapid growth methods used to synthesize potassium dihydrogen phosphate crystals (KDP) and their partially deuterated form (DKDP) have been shown to increase the level of their impurities. These impurities may have a strong impact on the optical performance of such crystals. Transmission measurements have thus been performed on rapidly grown KDP crystals in order to evaluate the influence of impurities on the crystal transmission properties. The latter data have been compared with a DKDP transmission spectrum. A difference appears that can be explained by an analogy with the transmission spectra of water and deuterated water. This study also enables us to obtain information about the value of the optical band gap.