Previously, the nanocrystalline grain boundaries were often contaminated by the “non-diamond phase” with the photo-ionization threshold at 0.8 eV. Here, we present the optical spectra of the NCD films grown on transparent substrates by the microwave plasma enhanced chemical vapor deposition (CVD) at a relatively low temperature below 600°C. The transmittance and reflectance spectra are useful to evaluate the film thickness, the surface roughness and the index of refraction. The direct measurement of the optical absorptance by the laser calorimetry and photothermal deflection spectroscopy (PDS) provides high sensitive methods to measure the weak optical absorption of thin films with rough surface. The optical measurements indicate the high optical transparency of our standard, nominally undoped 0.2-0.3 μm thick NCD film with low non-diamond content. However, the optical scattering is rather high in UV and needs to be reduced.