We present the study on optical quality of nanocrystalline diamond (NCD) films with submicron grain size grown from H2/CH4/CO2 gas mixture on alkali-free borosilicate glass substrates using the commercially available large area linear antenna pulsed microwave plasma chemical vapor deposition (CVD) system. The NCD films were characterized by Raman spectroscopy, atomic force microscopy, and scanning electron microscopy. The NCD layers are optically transparent and show low optical scattering. The defect states measured by photo-thermal deflection spectroscopy (PDS) and dual beam photocurrent spectroscopy (DBPS) are dominated by the non-diamond carbon phases localized at grain boundaries with the characteristic shape of the optical absorption spectrum and the photo-ionization threshold in a near infrared region. The sp2-related defect density increases for samples grown at higher pressure. From DBPS we estimate the concentration of substitutional nitrogen to be below 10 ppm. The optimized NCD layers show the photoionization threshold in UV region above 5 eV and a broad exponential tail below 5 eV.