Results from absorption measurements in semi-insulating 4H-SiC HTCVD-grown substrates show that the infrared absorption spectra is dominated by the UD-1 defect. The UD-1 spectrum has two sharp lines around I eV. The activation energy of the T-dependence of the resistivity in the semi-insulating material, which has a strong absorption by the UD-1 defect, is determined to be 1.4 eV. Photo-induced absorption measurements show that the absorption of the UD-1 defect increases when the sample is illuminated with 1.4 eV. When the energy of the exciting light reaches 1.85 eV the absorption intensity starts to decrease. In this way we have been able to correlate the electrical behavior (with a thermal activation energy of 1.4 eV) of the HTCVD 4H-SiC material with the UD-1 defect which we observe in luminescence and absorption measurements.