This study examined the effect of UV-spectrum light soaking on solar cell degradation tests. An indoor light soaking test was evaluated over three different spectral ranges: "UV only", "UV blocked" and "Full spectrum". a-Si: H and poly-crystalline silicon solar cell technologies were studied by light soaking tests with the same optical filter configurations. The I-V measurement results demonstrated that "UV only" irradiated solar cells exhibited the smallest output power degradation, which was only half of a percent variation compared with the full spectrum light soaking case. Using a filter that excluded the effect of the UV spectral range on light soaking, the "UV blocked" case also exhibited a significant output power degradation of the solar cells. A comparative analysis of the solar cell response, based on the I-V characteristics and the diode ideality factor under these different light soaking spectra, demonstrated the importance of the full spectrum light soaking test in the evaluation of the long-term performance of solar cells. (C) 2013 Elsevier B.V. All rights reserved.