Introduction: A range of phototherapy devices are commercially available. The American Academy of Pediatrics (2004) recommends routine intensity measurement of phototherapy devices to ensure that babies affected by hyperbilirubinemia receive effective phototherapy. Objective: The aims of this study were to calculate the irradiance decay velocity of phototherapy devices used in a tertiary care hospital to evaluate whether current maintenance procedures for phototherapy devices are effective, and to contribute to the improvement of a standardized maintenance procedure in daily practice, thus helping to ensure that all babies affected by hyperbilirubinemia receive prompt treatment. Methods: This research represents a prospective observational study conducted at Dr. Soetomo Academic Teaching Hospital in Surabaya, Indonesia from February 2019–July 2019. The intensities of 11 phototherapy devices were measured at specific times using a Bili Blanket Meter II. We calculated the Δ irradiance differences in μW/cm2/nm and calculated them as velocity μW/cm2/nm/hour of use. Results: Among the 11 phototherapy devices included in this study, nine were fluorescent and two were light-emitting diode (LED) machines. The mean (standard deviation) irradiance decay velocity of the fluorescent lamps was 0.02 (±0.03) μW/cm2/nm/hour of use, while that of the LED lamps was 0.015 (±0.007) μW/cm2/nm/hour of use. The fastest irradiance decay velocity was 0.08 μW/cm2/nm/hour of use, while the slowest irradiance decay velocity was <0.01 μW/cm2/nm/hour of use, both of which were from fluorescent-based devices. There was one fluorescent-based device that provided an intensity lower than the therapeutic level. Conclusion: Irradiance decay occurred in all phototherapy device lamps. It is important to perform routinely intensity measurements, regardless of manufacturer recommendations, to avoid ineffective phototherapy resulting from intensities lower than the required therapeutic levels.