A clear dose-response relationship has been demonstrated between the irradiance and the effectiveness of phototherapy. This study is designed to detect if there is any difference in irradiance between the inside and outside of different types of incubators. Forty-four incubators of two different types and with ages up to 18 years were included. In comparison with the measurement taken outside an incubator, the mean spectral irradiance detected within a single-walled incubator was significantly decreased (p < 0.001) while that detected within a double-walled incubator was slightly increased (p = 0.014). The reduction of spectral irradiance within an incubator correlated with the degree of incubator wall damage with a maximal loss of 15%. However, the irradiance did not correlate with the age of the incubator. In conclusion, the spectral irradiance received within an incubator was influenced by the type of incubator and the wall damage. Measuring the irradiance within an incubator prior to phototherapy should be emphasized, especially for those incubators with severely damaged walls.