Background: Phototherapy (PT) is the standard treatment of neonatal unconjugated hyperbilirubinemia. The irradiance footprint, i.e., the illuminated area by the PT device with sufficient spectral irradiance, is essential for PT to be effective. Irradiance footprint measurements are not performed in current clinical practice. We describe a user-friendly method to systematically evaluate the high spectral irradiance (HSI) footprint (illuminated area with spectral irradiance of ≥30 μW cm−2 nm−1) of PT devices in clinical practice. Materials and methods: Six commercially available LED-based overhead PT devices were evaluated in overhead configuration with an incubator. Spectral irradiance (µW cm−2 nm−1) and HSI footprint were measured with a radiospectrometer (BiliBlanket Meter II). Results: The average measured spectral irradiance ranged between 27 and 52 μW cm−2 nm−1 and HSI footprint ranged between 67 and 1465 cm2, respectively. Three, two, and one PT devices out of six covered the average BSA of an infant born at 22, 26–32, and 40 weeks of gestation, respectively. Conclusion: Spectral irradiance of LED-based overhead PT devices is often lower than manufacturer’s specifications, and HSI footprints not always cover the average BSA of a newborn infant. The proposed measurement method will contribute to awareness of the importance of irradiance level as well as footprint measurements in the management of neonatal jaundice. Impact: While a sufficient spectral irradiance footprint is essential for PT to be effective, some PT devices have spectral irradiance footprints that are too small to cover the entire body surface area (BSA) of a newborn infant.This study introduces a user-friendly, accessible method to systematically evaluate the spectral irradiance level and footprint of PT devices.This study supports awareness on the role of the spectral irradiance footprint in the efficacy of PT devices. Irradiance footprint can be easily measured during phototherapy with the proposed method.