An instrument designed to measure the radiant flux (radiant power) in the spectral range of 400 to 500 nm (i.e., blue) during bilirubinemia treatment for newborns and infants. It typically includes a pre-filter intended to remove wavelengths of light not in the 400-500 nm range (e.g., infrared light); a primary detector consisting of a temperature-stabilized, solid-state [e.g., selenium (Se) or indium-gallium-arsenide] device used to detect radiation; electronic circuits including an amplifier and a electric meter; a power source (e.g., a battery); and a display showing the results either in analogue or digital format.