Ideally, the PLQY value can be determined by directly measuring the emitted and absorbed photons. This method obtains what researchers commonly refer to as absolute PLQY. The absolute method directly reflects the exact PLQY and thus receives increasing attention and importance. Since the work of Vavilov was published in 1924, researchers have never stopped searching for a method to accurately measure the absolute PLQY. After nearly 100 years of development, integrating sphere spectroscopy has emerged as the most reliable and mature technique (Figure ). This technology measures changes in both incident light (blue color, absorbed photons) and photoluminescence (orange color, emitted photons) before and after sample loading (Figure ). The PLQY can then be directly calculated from these measurements. Recently, integrating sphere setups have become commercially available. Especially, integrating spheres from Hamamatsu Photonics can detect photons in a wavelength range from 350 to 1650 nm, greatly facilitating the measurement of PLQY of NIR-II materials. Unfortunately, most NIR-II organic fluorophores have very low brightness with their PLQY far below 1% (dashed line in Figure ). This leads to difficulties in accurately measuring the number of emitted photons with an integrating sphere, resulting in a significant error in the PLQY calculation.