Organic dyes with photoluminescence in the second near-infrared window (NIR-II, 1000-1700 nm) are promising for bioimaging and optoelectronic devices. Photoluminescence quantum yield (PLQY) is a direct measure of their performance. Integrating sphere technology is effective in determining the absolute PLQY. However, the low PLQY values of most NIR-II organic fluorophores lead to significant measurement errors. Therefore, the most common method for PLQY determination is a relative approach using a photoluminescence spectrometer and a standard reference like IR-26. Although the relative method enables precise calculation of the PLQY ratio between the sample and the reference, the specific PLQY value of IR-26 is not clearly defined and is reported to range from 0.05% to 0.50%. Such a deviation can cause significant errors in relative PLQY measurements. In this study, it is reported that a bright organic fluorophore called TPE-BBT exhibits a high PLQY of 3.94% in THF, which can be accurately measured using a commercially available integrating sphere. Using TPE-BBT as a standard, the PLQY values of IR-26 in 1,2-dichloroethane and IR-1061 in dichloromethane are accurately determined to be 0.0284% and 0.182%, respectively. It is hoped that using this reliable standard will unify the evaluation criteria for NIR-II organic fluorophores.