An alternative method is the relative method, which measures the PL spectra of a given sample and a specific reference with a known absolute PLQY (Figure ). PL spectrometer is believed to be more sensitive to the emitted photons than the integrating sphere. Thus, the relative method can accurately measure the ratio of PLQY more accurately even if it is less than 1%. The fundamental principle of the relative quantum yield method states that when two solutions absorb an equal number of photons, the ratio of their integrated fluorescence intensities under identical measurement conditions (excitation wavelength, excitation bandwidth, emission bandwidth, and integration time) corresponds to the ratio of their PLQYs (Figure ). Then, the relative PLQY of the sample can be calculated using Eq. 2. The subscripts S and R represent the sample and reference, respectively. ϕR is the known PLQY of the reference standard, I is the integrated PL intensity, A is the absorbance of the solution at the excitation wavelength, and n is the refractive index of the solvent used. Notably, When the absorbance value is small, the ratio of absorbed photon numbers in the equation can be approximated by the ratio of the absorbance values. Therefore, Eq. 2 can be rewritten as the commonly used formula Eq. 3. In principle, only a single solution of the sample and reference should be enough. However, to reduce the error, multiple solutions of the sample and reference at different concentrations are commonly required. Then Eq. 3 can be rearranged into a linear equation Eq. 4. By plotting IS versus AS and IR versus AS, the ratio of the "gradients" can be used to calculate the PLQY of the sample (Figure ).