Hey there Catarina Viola!

Quantum yields (QYs) and external quantum efficiencies (EQEs) are both crucial in understanding the efficiency of a photonic process, but they measure slightly different aspects.

Quantum yield (QY) refers to the efficiency of a photochemical reaction or process in converting absorbed photons into useful products, such as fluorescence or phosphorescence. It's expressed as the ratio of the number of photons emitted to the number of photons absorbed.

On the other hand, external quantum efficiency (EQE) measures the efficiency of a photonic device, such as a solar cell or LED, in converting absorbed photons into electrical current or light output, respectively. It's calculated as the ratio of emitted photons to incident photons.

Now, when it comes to measurement methods, both relative and absolute (integrating sphere) methods are used.

The relative method involves comparing the emission of a sample to a standard of known quantum yield or external quantum efficiency. This method is often simpler and faster but may lack accuracy.

The absolute method, using an integrating sphere, directly measures the total emitted photons from the sample and compares it to the number of absorbed photons, giving a more precise result.

However, it's essential to note that using measured values to calculate radiative constants might not always be accurate. This is because radiative constants involve other factors such as refractive index, cavity effects, and non-radiative processes, which may not be fully accounted for in these measurements.

In summary, while both QYs and EQEs provide valuable insights into the efficiency of photon conversion, their measurement methods and applicability in calculating radiative constants require careful consideration.