Spectroscopy and photometric error
Start with a reading about the types of errors that arise in spectroscopy. Clearly photometric error $\delta I_{pe}$ is a device uncertainty. The suggestion is, its magnitude depends on the number of "beams". What changes as the number of beams increases? Path length? Number of gratings or optical components? What else? How do each of those factors contribute to an analytical expression for photometric uncertainty?

At the end, a reasonable result might suggest that, as you double the number of beams, you double the relative uncertainty in signal $\delta I_{pe} / I$, where $I$ is the signal intensity. To obtain the same absolute confidence, you therefore have to double the measurement intensity $I$.