In a two-monochromator setup, which option best describes the purpose of the second monochromator?

• A. To increase excitation energy.
• B. To block all stray light.
• C. To minimize stray light and improve signal-to-noise by blocking stray light from reaching the detector.
• D. To calibrate the instrument.

Answer: (C)

In a two-monochromator arrangement, the key idea is to dramatically reduce stray light by adding a second stage of wavelength filtering. The second monochromator acts as an extra barrier that only passes the desired wavelength while rejecting off-band light, including second-order or scattered light, before it reaches the detector. This tightens the spectral purity and improves the signal-to-noise ratio without changing the excitation energy or serving as a calibration step.