Which instrument is known to require a highly regulated DC power supply?

The instrument that requires a highly regulated DC power supply is a spectrophotometer with a photomultiplier tube. This is because photomultiplier tubes (PMTs) are highly sensitive detectors that amplify the signal produced by incoming photons. Their operation is based on the photoelectric effect, and they convert light into an electrical signal through a series of dynodes that amplify the number of electrons generated by the initial photon interaction.

For optimal performance, PMTs require stable and consistent operating conditions, including a regulated DC power supply to maintain constant voltage levels. Variability in power supply can significantly affect the amplification process, leading to inaccurate readings and decreased sensitivity. Therefore, using a highly regulated DC power supply ensures precise and replicable measurements in spectrophotometry when utilizing a PMT.

Other options involve different detection methods or technologies that may not require such stringent power supply regulation. For instance, colorimeters generally use simpler detection mechanisms that don't need the same level of sensitivity and regulation as photomultiplier tubes, while other detectors mentioned could operate effectively with less stringent power requirements.