monochromator
/ mon-oh-KROH-may-tor /
A glass prism splits white sunlight into a rainbow fan; if you then place a narrow slit in front of that fan, you can let just one pure colour pass. A monochromator is essentially that trick built into an instrument: it carves a single chosen colour out of a broad mixed-up beam of light.
Inside a spectrophotometer, the monochromator uses a prism or, more commonly, a diffraction grating to spread incoming light into its component wavelengths, and a slit to select a narrow band around the wavelength the operator wants. Turning the grating scans the selected colour smoothly across the spectrum.
The monochromator is what makes Beer-Lambert measurements trustworthy, because the law assumes a single wavelength. In practice the slit passes a small band, not a perfect single colour; a narrower slit gives purer light and truer absorbances but lets through less light, so there is always a trade-off with signal strength.
To measure a green-absorbing dye, the operator turns the monochromator's grating until the slit passes light near 520 nm, then records the absorbance at that single wavelength.
Rotating the grating selects which single wavelength reaches the sample.
A diode-array instrument flips the order: it sends all colours through the sample first and disperses them onto a detector afterwards, so it has no scanning slit before the cell.