When a reaction responds differently to different wavelengths, the light source must be configured around the absorption profile rather than chosen off the shelf. Two technical routes dominate: tunable-wavelength systems and fixed monochromatic sources.
A xenon or deuterium source coupled to a grating monochromator outputs a narrow, adjustable band. Typical bandwidths of several nanometers allow continuous scanning across the UV-visible range, which is exactly what action spectrum measurements need. The trade-offs are limited output power, long exposure times, and strict slit-width management, because bandwidth and intensity trade off directly.
Single-wavelength LEDs and laser sources deliver high photon flux at a fixed wavelength with simple operation. Their limitation is coverage: you can only buy the wavelengths that exist, and binning tolerances mean each unit should be individually verified with a spectrometer.
For a monochromator, calibration drift shifts the center wavelength over time; recalibrate periodically against known spectral lines. For LEDs, the peak wavelength shifts with temperature and drive current, by up to several nanometers in the blue range, which is not negligible for a sharp absorption feature.
Normalize photon flux at every wavelength before comparing conversions. Report center wavelength, bandwidth and intensity together. Shield the detector from stray light, and when a filter-based configuration is used, check for filter fluorescence, which can contaminate the intended band.
Perfectlight's portfolio includes laser sources, LED modules and filters that support both routes; the light source solutions page lists typical wavelength configurations.
