Filters are how a broadband lamp is shaped into a narrow experimental band. Choosing and combining them correctly is a small piece of optical engineering that has an outsized effect on data quality.
Long-pass filters transmit above a cutoff wavelength and block below it. In photocatalysis they are the standard way to remove UV from a xenon beam, isolating visible-light response. Key parameters are the 50% cutoff wavelength, blocking depth and transition steepness. A shallow transition means the stated cutoff is not the effective cutoff for a sharp catalyst absorption edge.
Band-pass filters transmit a defined window, characterized by center wavelength and FWHM. They are used to isolate a single absorption feature or to build multi-band experiments. Check out-of-band blocking across the full lamp spectrum, not just near the passband, because xenon lamps radiate everywhere.
Verify transmission at the working angle of incidence, typically specified for 0 degrees and degrading as the filter tilts; thermal durability under continuous irradiation; and filter diameter relative to the beam footprint, edge clipping shifts the effective spectrum.
Xenon plus UV cutoff filter for visible-light photocatalysis; band-pass filter for wavelength-specific activation; stacked long-pass plus short-pass to synthesize a custom band; and cold mirrors or IR filters to manage heat without touching the visible band.
Fluorescence: filters can re-emit absorbed light at longer wavelengths, contaminating a supposedly clean band. Aging: UV exposure gradually degrades blocking. Whenever the filter changes, re-measure the transmitted spectrum, do not trust the datasheet forever.
Perfectlight provides matched optical filters for its xenon, LED and mercury systems; see light source solutions for standard configurations.
