On the pad and screen lines, color drift isn’t just a cosmetic glitch—it’s a hard failure in the curing chemistry. When the UV spectrum doesn’t line up with the ink’s photoinitiator package, the film cures unevenly. You get surface skinning that hides uncured material underneath, and pigments shift shade under different spectral conditions. The fallout is rejects, reprints, and jobs that get scrapped. What actually matters on the bench We build lamp systems around spectral output, peak irradiance, and delivered energy density. A high-pressure mercury vapor lamp hits strong output around 365 nm, with additional lines at 313 nm and 436 nm. For many pad-print inks, that 365 nm is what drives deep cross-linking—the kind that locks pigment orientation and keeps opacity stable. With screen inks, where the deposit is thicker, we often shape the UV envelope so there’s enough long-wave UVA (385–405 nm) penetration. That keeps you from baking a brittle top layer while the bottom stays under-cured. The socket choice—G5 or G13—sets lamp diameter and arc length. Those two numbers then dictate reflector geometry, focal distance, and the irradiance profile across the substrate. Match the socket to the lamp, match the lamp spectrum to the ink’s photoinitiators, then match the optical assembly to the dwell envelope. Why this plays the way it does Pad printing lays down a thin, precise ink film, so color shift shows up fast when the curing front is inconsistent. Screen printing lays down a heavier coat, and that demands deeper cure. Surface-dry only leads to adhesion problems and color non-uniformity after die-cutting or assembly. Run the right G5 or G13 socket with the correct lamp spectrum, and cure depth stabilizes along with polymer network density. That keeps pigment distribution uniform, reduces metamerism under daylight and shop lighting, and cuts makeready because the first sheet is repeatable. Energy density comes through predictably, so you can push speed without chasing marginal cure. The details that bite you G5 and G13 sockets are not interchangeable. Lamp diameter and arc length are different, and the reflector focal line changes with the fixture. Keep an eye on lamp end-of-life behavior—output decay and ozone generation have to be managed if you want spectral stability. And don’t ignore lamp-reflector alignment. A small offset can move peak irradiance off the print window and bring color drift back, even when you’ve got the right lamp type.