What's Actually Happening Inside the Lamp?
A mercury-vapor lamp is a gas-discharge bulb — an electrical arc excites mercury vapor inside a sealed tube, and that arc has to build up to a stable internal temperature near 800°C before the lamp puts out a consistent, usable UV dose, which is why these lamps need a warm-up period of roughly two to thirty minutes and generally stay lit continuously through a production run rather than cycling on and off. The spectrum that comes out is broad, spanning roughly 260 to 440 nanometers, and only a portion of that output — industry estimates put it as low as a fifth of total energy — actually falls in the range that cures ink. The rest becomes heat and, in open-air setups, some ozone.
An LED-UV array is solid-state — banks of UV-emitting diodes that reach full output the instant power is applied and can be dimmed or switched off between passes with no warm-up penalty. The wavelength is narrow and predictable, typically 385 to 395 nanometers, tuned close to what the ink is formulated to cure under, which is a large part of why LED wastes so much less energy as heat. See our how UV printing works guide for where curing fits into the broader print process, from inkjet deposition through the finished, cured film.
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