Every UV source loses output as it ages, and the drop is invisible because the light itself is invisible. Waiting for a bulb to fail outright means running an undercure risk for weeks first. The better approach is to watch a short list of indicators and replace on evidence, not on a guess. 1. Measured Output Has Dropped The definitive sign is a radiometer reading below the process minimum. UV bulbs typically fade gradually, and many arc lamps are considered end-of-life when irradiance falls to around 70 percent of the new value. If you take a reading at the part surface each week and log it, the decay curve tells you when replacement is due before any part fails. Without a radiometer, you are relying on defects to tell you, which is far more expensive. Our note on what causes UV light guide degradation over time covers how the delivery optics contribute to the same loss. 2. Visible Discoloration or Deposits Inspect the bulb envelope when it is cool. An arc bulb that has darkened, developed a brown or grey film, or shows a milky devitrified patch on the quartz has lost transmission and is near the end of its life. A blackened band near an electrode indicates electrode erosion from too many restarts. LED emitter windows can yellow or haze from heat and stray adhesive vapor, which also cuts output. 3. Physical Damage Any crack, chip, or bubble in a bulb envelope calls for immediate replacement. A cracked quartz arc bulb can fail violently when hot. Check the seals at each end for looseness. On LED heads, look for lifted or discolored lens material over individual emitters. 4. The Lamp Has Exceeded Its Rated Hours Manufacturers publish a rated life in hours for a reason. Once a bulb passes it, output is below spec even if the lamp still lights. Track run hours with the controller's counter or a simple log and replace proactively at the rating rather than pushing past it. 5. Rising Energy Use or Erratic Behavior An arc lamp that flickers, is slow to strike, or dims and brightens has an electrode or ballast problem and will not last. If the same curing step suddenly draws more current or the controller reports faults, the source is deteriorating. If you see these signs and are not sure whether the bulb, the ballast, or the light guide is the cause, Email Us with the lamp model and your radiometer log. What to Do When Replacement Is Needed Order the correct part. Match the bulb to the fixture and ballast by part number. A near-equivalent bulb can run at the wrong current and fail early or undercure. Handle quartz with gloves. Skin oil on an arc bulb bakes into a hot spot that shortens life. Wipe with the maker's recommended solvent before first use if handled bare. Reset the hour counter so the next replacement interval is accurate. Re-measure irradiance at the part surface with the new bulb and…