A welding arc is so bright because it creates a small pocket of plasma that reaches temperatures between 3,000 and 30,000 Kelvin, hotter than the surface of the sun. At those temperatures, gas molecules and metal vapors become supercharged with energy, and when their electrons drop back down from excited states, they release that energy as intense light across the entire spectrum, from ultraviolet through visible light to infrared.
What Creates the Light
When a welding arc strikes, electricity jumps across a gap between the electrode and the metal workpiece. That electrical energy heats the surrounding gas into plasma, a state of matter where atoms are stripped of their electrons. In a typical MIG welding arc using an argon-CO2 mix with iron vapor, the core temperature sits around 8,000 Kelvin (roughly 14,000°F). Some arcs push well beyond that.
The brightness comes from what happens inside that plasma at the atomic level. Electrons in the superheated gas atoms get knocked into higher energy states. When they spontaneously fall back to lower energy levels, they emit photons, individual packets of light. Billions of these transitions happen simultaneously across the tiny volume of the arc, producing an extraordinarily concentrated light source. At temperatures above 10,000 K, continuous radiation (a smooth glow across all wavelengths, like a lightbulb filament taken to the extreme) starts contributing significantly alongside the sharp bursts from individual atomic transitions.
The result is a light source roughly the size of a pencil tip that rivals the sun in intensity per unit area. Your eyes simply aren’t built to handle that kind of radiant power at close range.
More Than Just Visible Light
What makes welding light particularly dangerous is that the brightness you can see is only part of the story. A welding arc emits radiation across wavelengths from 200 to 1,400 nanometers. Visible light, the part your eyes can detect, occupies just a narrow band in the middle (400 to 700 nm). Below that sits ultraviolet radiation, and above it, infrared.
The UV output is especially intense and comes in three flavors. UV-A (315 to 400 nm) penetrates deeper into the eye. UV-B (280 to 315 nm) is the range responsible for most corneal burns. UV-C (200 to 280 nm), the most energetic type, is normally filtered out by Earth’s atmosphere from sunlight but is produced directly by welding arcs at close range. Meanwhile, infrared radiation (700 to 1,400 nm) penetrates deep into the eye and can heat the retina directly.
So the “brightness” you perceive is really just the tip of the iceberg. The invisible radiation on either side of the visible spectrum is doing damage you can’t even see happening.
What Happens If You Look at an Arc
Even brief unprotected exposure to a welding arc can burn your eyes. The most common injury is photokeratitis, essentially a sunburn on the surface of your cornea. It works the same way a skin sunburn does: UV radiation damages the thin outer layer of cells, but you don’t feel it right away. Symptoms, including pain, tearing, redness, and a gritty sensation, typically show up hours after exposure. Most cases resolve within a day or two.
The deeper concern is retinal damage. Visible light and infrared radiation from the arc can penetrate all the way to the back of the eye, where they trigger photochemical reactions that release destructive molecules called free radicals. These damage the delicate layers of cells responsible for central vision. A study of occupational welders found they carry a higher risk of phototoxic maculopathy, a condition where the central part of the retina deteriorates. Imaging could detect structural damage in welders’ retinas before they even noticed vision changes.
While most mild exposures heal on their own, severe or repeated damage can be permanent. One documented case of accidental arc exposure resulted in vision dropping to 15/100 in both eyes, with no recovery even 18 months later. The damage was traced to a persistent disruption in a critical layer of photoreceptor cells near the center of the retina.
How Welding Helmets Block the Light
Modern auto-darkening helmets use layers of liquid crystal cells sandwiched between polarizing filters. Light sensors on the helmet detect the sudden flash of an arc striking, and the system applies voltage to the liquid crystals, causing them to rotate and align in a way that blocks the overwhelming majority of incoming light. This happens in roughly 1/20,000th of a second, far faster than a human blink.
The level of darkness needed depends on how much current the welding process uses, because higher amperage means a hotter, brighter arc. OSHA sets minimum shade numbers for different processes and current levels. Low-amperage work (under 60 amps) calls for a shade 7 filter, which still blocks well over 99% of incoming light. Higher-amperage stick welding above 250 amps requires shade 11, blocking even more. TIG welding tends to need slightly lower shade numbers at the same amperage because the arc characteristics differ.
These aren’t just tinted glass. The filters must block UV and infrared radiation even in their “light” state, before the arc is struck. The auto-darkening feature handles visible light intensity, but UV and IR protection is always on. That’s why even a helmet that appears clear when not darkened is still filtering invisible radiation.
Why It Looks Bright Even From a Distance
You might notice a welding arc is uncomfortable to look at even from across a large room. This is because the arc is an exceptionally small, concentrated source of light. Your eye’s lens focuses that point source onto a tiny area of your retina, concentrating all its energy. It’s the same reason a laser pointer is dangerous while a room light at the same total power output is not: concentration matters more than total brightness.
Reflected arc light off shiny metal surfaces can also cause eye damage, which is why welding shops use screens and curtains around work areas. The UV component reflects efficiently off polished metal and concrete floors, meaning bystanders can develop photokeratitis without ever looking directly at the arc itself.