An automatic (auto-darkening) filter darkens the instant the arc strikes and clears again when it stops. That lets the welder see the workpiece without lifting the helmet, which reduces neck strain and improves weld quality.
The four-digit code
The filter carries four digits separated by slashes — 1/1/1/2, say. Each digit rates a separate optical property; 1 is best and 3 is the lowest.
| Position | Property | What it means |
|---|---|---|
| 1 | Optical class | Sharpness of the image |
| 2 | Diffusion of light | Haze or cloudiness |
| 3 | Homogeneity of transmittance | Absence of dark patches |
| 4 | Angle dependence | Quality when viewed off-axis |
What to look for when choosing
- Switching time — the faster the better; 0.1 ms or less keeps eye fatigue down.
- Light-state shade — usually 3 or 4; this determines how well you see between welds.
- Dark-state range — 9–13, for example, which must cover the processes you actually run.
- Number of sensors — two is enough for open work; four is better when the arc is partly obstructed.
- UV/IR protection must be permanent — a good filter still protects when the battery is flat.