Welding Safety · Buying Guide · Canada Shade numbers, the four-digit optical rating nobody explains, sensor count, switching speed and where CSA Z94.3 fits in. Everything you need to buy a helmet once and be happy with it. The short answer Judge an auto-darkening helmet on four things. Optical clarity, shown as a four-digit rating like 1/1/1/1, where lower is better on every digit. Shade range, fixed if you run one process, variable shade 9 to 13 if you switch. Sensor count, …