What is the difference between a fixed shade and variable shade on a welding helmet?

An auto darkening welding helmet provides protection to a welder with its ability to automatically darken and lighten based on brightness. This ability frees the hands and gives the welder more freedom to perform tasks without having to negotiate a faceplate. The auto-darkening filter lens, or ADF, is a liquid crystal display (LCD) that is similar in design to a digital alarm clock. The welder’s eyes are covered at all times so hazardous exposure is limited.