In the helmet there is a sensor which can be adjusted for sensitivity, so it doesn't react to ambient light, but does react to the arc of the welder. The lens itself recieves a small voltage to keep it clear, when the voltage is removeed it darkens very fast. The reason for this "backwards" operation is so dead batteries result in dark, not clear.
To be useful for making a variable-reflectivity pellicle mirror, you'd need a material that varied its *reflectivity* in response to an applied voltage, not its absorptivity. Also, I think those auto-darkening helmets use a large-area LCD panel, and LCDs are out as far as pellicle mirrors are concerned, for reasons previously given...
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