The bulb doesn't have to be clear, the filament vibration would make the light flicker visibly (for sensitive equipment that is) even if the glass isn't clear.

The bulb doesn't have to be clear, the filament vibration would make the light flicker visibly (for sensitive equipment that is) even if the glass isn't clear. So it may be that the filament is far more susceptible to vibration than the glass on any type of light bulb. As I said, this is my interpretation, the text doesn't make it clear to me.