 The current through the filament was adjusted until it was of the same colour (and hence temperature) as the object, and no longer visible; it was calibrated to allow temperature to be inferred from the current.The temperature returned by the vanishing filament pyrometer and others of its kind, called brightness pyrometers, is dependent on the emissivity of the object.With greater use of brightness pyrometers, it became obvious that problems existed with relying on knowledge of the value of emissivity.