Tri-traget vacuum tube or tri-element vacuum tube is made of 2 layer of borosilicate glass which is specially hard. The absorber layer is coated with stainless steel, aluminum and copper in the inner tube. This selective coating is more stable sith when stainless steel is added, and the absortance is higher.Since copper is more effective than aluminum in reducing infrared condection, heat loss greatly cut down when copper is coated as the infrared-reflection layer.
