This is what the tubes look like when they are viewed in the Red end of the optical spectrum. Notice that the lower 10 mm diameter Argon tube is much brighter in the Red than either the Hydrogen tube or the standard Argon tube at this power level. Apparently, the optical conversion efficiency of the Argon/Neon gas mixture is increased when the RF power density in the tube is increased by confining the discharge to a smaller volume. Even though the power level of the Hydrogen tube (shown in the center of the picture) is ten times the power applied to the lower Argon tube, the argon tube is considerably brighter in the red end of the optical spectrum.