Is a neon light actually a mercury/phosphor tube?
Most “neon” lamps are mercury lamps with a colored phosphor coating on the inside. However the true neon lamp (that special red glow) is really neon gas glowing directly. Take a close look at an advertising lamp that contains a variety of colors. The mercury/phosphor ones will seem to emit light from their frosted glass walls. You are seeing the phosphors glowing. But the real neon lamp will emit light from its inside. The glass will be clear and you will see the glow originate in the gas itself.