TruGlo USED, TruGlo Brite Site Tritium Pro Front/Rear Green Night Sight Set for HK VP9/VP40/P30/HK45, 3-Dot Configuration Front, White Focus Lock Ring, Steel, Black, TG-TG231H1W

Tritium is a form of hydrogen that emits a continuous light source when batteries are removed or exposed to light. Since light is a basic requirement for life on earth this feature is vital for a night sight since it is the light source that will give you the ability to see without light.