I just got an M&P 9mm. Love the pistol! I am considering getting night sights for it. But I just read that the trijicon type tritium night sights need to be exposed to light in order to glow. My concern is, what if the handgun is in my safe for weeks or a couple of months? Will the night sights glow after being in the dark all that time?

Tritium gas is radioactive, and the light is produced as it decays. No exterior light source is required. Over time (years), as the tritium decays, the light will fade.

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Pisgah

If it is a tritium night sight it does not need to be exposed to light. The Trijicon HD series has a orange or yellow ring around the tritium insert on the front sight. This yellow or orange ring will glow if exposed to light but the tritium in the center of the circle will always glow with no need to be exposed to light.

Thanks, I just re-read the article about these sights, and only the ring around the front sight needs light to glow:


"One of the main differences is a lack of white rings surrounding each lamp for daytime visibility. Instead, the HD sights use a photoluminescent paint ring over the front lamp only. This paint stands out in the day but also glows-in-the-dark for a short time period after being exposed to light."