Chemical light stick
From Halopedia, the Halo wiki
Chemical light sticks, also known as "chemlights", are plastic tubes containing two isolated elements that when combined, are capable of producing light through chemoluminescence.
They are currently used by the UNSC. When lit, they glow a sickly yellow color, and are used primarily to mark areas or as emergency lighting. The UNSC also utilizes glow rods for similar purposes.
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