WK21 U2 is a portable light as long as a finger. By using CREE XM-L2 U2 LED, maximum output is 500Lm. It is compatible with CR123A/16340 lithium batteries. WK21 has DIY mode (patented) to recognize modes what you want and memory function to remember the mode you used before. Also head rotary switch, tail disassemble strong magnet, and stainless steel pocket clip for your easy operation.