This stainless pen light is just a little gem of a flashlight! It is only 3.5 inches long so it slips easily into your shirt or jeans pocket and you will forget it is there until you need it. It is a simple single mode Nichia W219BT- V1 LED flashlight that produces a very nice neutral tint. It will run off of either a single AAA alkaline/lithium, rechargeable 10440 lithium-ion battery or NiMH rechargeable Eneloop batteries. The lumen output varies based on batteries used but ranges from 175 lumens with a lithium-ion battery to 50 lumens with a NiMH Eneloop battery. When using lithium-ion batteries you should expect around 30 minutes of runtime and around 1 hour with Eneloops. We recommend using NiMH Eneloop batteries in this light due to the longer runtime, reduced amount of heat generated when using the light and they don't leak like traditional alkaline batteries.