(coexisting on another dimensional plane)
A radiation meter was a small hexagonal pin worn by officers serving aboard Terran Empire starships that was used to monitor how much radiation the person absorbed, usually from the unsafe warp cores used aboard Terran Empire ships.
In 2155 aboard the ISS Enterprise, T'Pol and Commander Tucker were working on adapting the Suliban cloaking device to operate with Enterprise's systems. Tucker stopped her when he noticed that she had forgotten to wear a radiation meter, a safety precaution that he believed was vital. Tucker subsequently ordered Crewman Biggs to provide her with one so that she did not suffer the adverse effects of delta radiation exposure, an experience that scarred Tucker for life. T'Pol ignored the commander's warnings, but wore the radiation meter anyway. (ENT: "In a Mirror, Darkly")