Ralf Ottow, a Dutch optics engineer, has designed the Maxblaster – a flashlight powered by 54 batteries, capable of producing a beam of light four miles long.
Ottow stripped out the innards of a powerful commercial flashlight and switched in a mercury arc bulb, which generates light by creating an ultra-hot plasma between two closely spaced electrodes inside the gas-filled central chamber of the lamp.
That results in a brighter, more focused beam but also kicks out more ultraviolet light (hence the sunburn, a product of early testing). So he added a specially coated reflector and designed, ground, and coated a new glass window that would trap UV rays while still pumping out light.
Via a bunch of great blogs: