Busch And Muller Taillights use a fresnel lens to spread out the light from 2 LED's to have a Bright Light in a line and light up the rest of the surface as well .

Here's the Battery model Busch & Müller: TOPLIGHT Line

I use the Dynamo One..

I also use their 4D toplight senso Plus on another bike , the whole great big red Lens lights up with those 4 LEDs