The ornamental design for a led based lightbulb, as shown and described.
FIG. 1 is a front elevational view of a LED based light bulb showing our new design;
FIG. 2 is a side plan view of the design of FIG. 1 rotated 30° respective to the side plan view of FIG. 1;
FIG. 3 is a top plan view of the design of FIG. 1;
FIG. 4 is a bottom plan view of the design of FIG. 1; and,
FIG. 5 is a perspective view of the design of FIG. 1.
Any broken line illustrations in the drawings are not part of the design sought to be patented. Speckling indicates frosted glass or another translucent light diffusive surface, and a relatively higher density of speckling indicates the surface has relatively higher light diffusiveness (i.e., relatively reduced visibility through the surface).