I claim the ornamental design for a portable light bulb system, as shown and described.
FIG. 1 is a top perspective view of a portable light bulb system according to a new design.
FIG. 2 is a rear view of the portable light bulb system shown in FIG. 1.
FIG. 3 is a left side view of the portable light bulb system shown in FIG. 1.
FIG. 4 is a front view of the portable light bulb system shown in FIG. 1.
FIG. 5 is a right side view of the portable light bulb system shown in FIG. 1.
FIG. 6 is a top view of the portable light bulb system shown in FIG. 1; and,
FIG. 7 is a bottom view of the portable light bulb system shown in FIG. 1.
Those portions of the present invention which are presented in broken or dotted lines show the environment of the design only and form no part of the claimed design. Moreover, any shading presented in the accompanying drawings is provided merely to highlight the contours of the invention and is not intended to be illustrative of texture or gloss.