1. The pictures swow SHRINES which were stored in the crypts to be carried around in processions on certain festival days and but into festival rooms above. The texts give even the materials and dimensions of the parts of the shrine. The "Bulb" is made of wood coverend with gold, 2 cubits long.
2. The shrines show a creation myth, slightly deformed by Greek influence - the releif is from about 50 BCE).
In the original myth the first hill rises from Nun, the sea of chaos, the first lotus flower, representing Re, rises and opens, and from it rises Nefertem, the god of the rising sun, as snake.
Here it is Resomtus, another god of the rising sun, coming from the Lotus.
Another myth is that of Re driving in a barge across the sky. The picture combines both myths: the bow on which the persons stand is called "barge" in the texts, the lotus flower is on the stern of the barge, and Resomtus rises from it.
3. The bulb is called "the glory of Resomtus" in the texts - the best guess is, that it represents the sky. Since the god He (the small guy with the ball on its head) carries it on one picture, and He separates Geb (the Earrth) an Nut (the Sky) this seems pretty clear.
4. There are 3 "bulb" representations there, each one used for a special celebration, 2 on the first sunrise of a specific month, one for a new moon celebration of a month.
No light bulbs or plasma tubes in sight, also no sarcophagus lids...