The "Shaft of Light" only occurs once a year at the caverns natural entrance when the sun and the natural entrance are in alignment. This is a postcard sold by the caverns, in town at the Carlsbad Caverns National Park administrative headquarters and at the caverns visitor center.
While the beauty of the postcard is impressive, I prefer the historical picture of Margaret Boles, daughter of renown park superintendent Tom Boles. To date, I haven't found a date for this picture, one of my favorites. BH