Here is another zoom on a bulb with a very pronounced overlay from:

z = (((cos(z^2) - cos(2.0 * z^2)) * c) + sin(z)) * tan((z + c) * (4.0 + cos(c)))

So far, I think the tangent aides in the overlay effect. I wonder if the overlays are both externally and internally tangent to the bulbs. Humm...