1. A compound lens comprising:a first lens element;
a second lens element in alignment with the first lens element along a common optical axis, wherein the first and second lens elements are integrally joined to each other, wherein the first and second lens elements are co-molded from a silicone resin, and wherein the first and second lens elements are optically coupled to each other to reduce the optical path difference of center ray bundles and sagittal ray bundles propagating through the compound lens;
a third lens element integrally joined to the second lens element, the third lens element being a bi-concave frusto-conical lens, the bi-concave frusto-conical lens separating the center ray bundle from the sagittal ray bundle; and
a fourth lens element integrally joined to the third lens element, the fourth lens element being a bi-convex aspheric lens, wherein the third and fourth lens elements are in alignment with the first and second lens elements along the common optical axis.