Billet Bi-Lens

This device uses a primary source with a lens split into two half-lenses in accordance with its diameter, and a screen placed perpendicularly to the bi-lens optical axis. Figure 4 represents the Billet Bi-Lens device.

Each half-lens gives its own image of the primary source. Therefore each image constitutes a secondary source. The screen is placed at the distance from the secondary sources, namely at the distance from the bi-lens. If the bi-lens is moved away from the quantity and the primary source placed at the distance , then the secondary sources are located at the distance such as:

And they are moved apart from the quantity such as:

As before, we can observe that the interferences between two spherical wavefronts emitted by the same two amplitude sources and in all point , the interferences are written

And in the case where  :

And the interfringe is: