Bonded doublets, triplets, and beamsplitters are modeled in FRED using a Glue layer between bonded optical components.  The adjacent surfaces are placed in close proximity, but not touching, and a glue layer is applied between the appropriate surfaces.

The Glue command allows the user to select the glue material and the surface to be glued to. More than one surface may be glued to a surface and different glue materials maybe used between each surface pair.  The process for applying a Glue layer is explained in the Glue page dialog help file.

When a ray intersects a surface with a glue specification, FRED first checks to see if the ray, when propagated forward, will intersect the surface to which it is glued. If it will intersect, then the ray is traced assuming the glue material is present. If it does not intersect a glued surface, then it propagates assuming there is no glue.

The example below illustrates both of these conditions. The top surface of the L2 lens is glued to the bottom surface of the L1 lens with the material common to both lenses so that the rays between them do not bend when refracting into the gap (see ray bundle 1). The top surface of the L3 lens is not glued to the bottom surface of the L1 lens, so the rays bend when they refract into the gap (see ray bundle 2).  Note that the sides of all three lenses are absorbing.

There are no requirements for how thin or thick the glue layer must be, but the user should be aware that thick glue layers can lead to unexpected results. For example, the ray on the far left in the example above would not intersect the top surface of the L2 lens in the glue layer, so it was traced assuming an glass to air interface on the bottom surface of the L1 lens. The glass-to-air refraction causes the ray to bend in the gap and hit the left surface of the L2 lens. The dotted line shows the ray path that would have been taken if glue were present there.

Important Note:

There is one "pathological" condition in which the gluing algorithm will fail. The condition is best illustrated by the following example. The Pechan and Right Angle prisms shown below are constructed by extrusion of the in-plane segmented edge curves at their outlines. The extrusion is in-and-out of the page. Since the outlined edges form single surfaces in both cases, assigning a glue layer between the two prisms means that the glue assignment will be encountered at the initial intersection on the far right, not where the prisms are in close contact. Upon acknowledging the glue, the algorithm immediately attempts to intersect the surface to which it is glued, namely the vertical surface of the right angle prism. The algorithm does not check to see if the ray can intersect the initial surface (itself). As a result, the second intersection of the ray shown below would not be with the vertical surface of the Pechan prism but with the vertical surface of the Right Angle prism. The total number of intersections below would be three instead of the expected four. Therefore, the user should avoid gluing objects which are constructed from extrusions.