Structures within Structures As the 3D printing process is
performed additively layer by layer, geometric structures can
be fabricated inside other structures to create optical ele-
ments. For example, areas of transparent material can be
surrounded by a material with a different refractive index,
to transmit light from point to point using total internal re-
flection (TIR) through the inside of a model. Opaque mate-
rials printed within or around a transparent material can be
used to block the transmittance of light from one section of
the model to another. This can be used to seal internal com-
partments and avoid optical crosstalk, or to minimize light
leakage from the model that may be distracting to the user.