planparallel

Planparallel is a term describing a geometry in which two opposite faces of a object are parallel, resulting in a uniform thickness across the part. The concept is often used interchangeably with plane-parallel, particularly in optics and precision engineering. Planparallel components may be flat plates, substrates, or windows designed to maintain consistent thickness and predictable optical or mechanical behavior.

In optics, planparallel plates are used as windows, beam splitters, reference standards, or light-path components. The parallel faces help minimize angular deviation of transmitted light and ensure a stable optical path length. However, any departure from perfect flatness or parallelism can produce interference fringes (often called etalon fringes) and fringe patterns, which can be undesirable in some applications but useful in others for metrology and calibration.

In manufacturing and metrology, planparallelism and planarity are key tolerances. Planparallelism refers to how closely opposite faces run parallel to a specified tolerance, while planarity concerns the flatness of a surface relative to a reference plane. Achieving planparallel surfaces typically involves precision grinding, lapping, or polishing, followed by interferometric or coordinate measuring machine (CMM) verification. Temperature effects, residual stress, and mounting-induced bowing are important factors to control in maintaining planparallel integrity in service.

Planparallel surfaces find use across optics, semiconductor substrates, precision instrumentation, and other fields requiring uniform thickness and predictable optical or mechanical performance. See also plane-parallel plate, parallelism, and planarity.