Wedge & Prism Angle

The performance of optical systems such as telescopes, camera lenses, and laser systems rely on the quality of the optical components within the systems. Many plano optical components appear completely parallel from a side view, however there is typically some wedge between the sides.  In extremely precise systems, knowing the degree of a window’s wedge is crucial.

Non-Contact Methods

There are multiple non-contact methods to measure wedge or parallelism.  Autocollimators are a common device used in the optics industry to do this.  When light passes through an optical wedge, the beam is deflected by some angle, δ, which is defined by the refractive index of the glass, n, and the glass’s mechanical wedge angle, α. The calculation for this is given by the formula: δ = 2(n-1) · α.

While this method is useful, the accuracy and reproducibility of the results are dependent on having a skilled operator.  It is also necessary to have a target mirror that is flat to λ/4 or better to ensure that the return image is sufficiently well focused.  In addition to all of the measurement capabilities that an interferometer has, it is also useful in measuring the wedge of a window under certain conditions.

Wedge Angle Measurement Setup

Measuring Wedge with Interferometry

The setup for measuring wedge of a plano optic is shown in the setup in the image.  The transmission flat (TF) and reference flat (RF) make a cavity with the wedge optic in between.  To reduce environmental errors, the distance between all components should be as small as possible.


For optical systems containing windows, measuring wedge can reduce error and increase efficiency.  Autocollimators are useful in measuring a window’s wedge, but the results are much less precise than those of an interferometer and are dependent of operator variability.  For this reason, ZYGO recommends the use of interferometry to measure the wedge of a window.

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