Transmitted Wavefront Error (TWE) is an all-encompassing measurement of the performance of an optic or system containing transmissive pieces (e.g. lenses, filters, windows). This measurement includes the effects of surface form, homogeneity, and mounting stress, which is why AMS recommends this type of measurement when evaluating received optics. Some examples where the TWE test would show a different pass/fail result from the surface form test are:
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What should be a plane parallel window has some sag (power). It is shaped like a shallow bowl: one surface is convex, the other suface is concave, both surfaces failing their surface form specifications. But to first order, the thickness is the same over the entire part. In this case the TWE would pass, since each surface’s error cancels out the other for the transmitted
beam. -
A plane parallel window is made of glass with varying density (homogeneity error); the glass is more dense at the center than at the edge of the window. But the surfaces are accurately flat
and easily pass their specifications. As the wavefront travels through the glass, the center of wavefront travels more slowly through the dense glass, resulting in a highly concave output TWE which fails the specification.- There is a bright side: by careful measurement of surface errors and homogeneity, the manufacturer can calculate and polish one of the surfaces with a shape opposite that of the homogeneity error, bringing the TWE back into specification.
- An optic that passed its surface form specification is incorporated into an optical system, in which it is placed into a three-point mount. If the mount is overtightened, the transmitted wavefront will be distorted at the contact points and fail the TWE specification.
On the shop floor, surface form is easier to measure during the polishing process, requiring only a wipe-off of the surface. The TWE measurement requires both sides to be clean with no obstructing mounts or fixturing. It also requires a return optic, which takes additional time to set up and align. This is one of the main reasons surface form specifications are used during the manufacturing process, and transmitted wavefront is measured on the finished parts.
The Reflected Wavefront Error (RWE) measurement is used for all reflective optics, including telescope mirrors, satellite mirrors, reflective diffraction gratings, and more. Customers often request RWE tests to be performed for mirrors used at a nonzero angle of incidence (AOI), as the RWE becomes less sensitive to surface errors at higher AOIs. RWEs are also requested for coated optics to check for errors introduced by nonuniform coatings.