Material: BK7 glass or UV grade fused silica.
Based on the principle that total internal reflection
  produces a phase retardation between p- and s-
  components. The retardation is insensitive to wavelength.
Half wave rhombs are made of two quarter wave rhombs
  optically contacted.
Designed to give a retardation of λ/4 or λ/2 over an
  extremely broad wavelength range.
λ/4 retarder causes a beam displacement ~ 1.3 x the
  aperture. λ/2 does not.

 Learn more about waveplates ...

Specifications of Fresnel Rhomb Retarders
Dimensions Tolerance: +0.0, - 0.2 mm
Flatness: < λ/10 @ 632.8nm
Surface Quality: 20-10 scratch and dig
Coating: AR coating available upon request

Quarter wave Fresnel rhomb retarder

Half wave Fresnel rhomb retarder