Fresnel Rhombs

Standard Version

The Fresnel rhomb is used to rotate the plane of vibration of linearly polarized light. It consists of two optically contacted Fresnel parallelepipeds of crown glass BK 7 or quartz glass Suprasil which by total internal reflection together create a path difference of 180° between the components of light polarized perpendicular and parallel to the plane of incidence. Over a relatively wide range the retardation is only minimally wavelength dependent, see curve.
This polarization rotator can only be used for very small acceptance angles. The parallelism error of the prism combination is less than 2". The end faces are adjusted perpendicular to the rotation axis.
The usable aperture is 9.5 mm. The holder is provided with a graduated circle marked from 0° to 360° and is supplied with a mounting rod. The rhomb is mounted in easily rotatable ball bearings so that a motor drive can be fitted; however, the rotating parts are not balanced.

Version for UV (215 - 450 nm)

The parallelepipeds are produced from Suprasil. The retardation is 180° ± 5% between 215 and 450 nm (230 to 350 nm: ± 3%). The entrance and the exit faces are single layer antireflection coated with minimal residual reflectivity of 1.7%. 
→ RFR 215

Version for the visible (400 - 700 nm)

The parallelepipeds are produced from crown glass BK 7. The retardation is 180° ± 1% between 400 and 700 nm. Deviation of ± 0.5° from normal incidence changes the retardation by ± 0.5%. The exit faces are multilayer antireflection coated (minimal residual reflectivity is less than 0.2% in the visible). 
→ RFR 400

Version for 500 - 1300 nm

The parallelepipeds are produced from crown glass BK 7. The retardation is 180° ± 1% between 500 and 1300 nm. The exit faces are not antireflection coated. Antireflection coatings are available for partial ranges upon request.
→ RFR 500

Version for 650 - 1700 nm

The parallelepipeds are produced from crown glass BK 7. The retardation is 180° ± 1% between 650 and 1700 nm. The exit faces are not antireflection coated. Antireflection coatings are available for partial ranges upon request. 
→ RFR 650

With simple mounting

For applications with lesser demands on the mechanics of the holder the Fresnel rhombs can be supplied cemented onto a base plate (with a cylindrical attachment 25 mm in diameter, centered on the optical axis of entry).

With Lambda/4 retardation

A single one of the parallelepipeds as used in the λ/2 Fresnel rhomb (described above) has λ/4 retardation but produces a lateral beam displacement of 13 mm. λ/4  Fresnel rhombs are supplied in a simple cylindrical holder:

K-Prisms

The „K" prism with λ/4 retardation is a prism which is free of beam displacement. The following sketch shows the construction of the prism and the path of rays. The prism halves are optically contacted and the edges of the contact surfaces are sharp so that the edge running through the middle of the field of view is hardly visible.
The three reflections together produce a path difference of λ/4 the dispersion of which is given in the diagram for both versions in BK 7 and Suprasil. A deviation of ± 0.5° from the normal incidence changes the retardation by ± 1.7%. The prisms are supplied mounted in a cylindrical holder with a clear aperture of 9.5 mm, outer diameter 30 mm, with extensions of diameter 25 mm.

• Version for UV in Suprasil, without antireflection coating  → RFK 200
• Version for the visible and IR in BK 7, without antireflection coating → RFK 400

Available on special order

• Prisms with similar constructions can be made of other materials or for other wavelengths.  → on request
• Other holders  → on request
• Antireflection coatings  → on request