Achromatic Mica Retarders

The retarders are produced with a path difference of λ/2 or λ/4 in the spectral range of 430 - 680 nm with a calculated accuracy of the path difference ± 1.5% for the λ/2 plate and ± 3% for the λ/4 plate. The retarders consist of three mica sheets which are cemented between plane parallel plates of crown glass BK 7. The flatness of the outer faces is λ/10 and the prismatic deviation is less than 2'. The outer faces are multilayer antireflection coated. Minimal residual reflection is less than about 0.2% per surface.
Due tuo the considerable absorption of mica the transmission of the achromatic plates can be reduced to as little as 50% at 500nm.
The retarders are supplied in a cylindrical mount engraved with the wavelength range, the path difference and the average direction of the optical axis.
In contrast to the achromatic plates, the axis orientation of these plates is wavelength dependent. The change is about 3° with the λ/2 retarder and about 1.5° with the λ/4 retarder.

The following diagram (Retardation R and axis orientation Ψ) is a calculated example. Because of considerable variation of the optical properties of mica a variation of the retardance of up to a few percent may occur.

Available on special order

• This type of achromatic retarder can also be produced for other partial wavelength ranges between 400 nm and 2500 nm. → on request

Literature

• S. Pancharatnam, Proc. Indian Acad. Sci. A41 (1955) S.137