Pellicle beamsplitters are constructed from thin, nitrocellulose films that are stretched and cemented to rigid supporting rings. Pellicles are very delicate and liable to resonate to certain vibration frequencies. Unlike thicker beamsplitters, pellicle beamsplitters have the major advantage of eliminating second surface reflections by making them coincident with the original beam.
Pellicle beamsplitters are available either coated or uncoated. Uncoated pellicles yield an average reflectivity of 8% from 375–2400 nm. Uncoated pellicles reflect 10% of the incident light when used at 45 degrees, thus providing a useful, low-intensity side beam, thus they are often used in beam sampling applications. Coated pellicle beamsplitters provide equal reflectance and transmission over the same range. Coated pellicles are generally manufactured with two different coatings, dielectric and Inconel®. Dielectric coatings are designed for a specific wavelength. Inconel coating contains nickel, chromium, cobalt, and iron. A thin Inconel film balances the transmission of each element, resulting in a wavelength neutral coating.
Pellicle beamsplitters have several advantages over plate beamsplitters. Unlike plates, the thinness of pellicle beamsplitters causes very little ghosting. Additionally, they foster a very low incidence of chromatic and spherical aberrations. Finally, they cause only negligible alterations in the optical path.
However, pellicle beamsplitters are very fragile devices and are sensitive to environmental factors. They are easily damaged, and should never be touched. When cleaned, pellicles should never be wiped or bushed. They should simply be allowed to dry in clean, moving air. Their drum-like design (film stretched over a frame) makes them susceptible to acoustic interference. Care should be taken to minimize this type of disturbance while they are in use.
Inconel® is a registered trademark of the INCO family of companies.