What is a pellicle beamsplitter and when should I use it?

A pellicle is a very thin optical-grade nitrocellulose membrane (or film) stretched over an aluminum ring and bonded in place. In function, a pellicle beamsplitter serves the same purpose as a common plate beamsplitter. Upon close inspection, a plate (or "mirror-type") beamsplitter produces two reflected beams for a single input beam. One is a reflection off the first (or front) surface and the second is off the second (or back) surface. The result is what is called a "ghost image" or secondary reflection. In addition, plate beamsplitters slightly displace the transmitted beam laterally from the input beam due to the thickness of the glass substrate (1-3mm depending on size). For these reasons, plate beamsplitters minimize these effects and offer the best performance when the glass thickness is minimal, the coated surface is oriented toward the source, it is inserted in a collimated beam, and the back surface is AR (anti-reflection) coated. Pellicles on the other hand eliminate ghost images because of the thinness of the membrane (2 microns), since the first and second surface reflections are superimposed. In addition, absorption is imperceptible. Pellicles can be used uncoated as well as coated for several different reflection percentages. Also because of their lightweight, pellicles can be used when weight issues prevent the use of heavier plate or cube beamsplitters. However, since pellicles are extremely delicate, they have several drawbacks. The membrane can be easily broken (and cannot be repaired), thus they require caution in handling and mounting. Pellicles are also very sensitive to mounting stresses and atmospheric vibrations (including acoustic noise and compressed air), both can cause the film to warp or vibrate and therefore result in reflection and transmission distortions. Pellicles are highly desirable for interferometric applications since there are no ghost images, no change in the optical path length, and no chromatic aberrations with converging beams. However due to their cost and delicate nature, they are often not selected for the majority of applications requiring a beamsplitter.