We manufacture Birefringent Filter Plates in crystalline quartz to customer specifications. These are similar to our Retarders in that the crystal axis is parallel to the face of the plates. Wavefront distortion of each plate is better than 1/10-1/20 wave, with parallelism of the faces better than 1 second of arc. Birefringent Filters are used in dye laser tuning. We have many Birefringent Filter Plates in stock and can also produce plates on a custom basis. Please inquire.