Spherical mirrors are curved so that reflected beams converge on a focal point. Important parameters to consider when searching for spherical mirrors include diameter, radius of curvature, thickness, and focal length. The diameter of the mirror if viewed straight on can also be thought of as the height. If a mirror's curvature was extrapolated into a sphere, the radius of that sphere is the radius of curvature for that mirror. The thickness can be specified as center thickness or edge thickness. Focal length is the distance from the spherical mirror the light converges. Mirror material refers to the classification of the material used to make spherical mirrors. The material used influences the reflectivity characteristics of the spherical mirror. Choices include BK7 glass, copper, fused silica, nickel, optical crown glass, and UV grade fused silica.
Mirror coatings are used to enhance the reflectivity of spherical mirrors. Common coatings for spherical mirrors include none (uncoated), bar aluminum, enhanced aluminum, protected aluminum, dielectric, bare gold, protected gold, rhodium, and silver. Aluminum is an excellent reflector in the upper UV, visible, and near infrared regions; however, the surface is soft and must be handled with care. With enhanced aluminum coatings, a multilayer film of dielectrics on top of aluminum is used to enhance the reflectance in the visible or ultraviolet region. The multilayer film also provides the handling characteristics of the protected aluminum coating. In a protected aluminum coating, a coating is used as an overcoat to protect the delicate aluminum. This treatment provides an abrasion resistant surface while maintaining the performance of aluminum. Dielectric coatings are multi-layer coatings which offer excellent performance over a specific wavelength range and are relatively insensitive to small angle changes. Dielectric coatings tend to be more durable than metal coatings. Spherical mirrors with gold coatings provide consistently high reflectance in the near-IR to far-IR. Rhodium coatings have a reflectivity of approximately of 80% throughout the visible spectrum. Silver coatings are superior in reflectance to aluminum; however its strong tendency to oxidize and tarnish means that it must be thoroughly sealed from the atmosphere in order to avoid degradation.
Surface Quality Ratings
Surface quality ratings for spherical mirrors include 10-5 scratch / dig, 20-10 scratch / dig, 40-20 scratch / dig, 60-40 scratch / dig, and 80-50 scratch / dig. A dig is a defect on a polished optical surface that is nearly equal in terms of its length and width. A scratch is a defect on a polished optical surface whose length is many times its width. 10 / 5 indicates the average diameter of the digs to be .05 mm and the average length of a scratch is .10 mm. 20 / 10 indicates the average diameter of the digs to be .10 mm and the average length of a scratch is .20 mm. 40 / 20 indicates the average diameter of the digs to be .20 mm and the average length of a scratch is .40 mm. 60 / 40 indicates the average diameter of the digs to be .40 mm and the average length of a scratch is .60 mm. 80 / 50 indicates the average diameter of the digs to be .50 mm and the average length of a scratch is .80. Spherical mirrors with other, unlisted scratch / dig designations are also available.