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Cylindrical Lens manufacture
Cylindrical lenses are most commonly manufactured from a rectangular convex cylinder. The way in which the cylinder is mounted for fabrication is influenced by the radius of curvature and the dimension parallel to the cylindrical axis. If the substrate has a short radius of curvature, it will usually be mounted around a barrel shaped tool, whereas if the radius of curvature is long, the substrate is usually mounted onto a flat bottomed plate with a curved mounting face.
After deciding which tooling to use, cautious mounting of the substrate to the tool is critical otherwise the lens specification may be compromised. Because a cylindrical lens lacks the spherical symmetry of a spherical lens, and because the optical axis is a plane as opposed to a line, centring the lens is a much more demanding process.
In the majority of cases, the lens specification is process-controlled. It is essential to reference the cylindrical axis when mounting the substrate. The same mounting plate or cylinder must be used during the milling, smoothing and polishing processes for each lens, to ensure uniformity.
The surface figure and the radius of curvatures are two important factors when determining the surface of a cylindrical lens. The method of determining the radius of curvature is similar to that used for spherical lenses but due to the asymmetrical surface of cylindrical lenses, test equipment must be converted.
The surface figure is a measurement of how far the lens deviates from an ideal cylinder shape, and is determined by irregularity and power. Power is a measurement of conformity to the specified radius, and irregularity is a measure of the deviation from a perfect sphere.
In a cylindrical lens only the power in the perpendicular plane to the cylindrical axis is typically specified. The remaining surface form error, including the unwanted power in the parallel direction, is called the surface figure. It is usually specified in units of wavelengths and quoted separately along the parallel and perpendicular directions.