Considerations when Using Cylinder Lenses
In its most basic form, a cylindrical lens is a subsection of a cylinder of glass where the circumference of the cylinder is the powered (curved) optical surface. A typical cylindrical lens has one powered surface paired with a plano surface, which enables added manufacturing and testing flexibility. Much like a spherical lens, the powered surface is governed by the surface accuracy and finish after the fine polishing process. The surface accuracy is defined by the surface irregularity and the surface quality, both of which are specified identically to spherical lenses. Unlike spherical lenses though, cylindrical lenses have additional specifications due to their unique geometry, including a power axis and a plano axis (See Figure 1). These axes are used to define the angle between the surfaces within the plane.
Figure 1: Example of plano and power axis in a cylinder lens.
The following specifications will require specific consideration that is dependent on the target application:
Power Axis Wedge
Power axis wedge occurs when the power axis is not orthogonal to the thickness of the lens. Similar to decenter of a powered surface in a spherical optic, the power axis wedge is a displacement of the actual optical axis. The presence of a power axis wedge in a cylinder lens indicates mechanical errors and optical aberrations in a system such as:
- Uneven line thickness at the opposing ends of the focus plane
- Beam deviation in the power axis
Figure 2: Example of power axis in a cylinder lens.
Plano Axis Wedge
Plano axis wedge occurs when the plano axis is not orthogonal to the thickness of the lens. This wedge adds an angular beam deviation that creates significant pointing errors in a system. The presence of plano axis wedge in a cylinder lens indicates multiple mechanical errors and optical aberrations in a system including:
- Uneven line thickness at the opposing ends of the focus plane
- Beam deviation in the plano axis
- Focus plane tilt
Figure 3: Example of plano axis in a cylinder lens.
Axial Twist
Axial twist occurs when the plano axis is not parallel to the optical plane of the lens. Axial twist represents a rotation of the powered surface of the cylinder lens with respect to the outer dimensions. This is especially detrimental to an application when rectangular elements are secured by their outer dimensions. The presence of axial twist in a cylinder lens indicates the following optical aberration in a system:
- Rotation of the image about the optical axis
Figure 4: Example of axial twist in a cylinder lens.
| Cylinder Lens Selection Guide | |||
| Centration Error | Illumination Grade | Beam Shaping Grade | Ultra-Precision Grade |
| Plano Axis Wedge | N/A | < 5 arcmin | < 2 arcmin |
| Power Axis Wedge | N/A | < 5 arcmin | < 2 arcmin |
| Axial Tilt | N/A | < 3 arcmin | < 2 arcmin |
