Until relatively recently, optical designers were limited to devising systems that employed spherical or rotationally symmetrical aspheric optical surfaces. The availability of modern optical manufacturing machinery capable of producing advanced surface shapes means that designers can now make use of the optical equivalent of a potato chip (i.e., a surface of completely freeform shape) in their systems. The possibilities are currently limited by the designer's imagination, or at least his or her ability to devise a mathematical equation that will describe such a surface as a continuous shape. The availability of such surfaces is leading to solutions to difficult problems.
Most optical surfaces are first generated to a shape that is a close approximation to the requirement. For example, an initial freeform surface shape can be generated by grinding, direct turning on a multi-axis precision lathe, or by ‘slumping’ a heated part over supports. A precision freeform polisher can then operate on the surface to produce an accurate, highly polished surface. Zeeko Ltd. has developed the first truly freeform automated polishing process.1 Machine manufacture is shared between Zeeko and its partner Satisloh. One of these machines is installed at the OpTIC Technium (St. Asaph, UK) and is capable of producing freeforms in excess of 1m in size.2
The machine operates using the Precessions process, which involves a sub-diameter inflated bulged membrane tool, operating in the presence of polishing slurry. The internal pressure of the tool and its axial position can be modulated independently, thus giving control of both the tool pressure and the polishing spot-size. The tool is spun about its axis and moved with respect to the rotatable part in X, Y, Z, and two angles of inclination, all under computer numerical control. The tool can also attack the optical surface at different offset angles, these being chosen depending on the desired function (pre-polish or surface-form correction). Surface smoothness of a finished part can be to a radius of less than 1nm.
The Zeeko freeform polishing machine typically operates over a dynamic depth range of a few to tens of microns on an optical surface. Larger removals can be achieved using `grolishing', a family of processes using the same machine that are intermediate between grinding and polishing. The control system in the machine allows high slope angles to be produced on a surface when required. Figure 1 gives an example of the latter by showing the Zeeko logo polished at a height of approximately 0.5μm onto a nominally concave surface. The results in Figure 2 confirm the correct operation of the form-optimization code and the tool-path generator, and demonstrate the machine's ability to follow the prescribed tool path and velocity map.
Figure 1. Freeform logo polished at a height of approximately 0.5μm onto an optical surface
A simple type of freeform is a toroidal mirror that can be used to give an astigmatism-free image when light is reflected off of the mirror at a specific oblique angle. Off-axis sections of rotationally symmetric aspheric surfaces can be produced directly as freeform surfaces, thus avoiding the need to manufacture a much larger component and then select an off-center portion. Such off-axis aspheric surfaces are of great advantage in unobscured mirror optics. Non-imaging illumination systems benefit greatly from freeform surfaces because local areas of an optical component can be adjusted in slope to optimize the light energy distribution
More complex surfaces are those of optical windows that conform to an airframe shape dictated by aerodynamic or reduced-signature considerations. The form of such a window can have a degrading effect on the imagery of any optical sensor that has to look through it. The optical sensor may itself therefore require freeform surfaces in order to effect adequate optical correction. Where the optical sensor has to scan over a range of angles through a conformal window, several (possibly movable) freeform components may be required for compensation. Figure 2 shows the color-coded Talysurf profile measurement of a freeform surface that has no axis of symmetry.
Figure 2. Talysurf profile measurement of a freeform surface with no axis of symmetry
The ability to produce smooth, accurate freeform surfaces has opened up many prospects in optical design. The Zeeko freeform polisher can produce such surfaces at sizes greater than 1m, and results have demonstrated this machine's abilities and accuracy. Large off-axis aspherics for unobscured multi-mirror optics and also accurate, highly polished conformal windows are now possible. Optical designers are also free (both in spirit and form!) to use unusually shaped surfaces in their new designs. This is well illustrated by modern illumination optics.
A module on freeform surfaces is included in the newly created Masters Degree course in Ultra Precision Technologies and Applications. This annual program led by Cranfield University, in partnership with the University of Cambridge, University College London, and OpTIC, will be launched in October 2007.
Bodelwyddan, United Kingdom
Professor Phil Rogers is an optical design consultant and an associate of the OpTIC Technium in St Asaph, UK. Previously he was chief optical designer at Pilkington Optronics/Thales Optics for over 35 years. He is a Fellow of SPIE, the Optical Society of America, and the Royal Academy of Engineering, and is a visiting professor at Cranfield University
London, United Kingdom
Coalville, United Kingdom
Dr. David Walker directs the Optical Science Laboratory of University College London and is technical director of Zeeko Ltd. After some years developing instrumentation for large telescopes, his more recent research has been in the field of computer numerically controlled polishing and surface metrology, with a particular focus on large optics and extremely large telescopes. He chairs the Form Metrology Special Interest Group of the National Physical Laboratory, which promulgates best practice in measurement technology.