Figure: Comparison of the three most frequent surface form imperfections (form error, waviness, and surface roughness) according to shape and type of deviation
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The three most reported surface shape imperfections are:
• Surface form error,
• Waviness and
• Surface roughness.
They represent deviations of the real surface from the ideal surface, as for the aspheric lens. The parameters used to describe the surface profile allow a prediction of the quality of a manufactured lens profile after processing. A high surface quality can among other things be achieved by a high process stability.
Surface form error
The form error describes the difference between the lowest and highest point of the test surface. Metaphorically speaking, it refers from mountain to valley, therefore the form error is given by the PV value, peak-to-valley. The PV value is one of the most important surface specifications for inspecting the surface of an aspheric lens. It is evaluated in waves or in fringes. It is also possible to specify it as an RMS or micrometer deviation. The RMS value (Root Mean Square) describes the mean square difference between the ACTUAL and the TARGET surface, taking into account the area of the defect.
Waviness
Waviness errors on an aspheric lens can be caused, for example, by polishing tools during the machining process. This surface deviation is therefore application specific. The waviness has a longer wavelength than the roughness, which is why the short wavelengths are filtered out for their examination. Only low frequencies may pass. It is often also referred to as the inclination error, which is examined over a defined length. A specification of waviness tolerances is only necessary if the waviness has an effect on the optical task of the aspheric lens.
Surface roughness
Surface roughness describes smallest irregularities on the optical surface. Therefore, only the short wavelengths are examined for analysis and low frequencies are filtered out. Surface roughness is a dimension for the quality of polishing processes. The effect on optical applications of the aspheric lens can often be decisive. For example, a high degree of roughness can lead to a faster wear of the aspherical lens as soon as high powers, such as those of a laser, act on it. In addition, scattering reduces the quality of the measurement results, which is why low surface roughness is considered a high-quality feature. In industries such as metrology or aerospace this is of importance. The determination of surface roughness is part of the manufacturing process, especially for high-quality aspheric lenses.
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asphericon has specialized in the production of aspheric lenses by grinding, polishing, diamond turning and high-end finishing. In this process a blank is subjected to various work steps:
• Grinding or diamond turning for shaping,
• Polishing the ground aspheric lens,
• Measurement for form and surface inspection,
• Measurement and processing by means of high-end finishing.
Grinding and polishing
Blanks are already shaped lenses and the starting material for the further process to produce an aspheric lens. In the first work step, the blank is ground to give it its desired shape. Various grinding tools and technologies are used for this complex process. The ability to simulate the individual process steps using asphericon’s unique CNC control software allows for an unprecedented realization, for high flexibility and reliability during the entire process. In the following, the polishing process is an important part in the production of an aspheric lens. Step by step, the surface is reworked to achieve the desired requirements (e.g. the surface shape deviation). Polishing can be done by machining with geometrically undefined, very fine grain, but also by chemical removal. A finished polished lens has a bright surface without pores and depth cracks as well as the desired shape accuracy and surface quality.
Diamond Turning
The diamond turning process is an alternative machining method for shaping an aspheric lens. A monocrystalline diamond is used to machine the lens surface. In contrast to grinding tools, this is much smaller and more filigree. Due to its high hardness, ultra-precise machining of the lens is possible, resulting in an improved surface quality. By means of diamond turning, non-ferrous metals, nickel-phosphorus layers, crystals, and IR-glasses can be machined, in addition to an aspheric lens made of plastic.
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