Evaluation of the Form of Aspheric Lenses

John W. McBride

TaiCaan Technologies Ltd.
Southampton, United Kingdom
J.W.McBride@taicaan.com

Using BODDIES software Fig. 1 shows a commercial aspheric lens surface.

Fig. 1: A measured aspheric lens surface.

The surface has been measured with a grid of 160 x 160 data points. To analyse the form of the lens, the first stage is to cut down the surface such that only the aspheric surface remains. This is shown in Fig. 2.

Fig. 2: Aspheric lens after data reduction.

The surface must now be pre-processed to obtain a normalised representation of the lens. This removes off-set and tilt and the data can then be processed for form analysis. Fig. 3 shows the new surface after pre-processing.

Fig. 3: Data set after pre-processing (please
note that the vertex is now at (0; 0; 0).

The pre-processing of the data leads to an inverted data set. The following parameters have been determined:

In Fig. 4 an 8th order polynomial fit is used to fit the data set. The figure shows a range of information including the 2D fit in the two main axes and the removal of the polynomial surface from the measured surface.

Fig. 4: Result in BODDIES after 8th order polynomial fit.

Within the BODDIES software there are a range of data fitting and data analysis methods available. Fig 5 shows the final surface after form removal.

Fig. 5: Final surface after form removal,
showing a number of surface defects.

Further Information

Jung M., Cross K.J., McBride, J.W. and Hill, M., A method for the selection of algorithms for form characterisation of nominally spherical surfaces. Precision Engineering, 24/2(2), 2000, 127-38

McBride, J.W. and Cross, K.J., The measurement and analysis of the three-dimensional form of curved surfaces. International Journal of Machine Tools Manufacture, 36(5), 1996, 597-610