Cameras and imaging
Crystran Newsletter November 2016 Surface Analysis
August 13, 2018
Crystran has extensive capabilities in testing and measurement, and one of the inspection tools at our disposal is the Taylor Hobson CCI 6000 Talysurf which allows us to accurately analyse the surface roughness of optics to ensure our customers’ exacting specifications are met.
It is a non-contact white light instrument for precise measurement of roughness, waviness and form error of an optic. Mounted on special anti-vibration mounting, it can measure Roughness Average (Ra) with a resolution down to 0.1 Angstrom (0.01nm).
The Talysurf’s camera scans thousands of adjacent points over the surface of a sample area of the optic, which can be from 0.36mm2 to 3.6mm2 depending on which lens is used, and over a preselected depth range, to capture the surface coordinates of each point within the sample area as raw data. This highly versatile machine includes various digital filters that can be applied to the raw data, for example, to help separate out the surface wavelengths associated with roughness from those longer wavelengths associated with waviness and form error of the optic surface, so that only roughness wavelengths appear for measurement.
Software then uses this captured data to calculate a range of standard measurement parameters for average and peak-to-peak roughness.
Parameters we typically measure for general surface finish include Ra (Roughness Average, or arithmetic mean deviation of the roughness profile) and Rq (the RMS root-mean-squared value of roughness). For many International Standards, the Ra or Rq value is usually determined by taking data over 5 consecutive sampling lengths, and averaging these, to ensure the roughness value is typical of the whole surface.
Sometimes maximum surface height/depth measurements are required instead of average height, typically to identify particular defects, e.g. Rt (maximum vertical peak- to-valley height, measured between the highest and lowest points of the profile within the evaluation length).
Surface roughness measurement is complex, and is defined in different ways by different International Standards. To accommodate this, the Talysurf can be configured to output results to any one of a range of different International Standards. This leads to a wide range of “standard” parameters that the Talysurf can measure, e.g. amplitude (Rp, Rv, Rz, Rzi, Rmax etc.), surface average (Sa, Sq, etc.), unfiltered (Pa, Pq; etc.); the parameter definition being in accordance with whichever International Standard option is selected.
The Talysurf can generate on-screen both 2d and 3d colour-coded contour maps of the texture of the sampled surface of the optic. Any anomalies can be easily seen among the microscopic peaks and valleys of the surface. Scales and orientation can be adjusted to aid visualisation of surface defects.
2D profiles can also be extracted and presented graphically to examine the amplitude of the surface along specific user-defined paths.
The Talysurf can compile its measurements into accurate surface analysis reports. At Crystran we use these reports typically in internal quality control, although they may also be provided to customers, on request, as an additional service.
Please feel free to get in touch for further details if you would like to get some samples measured.