Measuring Surface Roughness on Challenging Large Optical Components

Measuring surface roughness on large optics has long been a challenge for manufacturers. Workstation based optical profilers do not have sufficient clearance to allow access to the central portions of larger components. Replication methods are messy and can potentially lead to damage of the optic.

4D's NanoCam Sq Dynamic Surface Profiler was developed to measure surface roughness on large optics and in other difficult applications. In addition to a standard workstation stand, several other measurement configuration have been developed.

The NanoCam tripod stand, shown below, enables the NanoCam to be placed directly on the optic for rapid sampling across the surface.

Measurements close to the edge of the optic can be accomplished by placing one leg of the tripod on a riser. The image below shows the NanoCam positioned with one leg off of the surface in order to measure the edge region. Roughness data from the surface is shown to the right.

The tripod below was developed for an application in which space constraints precluded the use of the riser method. The extended tripod was developed to image and measure the regions of a mirror blank that will eventually become the edges and vertices of a hexagon mirror, following trimming operations.

Some test configurations and optic geometries preclude on-optic measurement entirely. The images below show a custom gantry system which incorporates a NanoCam Sq to measure large optics with significant form. The NanoCam Sq is mounted on the overhead gantry and can be tilted to align to, and focus upon, the test surface. The test optic is mounted to the cart below for precision positioning below the measurement head. Dynamic Interferometry enables accurate measurement despite vibration, even though the metrology and test piece are not directly coupled.

In one more case, shown below, a NanoCam Sq was mounted on an industrial robot for freedom of movement in six axes. This flexibility enabled measurement of surface roughness over complex part geometries.