There are two classes of instruments for measuring surface roughness: mechanical and optical. Each of these has its advantages. The primary advantage of the mechanical instruments is their cost. Optical surface roughness instruments image an area of the surface, so you can glean much more information.
A mechanical instrument for measuring surface roughness is known as a stylus profiler or profilometer because it drags a sharp stylus across the surface. In a properly designed instrument, the force on the stylus is small enough so that no visible scratch is produced, even on plastic. Our Mitutoyo SurfTest SJ210 has a 2 micron tip radius with 0.75 mN force and resolves 2.5 microns horizontally and 1.6nm vertically. It produces no visible scratch on acrylic, but scratches are visible on aluminum. The cost for an instrument like ours is on the order of 2k USD. For instructions on how to set up your instrument, go here.
If you do not want to touch the surface you are measuring, you need a non-contact profilometer. These are typically optical instruments. The earliest optical surface roughness instruments were confocal microscopes. Because they can scan a surface to provide 2D information rather than just a 1D trace, they provide a lot more information on the surface than a stylus can. However, technology moved on and CCD detectors combined with white light interferometry made measuring surface roughness of an entire area without scanning possible. The height data is still obtained by moving the sample vertically, though, which means that it takes on the order of a second to acquire the data. Vertical resolution is on the order of 0.1nm and horizontal resolution is set by the magnification of the microscope objective. An NA 0.5 objective should have a horizontal resolution on the order of 0.5 microns. State of the art instruments are made by companies like Zygo and Wyko (now owned by Bruker). There are other companies that make optical profilometers, but we have friends at both of these companies and have used their products, so we feel comfortable recommending them. Used Wyko optical profilometers are going for 20k USD on Ebay, so you can guess what a new one might run.
Eckhardt Optics LLC does not make an instrument that directly measure surface roughness. You can infer it from the optical scattering that can be measured by one of our scatterometers, but if your major concern is the surface roughness, you would probably be better served by one of the above instruments.
Contact or call us if you have questions about whether we can design an instrument to meet your surface roughness measurement needs. Schedule a consultation with our expert engineers to help figure out the best course of action.