Scatterometry: Technical Resources for Optical Scatter & Surface Roughness
Eckhardt Optics designs and builds custom scatterometer systems, including conoscopic scatterometers for BRDF/BTDF measurement. These resources cover optical scattering theory, measurement methods, surface roughness specification, and practical instrument guides, written by the engineers who design these systems.
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Optical Scattering
What is Optical Scattering?
Optical scattering is caused by two mechanisms: surface scattering and volume scattering.
With the former mechanism, scattered light comes only from the interaction of light with the surfaces of the optical element. When surface scattering is present, light will scatter from an optical element even when the optical material is completely clear and homogeneous.
For example, an astronomical telescope mirror reflects light before it even enters the substrate material, so no volume scattering is possible, but telescope mirrors are still susceptible to surface scattering.
Volume scattering, on the other hand, occurs only in the bulk of the optical material and is completely independent of the surfaces. A good example of bulk scattering is earth’s atmosphere. Even on the clearest day the sky will still be blue and sunsets red because of Rayleigh scattering.
In precision optical systems, both mechanisms may contribute to stray light, reduced contrast, or degraded image quality. Quantifying how light is redistributed in angle is therefore essential.
Several instrument types can measure optical scattering from surfaces or thin volumetric samples, each with different tradeoffs in speed, angular resolution, and dynamic range. Scanning scatterometers offer the highest dynamic range (up to 20-bit) and finest angular detail, but can take hours for a full hemisphere.
Imaging spheres, which are no longer available, capture nearly a full hemisphere in real time but at lower resolution. Conoscopic scatterometers split the difference, sub-second measurement with adjustable resolution from 0.005° to 0.1° depending on half-angle. Learn more about measuring optical scattering.
How Conoscopic Scatterometers Work
conoscopic scatterometer captures the full angular scatter distribution of a sample in a single sub-second exposure, no mechanical scanning required. The key is the conoscopic lens at its core, which maps the angle of scattered light to a position on an image sensor.
If you want to understand how the instrument works, start with how conoscopic lenses work, the optical layout of the lens and the scatterometer are nearly identical. Then explore how conoscopic scatterometers work for the full instrument design.
Optical Scattering and Surface Roughness
Rough surfaces scatter light. In many circumstances, this is bad, so you’ll probably want to understand the relationship between optical scattering and surface roughness. Visit our page on optical scattering vs surface roughness to gain clarity.
Surface Roughness
Measuring Surface Roughness
There are two classes of instruments for measuring surface roughness: mechanical and optical. Each has its advantages and disadvantages, which we’ll discuss further. But what will ultimately control your choice is whether or not you can contact the surface you need to measure with a stylus. Mechanical instruments measure by dragging a stylus with a very small tip across the surface, which can damage soft materials.
If you can’t touch the surface, you do have other options: non-contact profilometers use various methods to detect surface roughness without coming into contact with the surface itself. White light interferometers, phase shifting interferometers and confocal microscopes fall in this category. They sound like the perfect solution, but price can be an issue.
You can read a more detailed explanation of the issue on our guide to measuring surface roughness, as well as get some recommendations for trustworthy companies who make profilometers.
Our Tools for Surface Roughness
We don’t like leaving anything to chance, so we use several instruments to directly measure and infer optical measurements. Learn about our surface roughness instruments.
Measuring Roughness with a SJ-210
If you’re using a Mitutoyo SJ-210 to test surface roughness, you may benefit from our guide on setting it up. We’ve put together a list of extra parts you’ll need to hold the instrument while you make measurements, as well as a step-by-step guide on using them. Go to our page to read the full article → our Mitutoyo SJ-210 setup and measurement guide.
Specifying Surface Roughness
You may need to be able to specify the precise amount of surface roughness that’s present on your optical component. For more information on the length of measurement, interpreting ANSI/ASME B46.1 standard equations, and appropriate ISO standards, visit our page on specifying surface roughness.
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