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Definition
Light scattering is the way light behaves when it interacts with a medium that contains particles or the boundary between different mediums where defects or structures are present. It is different than the effects of refraction, where light undergoes a change in index of refraction as it passes from one medium to another, or reflection, where light reflects back into the same medium, both of which are governed by Snell’s law. Light scattering can be caused by factors such as the nature, texture, or specific structures of a surface and the presence of gas, liquid, or solid particles through which light propagates, as well as the nature of the light itself, of its wavelengths and polarization states. It usually results in diffuse light and can also affect the dispersion of color.
What Causes Light Scattering?
Surface scattering occurs when surface discontinuities and rough features in the micro and nano scales modify the behavior of light. Depending on their size, bumps, holes, and other geometric features on a surface can cause scattering geometrically, because light is reflected, refracted, polarized, or diffracted if the structures are smaller than 10 times the wavelength of the light interacting with them. Coatings such as paints, coatings, and other deposits like contaminants can also change the behavior of light due to either nature, even if very thin or sparsely spread out.
Volume scattering occurs when light passes through a material that contains particles that scatter light because they are reflective or refractive. The particles can be active or passive, absorbing, or reemitting lighting at lower energy levels in a certain angular distribution within the material, and they vary in size and concentration throughout the medium.
Ghost example: Photo taken with a cell phone camera that clearly shows three sharply focused ghost images of the candle flames. There is also a fourth, extended ghost image centered on the middle sharp ghost image.
Ghost example: A sequential ray trace of a single ghost image path. The light from an object in the field of view passes through the lens and forms an image on the detector at right. Some of the light is then reflected by the detector back into the lens. One of the lens surfaces then reflects the light back to the detector at a different location. This ghost image is of interest because the ghost light is nearly focused on the detector, which will lead to a much brighter ghost image than if the light were spread out over a larger area.
Why is Light Scattering a Problem in Optical Design?
While light scattering can be a tool for many illumination applications, it can also be a problem for imaging applications. Surfaces may scatter light in a way that creates additional paths for the light to travel back into the optical system, creating visual artifacts known as ghost images on the final image. Mechanical housing also can cause unwanted scattering within the system, and those are usually accounted for in illumination ray tracing applications, which track scattering events.
What is a Real-World Example of an Optical Design that was Dependent on Minimizing Light Scattering?
Camera systems of all sorts depend on optical design to produce a great, clean image, but also to ensure that there are no ghost images. The more complex the lens, the more important it is to factor in light scattering and eliminate it. After creating an optical system in an imaging design software tool, a designer can analyze the design in an illumination design tool to identify the possible paths reaching the image and quantify how likely they are to affect the image quality. Eliminating the supplemental paths without affecting the overall performance of the system can require changes from the main design, but slight modifications to the baffle structures already built in can also achieve significant improvements and clean up the final image.
What is Needed to Analyze and Minimize Light Scattering?
When using structures such as baffles to minimize ghost images, ray tracing the system to perform stray light analysis can help to rid the image of visual artifacts in the design. To do this, you can use illumination design software that provides a ray path analysis, which tracks every possible path the light can take in an imaging system, accounting for every structure in the design, including the mechanical housing of a system. The software can look at the power level of the ghost images generated, pinpoint where they come from, and enable designers to eliminate the offending surfaces or create structures that do the same, either manually or using optimization techniques.
How is Light Scattering Measured?
Light scattering can be measured using a scatterometer, which takes 2D or 3D measurements from one or more sources emitting onto a surface or into a medium and records the angular distribution of light (intensity) that is reflected back and/or transmitted through. The results can be recorded in many ways, but one common method is to record reflected light is as Bidirectional Reflected distribution Function (BRDF) and transmitted light as Bidirectional Transmitted Distribution Function (BTDF). Together, these two functions become the Bidirectional Scattering Distribution Function (BSDF). Depending on the measurement method, this function can define the light scattering of a surface, a medium, or both combined. Depending on the nature of the sample measured, it is possible to separate which of the two is the main contributor to the scattering effect. After the measurements are done, it is possible to establish a mathematical fit to the scattering profile of the material or surface, which can be used in simulations.
Optical Scattering Measurements and Equipment
Purchase solutions to measure optical samples and import custom data into Synopsys optical software tools.
How is Light Scattering Simulated?
Synopsys simulation software products LightTools and LucidShape provide capabilities for using light scattering measurements files, as well as for modeling scattering effects. Depending on the nature element(s) of the scattering effect, the distribution of light for surface scattering can be modeled using mathematical models such as Harvey-Shack, ABg, Gaussian, Lambertian, and interpolated angular distribution models. Volume scattering particles can be modeled using Mie theory, Henyey-Greenstein, or Gegenbauer methods, as well as several other mathematical models. Gradient materials can also cause an apparent scattering effect, even if it is the result of progressively changing index of refraction in the material, depending on where the material changes.
Coupled with optimization capabilities and advanced features such as ray paths, receiver filtering, and features in the Advanced Physics Module, LightTools can accurately simulate light scattering effects for stray light analysis, where the study of scattering is critical. In LucidShape, light scattering data can be used to design stylish automotive lighting systems and also deliver the accuracy and function required to meet strict regulations.
See also this related term: Total integrated scattering
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