Ray tracing is time consuming, sometimes, it takes hours to days, especially in some complex optical illumination system, and now here is a good case where it improves ray-tracing speed with little loss of accuracy.
A brightness enhancement filter (BEF) is used in LCD backlighting to improve the coupling of light to the outside world. It is made up of a series of prisms on the rear side of a plastic sheet. In this file, the BEF is modelled as a polygon object (POB):
The ray bending produced by the BEF causes light to flow out of the filter on the opposite side, and therefore improved brightness and illumination uniformity. Note that the plastic sheet is being illuminated by a single cylindrical source (inside a parabolic mirror) along one edge only.
When light travels from one refractive index to another, a partial reflection occurs because the speed of light is different in the two media. This means that some fraction of the energy is transmitted, and some fraction is reflected. Additionally, some energy may be lost (absorbed) especially if the interface has a metallic coating on it.
Partial reflections are sometimes known as Fresnel reflections. OpticStudio has sophisticated models of Fresnel reflections from bare and coated surfaces, including complex multi-layer coatings.
When a ray intersects the surface of an object, OpticStudio computes the fraction of energy transmitted, reflected and absorbed at the interface. It can then split the ray into two: a reflected and a transmitted ray, with the correct relative energy.
In this illumination systems, Simple Splitting tool is used and it can often give big productivity gains. However, the results should always be tested against full splitting, in order to establish confidence. In stray light work, where ghosts of ghosts of ghosts are important, full sampling will be both faster and more accurate.
Because we are interested only in the out-coupled light, we get a ray-tracing speed improvement of a factor of 6, with no loss of accuracy.
Reference Source:
- https://www.zemax.com/
- Zemax Optical Design Program User’s Guide, Zemax Development Corporation
- https://en.wikipedia.org/wiki/Main_Page
