Ray Splitting
Generally, when a ray strikes a surface, part of the energy will be reflected, part will be transmitted, and depending upon the surface properties, part may be absorbed. Ray splitting refers to the ability of OpticStudio to compute both the reflected and refracted paths, and then continue to trace both rays. OpticStudio also supports an option to randomly choose either the reflected or the refracted path rather than both, see "Simple Ray Splitting". Rays reflected from otherwise refractive interfaces are commonly called ghost reflections.
Once a ray splits, each of the "child" rays will in general strike another object, and the rays may split again and again. After many ray-object intersections, the total number of rays can become extremely large, so controls must be placed on the ray tracing to ensure that the computation will eventually end.
There are several ways to restrict the amount of ray splitting that occurs:
Maximum number of ray-object intersections: This control defines how many times a ray along any path, from the original source parent ray to the final ray-object intersection, may intersect another object.
Maximum number of ray segments: A ray segment is that portion of a ray path from one intersection to the next. When a ray is launched from the source, it travels to the first object. That is 1 segment. If the ray then splits into 2 rays, each of those are another segment (for a total of 3). If each of those rays split again, there will be 7 segments. Generally, the number of ray segments grows far faster, and needs to be set much larger, than the number of ray-object intersections does.
Minimum relative ray intensity: As each ray splits, the energy decreases. The relative ray intensity is a lower limit on how much energy the ray can carry and still be traced. This parameter is a fraction, such as 0.001, relative to the starting ray intensity from the source. Once a child ray falls below this relative energy, the ray is terminated.
Minimum absolute ray intensity: This parameter is very similar to the minimum relative ray intensity, except it is absolute in system source units rather than relative to the starting intensity. If this is zero, the absolute ray intensity threshold is ignored. The initial intensity of each ray is always given by the source intensity divided by the total number of analysis rays for that source. The number of layout rays is not used to determine the initial intensity of rays, even for those rays drawn on layout plots. Source units are defined on the Units tab of the System, General dialog box. See "Source Units" for details.
All of these settings are defined on the Non-sequential (system explorer) section of the System Explorer.
Ray splitting and polarization
Because accurate reflection and transmission computation requires polarization information, ray splitting is only allowed when performing polarization ray tracing.
Ray splitting can be turned off, and in this case the transmitted path is always taken at a refractive interface unless the ray TIRs; the reflected path is of course always taken if the surface is a mirror.
The following figure shows some of the ray paths possible when rays are split. There is only 1 input ray drawn.
When using ray splitting, the number of rays being traced can become enormous if the minimum relative ray energy is set too low. For example, for rays traced inside a cube of glass with 50% coatings on all surfaces, a relative ray intensity of 0.01 will yield about 256 ray segments. If the relative ray intensity is set to 1E-8, about 270 million ray segments will be generated for EACH ray traced into the cube! Systems with relatively low reflectivity (such as AR coated optics) will not generate as many ray segments because the intensity of the reflected paths falls so much more quickly. Still, it is advised to set the relative ray transmission reasonably high, around 0.001, until the model is working well and more detailed results are needed.
Because the total number of rays gets so large, the 3D Layout plots can become very cluttered. One way to reduce the amount of rays drawn is to use the "filter" string to suppress drawing of some of the rays, see "The filter string".
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