Home/Analysis Technologies/Optical analysis/Non-Sequential Ray Tracing
Analysis Technologies | Optical Analysis
A method that assumes no order in which surfaces are passed and decides the next intersection from the position and properties of the objects and the direction of the ray alone.
Systems where the ray path is set by geometry
Forward Monte Carlo ray tracing
Ansys Zemax OpticStudio
What the method is
Objects are placed as surfaces or solid bodies, each with its own position and orientation in absolute coordinates. Which object a ray strikes, and where, is decided by the position and properties of the objects and the direction of the ray alone. A ray may strike the same object any number of times, or none at all.
At an interface a ray can be split into reflected and transmitted components. That splitting requires polarization calculation. As an alternative to splitting, there is also the option of randomly choosing either reflection or transmission with probability proportional to each component, which amounts to Monte Carlo branch selection. At a scattering surface, the number of scattered rays you set is generated.
The number of rays is specified per source. Analysis and layout ray counts are held separately, so the clarity of the diagram and the accuracy of the analysis can be managed independently.
How it works
No path is entered. From its current position and direction, a ray looks for the next object it intersects and reflects, refracts or scatters according to the optical properties of that surface. This repeats until a termination condition is met, and the result accumulated over many rays is the distribution on the detector.
That way of accumulating is why it is called forward Monte Carlo ray tracing. Splitting and scattering branch the rays, so the number of segments arising from one ray grows rapidly. That is why there are limits on intersections and segments per ray, with a ray terminated where it reaches the limit.
Strengths of this method
Because no surface order is supplied, paths the designer never anticipated, such as multiple reflection, total internal reflection and scattering from mechanical parts, appear in the result as they are.
Lambertian scattering, Gaussian scattering, the ABg model, tabulated BSDF data and user-defined models can be assigned per surface, so roughness and paint effects can be handled from measured values.
Importance sampling, which always scatters toward the target, greatly increases the number of rays reaching it for the same run time. Intensity is weighted by the mean of the scattering distribution within the solid angle the target subtends.
Where it fits
→ when handling systems where the order light passes through cannot be fixed in advance, such as illumination systems and light guide plates
→ when the system includes prisms, corner cubes, light pipes or geometry imported from CAD
→ when you need to include multiple reflection, total internal reflection and scattering from mechanical parts
→ when you want to identify stray light paths exhaustively
Imaging systems with a defined surface order: imaging and afocal systems that proceed in order from object plane to image plane are the territory of sequential ray tracing.
Propagation of coherent fields: in systems containing elements where no path is assumed, the accuracy of complex amplitude propagation is generally not guaranteed. Use physical optics propagation on an equivalent model that does assume surface order.
Getting a first read on ghosts: if you only want to know quickly which combinations of surfaces matter, an inventory of single and double reflections assuming surface order is faster.
The effect of assembly variation: to see the spread of performance against errors, set it up as a tolerance analysis.
Applications
Design the illuminance distribution and light distribution from source to detector.
Handle elements that guide light by total internal reflection and multiple reflection.
Evaluate ghosting and scattering, including reflection from the detector surface itself.
Evaluate with measured scattering data or ABg coefficients applied to the surfaces of mechanical parts.
Inputs and outputs
INPUT
| Three-dimensional geometry | Optical elements and mechanical parts, including geometry imported from CAD |
|---|---|
| Light source | From point sources to three-dimensional emission distributions. Analysis and layout ray counts are specified separately |
| Surface properties | Coatings and scattering models (Lambertian, Gaussian, ABg, BSDF, user-defined) |
| Trace settings | Enabling splitting and polarization, and the limits on intersections and segments per ray |
OUTPUT
| Distribution on the detector | Incoherent illuminance, coherent illuminance, coherent phase |
|---|---|
| Radiometric and photometric quantities | Radiant intensity, radiance, and photometric results including color |
| Volume distribution | The distribution of luminous flux accumulated in three-dimensional voxels |
| Path data | Ray paths that can be filtered by condition, so the largest contributors can be identified |
How it works
Place sources, optical elements, mechanical parts and detectors in three-dimensional space. Conversion from a model assuming surface order is also possible.
Give each surface a coating and a scattering model. Where a target is hard to reach, specify it for importance sampling.
Enable ray splitting and polarization, raise the limits on intersections and segments to the level needed, and trace.
Check the distribution on the detector, then filter the paths to identify the largest contributors.
Enabling splitting and scattering makes the segment count grow sharply, and a ray that reaches the limit is terminated there. A trace run without raising the limits cannot be trusted. Step 03 is where most of your time should go.
Comparison with related methods
| Method | Relationship | Main targets | When to use which |
|---|---|---|---|
| Non-sequential ray tracing (this method) | This method | Systems where the ray path is set by geometry | Intersections are decided by object placement and ray direction alone, and reflection, refraction, splitting and scattering are handled as they are and accumulated on the detector. |
| Sequential Ray Tracing | Alternative | Imaging systems with a defined surface order | On the premise that each surface is passed once in order, runs aberration evaluation and optimization quickly. |
| Stray Light Analysis | Downstream | Identifying unintended paths | An approach built on this trace, combining splitting, scattering, importance sampling and path filtering. |
| Physical Optics Propagation (POP) | Complementary | Propagation of coherent fields | Propagates a complex amplitude field rather than accumulating flux along rays. In systems containing elements where no path is assumed, accuracy is not guaranteed. |
| Tolerance Analysis | Downstream | Performance variation against tolerances | In systems where no path is assumed, the effect of errors is estimated against a merit function built from detector values. |
Supporting products
Non-sequential ray tracing is provided as one of this product’s analysis modes. Conversion from a model assuming surface order, and systems combining the two, are also supported. See here for licensing, system requirements and deployment.
FAQ
Related technical information
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