The Raytrace Properties folder contains the default and optional user entered raytrace control settings that are applied to every surface in FRED. Raytrace Properties tell FRED how to propagate every ray that intersects a surface. The default controls are Halt All, Transmit Specular, Reflect Specular, and Allow All. The user can edit any one of these or create additional controls.
WARNING: Changing the default raytrace controls can have unexpected consequences. It is recommended that the user create a new control if different functionality is required.
The Raytrace Properties folder is found on the left hand side of the main FRED document window in the Object tree.
Right mouse clicking on the Raytrace Properties folder on the object tree exposes a list menu with the following options:
Summary Report When this option is selected, FRED prints a list of all the Raytrace Property controls in the model as well as any descriptive text associated with each.
Detailed Report When this option is selected, FRED prints a detailed summary of all of the Raytrace Property controls including allowed transmission and reflection, specular and scatter levels, intersection counts and power cutoff levels.
Create a New Raytrace Control Set Selecting this option opens the Raytrace Control Dialog allowing for definition of a new raytrace control. See the Create a New Raytrace Control Set help topic for further information.
Raytrace controls and coatings Raytrace Controls are applied before Coatings, but both together determine ray propagation. To continue propagating a reflected ray, both the Raytrace Control and the Coating must allow the reflection. To continue propagating a transmitted ray, both the Raytrace Control and the Coating must allow the transmission. Scattered rays are subject to the same limitations.
Intersection cutoffs Total and consecutive intersection cutoff thresholds apply to both coherent and incoherent rays and are evaluated at every surface intersection event. Rays leaving a surface interaction must satisfy both the total and consecutive Cutoff thresholds of the raytrace property assigned to that surface. A ray not satisfying the intersection cutoff threshold requirements will not be generated during the raytrace.
The Total intersection cut off represents the total number of times a ray may intersect with any geometry surface in the model. A rays total intersection count is incremented with each intersection.
The Consecutive intersection cut off represents the total number of times a ray may consecutively intersect with a particular geometry surface in the model. A rays consecutive intersection count is reset to zero each time the ray hits a different surface in the model, and only increments if the ray keeps intersecting the same surface.
Relative and Absolute power cutoff thresholds apply to both coherent and incoherent rays and are evaluated at every surface intersection event. Rays leaving a surface interaction must satisfy both the Absolute and Relative Power Cutoff thresholds of the raytrace property assigned to that surface as described below. A ray not satisfying the power cutoff threshold requirements will not be generated during the raytrace. Note that the specular power thresholds can interact with specular ancestry levels to generate parent rays where child rays would otherwise be expected, see the Ray Ancestry section below for more information on the rules that apply to ray ancestry.
Absolute Power Cutoff A ray leaving a surface must have a power value greater than the specified absolute power cutoff threshold.
Relative Power Cutoff Consider a ray incident on a surface with power P_in, which would generate a ray leaving the surface with power P_out. For the departing ray to be generated during the raytrace, (P_out / P_in) must be greater than the Relative Power Cutoff threshold.
With the exception of Lens, Mirror, and Prism components, the Raytrace Control for every surface created in FRED is Halt All, which stops ray propagation.
"Absorbed" rays represent the power "absorbed" at a surface. The "absorbed" power is computed by subtracting the power of all rays created at the surface from the power of the incident ray. Note that, according to the above definition, all rays prevented from being created at the surface by the raytrace property assigned to the surface would be counted as absorbed. "Absorbed" rays are physical rays that behave just like any other child ray generated at a surface. They can be used for analyses such as irradiance plots, spot diagrams, etc. They offer a convenient way to show the spatial distribution of absorbed power on a surface. A new ray selection operation has been added on the Analysis Surface Ray Selection list for selecting "absorbed" rays. Finally, "absorbed" rays are marked as having completed their raytrace and will not propagate away from the surface during the raytrace.
This example illustrates power distribution amongst rays when the "absorbed rays" option is selected under Allowed Ray Operations.
A) R + T < 1 (no scatter): An absorbed ray will be created with flux equal to 1 - R - T. This ray remains on the surface.
B) R + T + TIS < 1: Case 1: Scatter in reflection An absorbed ray will be created with flux equal to (1 - TIS)[1 - R - T] Case 2: Scatter in transmission An absorbed ray will be created with flux equal to (1 - TIS)[1 - R - T] Case 2: Scatter in reflection and transmission An absorbed ray will be created with flux equal to (1 - 2*TIS)[1 - R - T]
The Ancestry levels in FRED are used to determine whether or not a ray can be split at an interface. On creation, the Ancestry Level is automatically set to 0. That is to say, every ray is a Parent ray. When a propagated ray intersects a surface, if allowed, the ray can be split into reflected and transmitted specular components and 1 scatter component. The Parent ray specifier tells FRED which component maintains the Ancestry Level of the incident ray. The Ancestry level for any split ray that is not the Parent is incremented by 1. If the Ancestry level exceeds the cutoff, then the ray is halted, otherwise the ray continues to propagate. The following rules apply to ancestry levels:
The following figures illustrate Parentage and Ancestry Level splitting. In the first 2 figures, the Specular Ancestry Cutoff has been set to Level 2, which means that Grandchildren rays can no longer be split into specular components. Because the Scatter Ancestry Level has been set to 0, the Raytrace Control will not allow any ray to scatter, even if a Scatter Property has been assigned to the surface. The third figure shows a scattering surface. Note that since the reflected specular ray is the Parent, all of the scattered rays will maintain the Specular Ancestry Level of the incident ray.
Figure 1
Figure 2
Figure 3
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