Weight

The page provides advanced information on the Weight parameter.

Note: It is highly recommend to set this parameter to true excepted in interactive simulation.

Deactivating this option is useful to understand certain phenomena as absorption.

Weight and Minimum energy percentage

The Weight represents the ray energy. In real life, a ray looses some energy (power) when it interacts with an object.

The Minimum energy percentage parameter defines the minimum energy ratio to continue to propagate a ray with weight. This parameter helps the solver to better converge according the simulated lighting system.

If you do not activate weight, rays' energy stay constant and probability laws dictate if they continue or stop propagating.

If you do activate weight, the ray's energy evolves with interactions until they reach the sensors.

Specific scenarios

According to the configuration, activating the weight parameter may have some impacts on the simulation calculation.

The following configurations illustrate the behavior of the rays depending if the weight has been activated or not.

  • Ray/Face interaction Consider rays reaching an optical surface having a 50% reflectivity.

    • If you do not activate weight, rays have 50% probability to be reflected.

    • If you do activate weight, all the rays are reflected with 50% of their initial energy.



  • Ray/Volume interaction Consider rays propagating inside an absorbing material.

    • If you do not activate weight, rays have a probability to be absorbed or transmitted according to their path through the material.

    • If you activate weight, rays' energy decreases exponentially according to the material absorption value and the path of rays through it.



Tip: Practically, using weight in simulation improves results' precision as more rays with contributing energy reach the sensors. So, to get the same amount of rays on sensors without the Weight parameter, you need to set more rays in simulations, which tends to increase simulation time.

Direct simulation result when weight is activated

Direct simulation result when weight is deactivated

Weight Deactivation

Deactivating the weight is useful in two specific cases.

  1. When you analyze phenomena such as absorption. Considering a material with absorption, here is the observation of the absorbed rays using an interactive simulation.

    Interactive simulation result when weight is activated

    Interactive simulation result when weight is deactivated

  2. If you want a simulation performance improvement in a closed system.

Let us consider an integrating sphere with a light source and sensor inside of it.

The surface inside the sphere has a high reflectivity value. The system is set so the sensor is protected from direct illumination from the light source.



In this context, activating the Weight would highly extend simulation time.

When weight is activated, simulation time corresponds to 1747.

When weight is not activated, simulation time corresponds to 440.

This difference is due to the fact that low energy rays are still propagating after several bounds in the system for simulations using weight whereas the probability the rays still propagate decreases each bound they make for simulations not using weight.