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Material models from the optics laboratory lead to realistic simulations

Set in the right light

An optical simulation is only as adequate as the description of the material on which it is based. If a demanding development project requires high-precision data in order to achieve realistic results using simulation, these are ideally based on material models obtained through measurements. The new CADFEM Optics Lab provides such material models which are oriented towards the simulation requirements so that the calculation results correspond to reality.

Many developers ask themselves how their planned product will affect potential customers and whether it can really fulfill the desired functions under extreme lighting conditions – for example, at night or in bright sunshine. This is why optical simulations are carried out in many areas, including medical technology, the consumer goods and automotive industries, as well as in the planning and design of lighting concepts.

Continue to invest in expensive prototypes?

Simulation solutions enable product developers to optimize the desired performance without investing in expensive physical prototypes. For example, the display quality of a dashboard or a rear light should not blind the following traffic but should also be clearly visible day and night even in adverse weather conditions. For developers of medical products, for example for the detection of skin cancer, the correct illumination of the body parts to be examined plays a decisive role.

For this purpose, software for the science-based simulation of light is necessary in order to reproduce human vision with the aid of physical models. Ansys SPEOS is such a precise simulation tool for optical systems, which, integrated into virtual product development, simulates a realistic representation that corresponds to the individual user experience. In this way, the design of planned products can be designed and optimized even better.

Proper physics-based representation of the material

The accuracy of optical simulation, especially for complex macroscopic systems, is highly dependent on the quality of the material models used. Regarding surfaces, for example, the correct physics-based representation of the angle-dependent reflection is often important. Furthermore, properties regarding transmission and scattering are also of great importance.

In addition to powerful software such as ANSYS Speos and a precise CAD model from the design department, a precise material model is required for successful optical simulations. A verification of the simulation results is of course also valuable, among other things by comparing the luminance image created by simulation with a real luminance camera image.

Similar models can also work, but not necessarily ...

Although Ansys supplies a number of material models with the simulation software, it is not always certain whether the material model selected from the integrated library corresponds exactly to the material being used. The use of similar materials can produce useful results, but not necessarily.

An accurate optical material model can only be derived from specific measurements with corresponding material samples. For this purpose, CADFEM has set up a well-equipped optics laboratory at its new headquarters in Grafing-Schammach, headed by Dipl.-Ing. Matthias Noak. He studied information technology and has over 20 years of laboratory experience working at the lighting specialist OSRAM, including in the central laboratory for light measurements.

Measurement of spatial light distribution

“We conduct optical material measurements and modeling for our customers with samples they have suppled so they can perform correct optical simulations with Ansys Speos and similar programs,” reports Matthias Noak. “In our measurement laboratory we have a spectral measurement system with which we determine the reflection or transmission spectrum of the material sample supplied. In addition, we have a so-called Ansys Optical Measurement Device, a goniometer, for measuring the spatial light distribution. This enables us to scan the spatial reflection or transmission and also to record how the light is scattered within the material. In this way, we provide our customers with the information necessary for an exact model of their material sample. The customer therefore receives simulation results that correspond to the real conditions.”

Goniometric measurement
  • Receiver moves around the object
  • Measurement with various sources and irradiation angles as well as variable angular resolution
  • Measuring time depending on the surface condition or customer requirementson, average 6 - 8 hours
  • Geographic coordinates, containing lines of Latitude and Longitude
  • Light source: laser RGB and NIR and white light with filter

If, for example, the absorption or the scattering within a body cannot be represented correctly, the results of the optical simulation will be too light or too dark. Furthermore, if the characteristic values of the surface are inaccurate, reflections cannot be represented properly and, as a result, too much or too little light is reflected in a certain direction. Therefore, an accurate material model is required to perform sufficient simulations and to correct design imperfections already in the development phase.

Time and cost advantages with accurate material models

Many automobile manufacturers want to design unique rear lights for their vehicles that stand out. The challenging question is whether these lights actually work with the effects that developers have envisioned. And if not, what exactly needs to be changed so that the desired result can be achieved from every angle and in a wide range of lighting conditions? At the same time, legal requirements must be complied with at all times. Building various prototypes and testing them in detail is very time-consuming and costly.

Optics simulations using exact material models quickly provide the desired time and cost advantages, and often enable design optimization. This is possible if it is known at an early stage how the light passes through the red plastic material and which wavelengths are more or less blocked. It must also be clear to what extent the metallized surfaces reflect light in the device. Very precise information about the applied materials is required, especially in the case of multiple reflections.

This not only apply to automobile rear lights, but also, for example, to dashboards. The focus is on how the material reflects the sun and might create blinding effects. In the case of household appliances, developers want to know how the surface of an espresso machine, for example, which shines like piano lacquer, matches the brushed aluminum of the cup tray. Another application example is the roughened glass pane of an office door. Here, not only is the visibility from inside to outside of interest, but also the opposite direction.

Portfolio of the CADFEM optics laboratory

Purely absorbing bodies
Based on the transmission and reflection spectrum, we determine the wavelength-dependent absorption coefficients according to the Beer-Lambert law.

Diffuse scattering bodies
Here we scan the scattered light using our goniometer and determine the scattering parameters according to the Henyey-Greenstein and Gegenbauer models.

Reflection behavior on surfaces of non-transparent bodies (BRDF)
We scan the spatial light distribution at various angles of incidence with our goniometer and create a corresponding surface file.

Reflection behavior on surfaces of transparent bodies (unpolished)
For this, we need a metallized surface, or a CAD model of the surface, and we create a corresponding model based on a scan or simulation.

Reflection and transmission on transparent bodies (BSDF, fixed thickness)
We scan the reflectance and transmission and create a surface file that also represents the volume. This model may only be used with the thickness in which it was measured.

Finally, Matthias Noak emphasizes that “all generated material models are intensively validated for their suitability on an optical bench. The customer can therefore be certain that they are working with a material model that provides them with realistic simulations.”

Author: Gerhard Friederici (CADFEM Germany)
Images: © CADFEM
Published: November, 2022

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