In this example, we use Ansys Lumerical FDTD and Ansys Speos to model a diffusive scattering film used on an automotive display. The Bidirectional Scattering Distribution Function (BSDF) calculated in FDTD is used in Speos for photometric simulations and Human Vision experience.
Understand the simulation workflow and key results
In automotive displays, protective films are used to prevent the screen from scratches and damage. It can also serve additional purposes of reducing the glare, and reflection and improving the readability of the display. In this example, we use a rough surface to model this film and analyze its impact on the optical performance of the display.
Step 1: Calculate the BSDF with FDTD
In this step, we run a parameter sweep to calculate the BSDF of the diffusive film as a function of the incident angles, wavelengths, and directions (forward vs backward). The BSDF is then exported for further analysis in Speos.
Step 2: Speos analysis
In this step, we use the BSDF calculated in FDTD to define the diffuse scattering film applied on the screen. We run a photometric simulation to extract the key metrics such as luminance and color.
Note: the Speos simulation files are located in the SUV_Dashboard.zip file.
Run and Results
Instructions for running the model and discussion of key results
Step 1: Run FDTD simulations
- Open and run the script file [[BSDFExport_workflow.lsf]].
The simulation file [[bsdf_surface_roughness.fsp]] includes a nested parameter sweep over the following parameters:
- Direction: forward (from the substrate to air), backward (from air to substrate)
- Angle of incidence (theta) and anisotropy (phi)
- Polarization (S and P)
- Seed (used to reset the random number generator – see randreset - Script command )
- Rotation (0 and 90 degrees)
In this example, we consider the surface scattering has azimuthal symmetry, so we can rotate the structure by 90 degrees (rotation sweep) and average the 2 angles. Additionally, we average over different surfaces generated with the same parameters (seed sweep). Lastly, we average the S and P polarization to obtain the unpolarized results. Overall, the total number of simulations will be:
- 2 directions x 1 azimuthal angle Phi x 9 incident angle theta x 2 wavelengths x 2 polarizations x 5 seeds x 2 rotation angles = 720 simulations
The results are calculated by the analysis group "BSDFexport". It returns the BSDF as well as the transmission and reflection.
The additional flag "do_plots" allows to generate and plot the far-field intensity distribution on a 1m hemisphere, calculated on an unstructured mesh. The following plots show the forward (illumination from the substrate) and backward (illumination from the air above) transmission and reflection with an incident illumination at 30°.
The 2 BSDF (forward and backward) results are exported to .brdf SPEOS files ( bsdf_surface_roughness_forward.brdf and bsdf_surface_roughness_backward.brdf ) for the next step.
Step 2: Speos analysis
- Open "BSDF - BRDF - Anisotropic Surface Viewer" to generate a BSDF180 using bsdf_surface_roughness_forward.brdf (substrate-to-air) and bsdf_surface_roughness_backward.brdf (air-to-substrate) from the previous step. Save the file as LCD Anti-Glare Film.bsdf180 .
- Open SUV_Dashboard.scdoc in Ansys SPEOS and apply LCD Anti-Glare Film.bsdf180 to "LCD Protective Film".
- Compute the "LCD.Lit" simulation. The simulation results include one XMP result file and a simulation HTML report.
- Double-click the XMP result to open it in the Virtual Photometric Lab.
- Inside the Virtual Photometric Lab, change the "True Color" to "False Color" to review the luminance values at different positions.
- Click the "Measures" tool to measure the average luminance value of the LCD display.
Result with antiglare film:
It shows the measured luminance value with a legend bar on the right side. A square shape defines the interest measurement area at the main display with antiglare film (i.e. mainly coming from the reflection of the environment light sources).
Inside the measurement information table, it shows the average luminance value at the main display position is about 343.965 cd/m2.
Result without antiglare film:
It shows a comparison luminance value result at the main display without antiglare film. The interest measurement area is identically defined.
Inside the measurement information table, it shows the average luminance value at the main display position is about 424.915 cd/m2.
Important Model Settings
Description of important objects and settings used in this model
Step 1: FDTD Simulations
Simulation size: In this example, we took a “super-cell” approach to calculate the BSDF. Bloch boundary conditions were used together with a plane wave illumination. As such, the angular distribution of reflection and transmission is only calculated for directions corresponding to the grating orders of the periodic structure. As long as the period is large enough, this provides enough angular resolution.
PML: we use the “steep angle” profile to increase the efficiency of the PML boundaries as we vary the angle of incidence of the source.
Step 2: Speos Simulations
BSDF180 generation: The BSDF results of the transmission and reflection are generated at discrete angles. To manage a ray arriving at an intermediate incidence angle, SPEOS requires the BSDF data to be interpolated. When building the “LCD Anti-Glare Film.bsdf180”, interpolation can be applied by using interpolation enhancement inside the “BSDF - BRDF - Anisotropic Surface Viewer”. A detailed explanation can be further found in Interpolation Enhancement Overview .
BSDF180 material application: Because the BSDF results of the transmission and reflection are different, it is important to apply the face optical property in the correct direction. In SPEOS surface material properties application, a blue arrow indicates the direction of forward face optical direction. Thus, when applying the surface properties to “LCD Anti-Glare Film.bsdf180”, make sure the blue arrow is pointing outwards of the LCD display.
Simulation Meshing Settings: Meshing settings are critical for getting the correct simulation results. Meshing settings define the quality of geometries that will be simulated. The finer mesh gives better results but also requires longer simulation time. Rough mesh can lead to poor results, especially at the precise optical component. Meshing in this project is set to be proportional to the body size. Further details about mesh settings can be found in Meshing Properties .
Geometrical Distance Tolerance (GDT) Management: GDT defines the maximum distance to consider two faces as tangents. When the distance between two faces is smaller than the maximum distance defined, the faces are considered a tangent. Such a setting is especially important between the optical components which are very close to each other. More detailed explanations can be found in Tangent Bodies Management .
Stop Conditions/Number of Passes: such parameters define the criteria to reach for the simulation to end. A simulation with a higher number of passes will take much longer than a lower one. In this project, we used 500 passes to achieve a good result without too much noise. Details can be found in Creating an Inverse Simulation .
Updating the Model with Your Parameters
Instructions for updating the model based on your device parameters
Step 1: FDTD Simulations
Surface: the random surface is defined by the “rough_surf” structure group to allow easy modification of its properties:
- Refractive index or material
- Surface width
- Sigma rms, delta, correlation length
Analysis: the "BSDFexport" analysis group allows to set parameters such as the central wavelength, wavelength range, direction, angle of incidence, and polarization angle. These parameters are used in the sweep we run to calculate the BSDF. To modify them, update the corresponding parameter sweep.
Step 2: Speos Simulations
Light source direction: in the definition of natural light, the sun's position is defined as at 3 PM on 1st May 2021 in France. Users can choose a different zone and time to evaluate the design in other specific scenarios. In addition, the user can set the sun position to a specific direction by changing the “Sun type” into “Manual” and choosing a line/axis/direction in Ansys SPEOS. More information can be found in Creating a Natural Light Ambient Source .
Virtual Light Controller: user can evaluate the design under the different intensities of each light source. By changing the ratio/power value inside the virtual light controller. More details can be found in Using virtual lighting controller .
Taking the Model Further
Information and tips for users that want to further customize the model
Step 1: FDTD Simulations
Use BFAST: When using Bloch boundary conditions in a broadband simulation, the angle of incidence depends on the wavelength and is only correct at the central frequency. In this example, we run single wavelength simulations and calculate the BSDF at 2 wavelengths. To get the BSDF over more points, we can run broadband simulations and use BFAST to ensure the angle is the same over the full bandwidth. Note this has some drawbacks and limitations (see Broadband Fixed Angle Source Technique (BFAST) ).
Surface: Here we use a random surface generated by script, but the same workflow can be applied to any surface. That said, with the “super cell” approach and the use of Bloch boundaries, the surface should be periodic to avoid artifacts due to the discontinuity at the edges of the simulation region. The “rough_surf” analysis group enforces this.
Step 2: Speos Simulations
In addition to photometric analysis, you can run legibility and visibility analysis in Human Vision Lab. Right-click the XMP result and open it in Human Vision Lab. In the Human Vision Lab:
- Open the “Legibility and Visibility Analysis” function to check the perceived LCD display performance. Detailed instruction about the “Legibility and Visibility Analysis” function in Human Vision Lab can be found in Using legibility and visibility tools
- Legibility result without antiglare film:
This illustrates simulation results in the human vision lab, which shows how humans see the product design. The main interest is to check the legibility of the content from the display with antiglare film.
The result shows that for people of an age higher than 40, in sunlight conditions, it will be very difficult to clearly distinguish the content from the display:
- Legibility result with antiglare film:
This represents a comparison result with antiglare film applied on the display.
The legibility result shows that most people can easily distinguish the content from the display. For people at age of 60 or higher, it becomes difficult for them to read the content.
As a comparison, without the anti-glare surface, you can check the result within the virtual human lab with glare effect activated:
This represents a direct sun glare reflected by the display and seriously affects the driver. Users can also change the “Natural Light” direction with a design table in SPEOS, to review the perceived LCD performance with different sun positions
The animation above shows a washout study with a sun source rotating from different position angles. This shows a use case that SPEOS can help designers to understand the best/worst lighting behavior.
Additionally, running the “LCD.Lit” simulation in “Live Preview” mode will open a separate window and run the simulation on GPU. This allows the users to review the design from different view positions:
- Single-click the simulation you would like to preview, i.e. LCD.Lit
- Choose "Preview" function from the Light Simulation Ribbon.
- A separate Preview window will open.
- On the top of the "Preview" window, activate "Human Vision" and choose the corresponding display settings as presented below.
- Navigate within the "Preview" window to explore design and change view positions via moving mouse.
- In Ansys Speos 2022R2, you can click "Save" button on the "Preview" window to save your current preview result as an xmp file.
Additional documentation, examples and training material